AB-183 [ETC]
AB-183 - NEW ULTRA HIGH-SPEED CIRCUIT TECHNIQUES WITH ANALOG ICs ; AB - 183 - 带有模拟IC的新型超高速电路技术\n
| 型号： | AB-183 |
| 厂家： | 
ETC |
| 描述： | AB-183 - NEW ULTRA HIGH-SPEED CIRCUIT TECHNIQUES WITH ANALOG ICs
|
| 文件： | 总17页 (文件大小：216K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
®
AP P LICATION BULLETIN
Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706
Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132
NEW ULTRA HIGH-SPEED
CIRCUIT TECHNIQUES WITH ANALOG ICs
By Christian Henn, Burr-Brown International GmbH
With the increasing use of current-feedback amplifiers, the
TECHNICAL PROCESS REQUIREMENTS
Diamond Structure has come to play a key role in today’s
analog circuit technology. Two new macro elements that
function in this structure are the Diamond Transistor and its
abridged version, the Diamond Buffer. These elements can be
used for both voltage and current control of analog signals up
to several 100MHz. The OPA660 combines both of these
elements in one package. Starting with a discussion of the
technical process requirements for complementary-bipolar
circuit technology, we would like to focus on the basic and
functional circuits of the Diamond Transistor and Buffer.
These circuits can be used in areas ranging from video signal
processing and pulse processing in measurement technology
to interface modules in fiber optic technology.
FOR COMPLEMENTARY CIRCUIT TECHNIQUES
Circuits implemented in push-pull arrangements, in which
both NPN and PNP transistors are located in the signal path,
demand a particular high level of symmetry in the electrical
parameters of complementary transistors. See Figure 2.
The most important requirement is that the bandwidths be
equal, since the slower transistor type determines the perfor-
mance capability of the entire circuit. The bandwidth of an
integrated bipolar transistor is dependent both upon the base
transit time and upon various internal transistor resistances
and p-n junction capacitances.
Another important point is the DC performance, which can
be described best by the parameters saturation current IS,
current gain BF and early voltage. The Diamond Transistor
and buffer are manufactured using a complicated process
with vertically structured NPN and PNP transistors. Table I
shows the most important parameters of a transistor of size
111. Two metallization layers with a gold surface simplify
the connection between the circuit parts.
SYMBOLS AND TERMS
In technical literature, various symbols and terms are used
to describe the same circuit structure, see Figure 1. Burr-
Brown has chosen the transistor symbol with opposed
emitter arrows. The symbol calls attention to the functional
similarity of the bipolar and Diamond Transistors, and the
double arrows refer to the Diamond Transistor’s comple-
mentary construction and the ability to operate it in four
quadrants. Regardless of how it is depicted, this type of
structure has a high-impedance input, a low-impedance
input/output, high transconductance and a high-impedance
current source output. The voltage is transferred with very
low offset of +7mV from the high-impedance input to the
low-impedance input/output.
APPLICATION EXAMPLES
•
•
•
•
•
•
Wide-bandwidth amplifiers
Video signal processing
Pulse processing in radar technology
Ultrasonic technology
Optical electronics
Test and communications equipment
3
2
C
E
VIN1
VIN2
1
B
PNP Transistor
NPN Transistor
gm
IOUT
B
E
B
C
p
B
E
B
C
p - Epi
n - Epi
n
Diamond Transistor
Voltage-Controlled Current Source
Transconductor
p - Substrate
Macro Transistor
Current Conveyor II+
3
n+ Doping
n– Doping
p+ Doping
p– Doping
Silicondioxide
Poly-Silicon (implanted)
1
Z
X
Y
CCII+
IOUT
2
FIGURE 1. Symbols and Terms.
FIGURE 2. Complementary Bipolar Technique (CBip).
©1993 Burr-Brown Corporation
AB-183
Printed in U.S.A. May, 1993
The Operation Transconductance Amplifier section, or OTA,
will be referred to as the Diamond Transistor in the follow-
ing. Its three pins are named base, emitter, and collector, like
the pins of a bipolar transistor. This similarity in terms
points to the basic similarity in function of the two transis-
tors. Ideally, the voltage at the high-impedance base is
transferred to the emitter input/output with minimal offset
voltage and is available there in low-impedance form. If a
current flows at the emitter, then the current mirror reflects
this current to the collector by a fixed ratio. The collector is
thus a complementary current source, whose current flow is
determined by the product of the base-emitter voltage and
the transconductance. Because of the PTAT (Proportional to
Absolute Temperature) power supply, the transconductance
is independent of temperature and can be adjusted by an
external resistor.
PARAMETER
NPN
PNP
DIM
Current Gain
Early voltage
CJS
CJE
RB
RC
220
66
0.26
0.02
540
46
115
30
0.50
0.02
429
43
V
pF
pF
Ω
Ω
GHz
Transit Frequency
3.5
2.7
• symmetric NPN/PNP pairs
• n-epitaxy, p-collector implantation
• complementary B/E structures
• isolation variations
• complex process sequence
• n-cube
• p+ and n+ buried layer
TABLE I. Process Parameter.
SIMPLIFIED CIRCUIT DIAGRAM
OF THE DIAMOND TRANSISTOR
The OPA660, Figure 3, is a new type of IC which can be
used universally. It consists of a voltage-controlled current
source (Diamond Transistor), a complementary offset-com-
pensated emitter follower (buffer amplifier, Diamond Buffer),
and a power supply. This new IC enables users to adjust the
quiescent current, and through its temperature characteris-
tics, it maintains a constant transconductance in the Dia-
mond Transistor and Buffer. The emitter follower functions
without feedback. For this reason, its gain is somewhat less
than one and is slightly dependent upon the load resistance.
The main task of the emitter follower is to decouple signal
processing stages.
Besides these features, the Diamond Transistor and Buffer
can process frequencies of up to several 100MHz with very
low errors in the differential phase and gain. Thus, they are
universal basic elements for the development of complex
circuits designed to process fast analog signals. Current
control, voltage control, and operation with or without
feedback are all possible with the Diamond Transistor and
Buffer. See Table II.
PARAMETER
UNIT
DT Transconductance
Offset Voltage
125mA/V
+7mV
It is distinguished by its extremely short delay time of 250ps
and an excellent large-signal bandwidth/quiescent current
ratio. When comparing the Diamond Buffer with the Dia-
mond Transistor, it becomes apparent that all aspects of the
components are identical except for the current mirror. The
Diamond Buffer can thus be called an abridged version of
the Diamond Transistor.
Offset Drift
50µV/°C
2.1µA
±10µA
1MΩ || 2.1pF
25kΩ || 4.2pF
0.06%
Input Bias Current
Output Bias Current
Input Resistance
Output Resistance
Differential Gain
Differential Phase
Quiescent Current
0.02%
1-20mA
TABLE II. Diamond Transistor Parameter.
(7) +5V
DB
DT
VIN
(5)
VOUT
(6)
B
E
C
(3)
(2)
(8)
100Ω
50kΩ
RQ
–5V (4)
(1)
FIGURE 3. OPA660 Schematic.
2
PTAT POWER SUPPLY
EQUIVALENT CIRCUIT DIAGRAM
PTAT biasing-controlled current source with adjustable qui-
escent current, Figure 4.
The user can construct a simple equivalent circuit diagram,
Figure 5, for the Diamond Transistor based on the previous
descriptions. The complementary emitter follower with an
input impedance of 1MΩ || 2.1pF at the base pin can be
regarded as a controlled voltage source. Using the fact that
an emitter follower is, in principle, a voltage-controlled
current source whose current flow is dependent upon the
voltage difference between base and emitter, it is possible to
determine the output resistance of the emitter. The output
resistance can best be approximated as the reciprocal value
of the transconductance. A quiescent current set at 20mA
results in a transconductance of 125mA/V and a low-imped-
ance output resistance of 8Ω, which is adjustable but stable
with temperature. The collector pin performs like a comple-
mentary current source, with output impedance
25kΩ || 4.2pF. The possible positive or negative current flow
results from the product of the input voltage difference times
transconductance times current mirror factor, which is fixed
at AI = 1 in the OPA660. The model presented here shows
the similarity to the small-signal behavior of the bipolar
transistor.
Each individual transistor stage has a current source as the
load impedance. Thus, control of the current source allows
adjustment of the quiescent current. The adjusted quiescent
currents and the transistors used determine the
transconductance of the entire circuit. An external resistor,
RQ, fixes the quiescent current. We will discuss the exact
equations for the ratio of the quiescent currents RQ and
transconductance to IQ in detail later in this paper.
+VCC
1
1
1
Vt
I’Q
I’Q
=
In (10)
RQ
I’Q
I’Q
2 In 10
RQ
gm =
I’Q
10
OPA660 BASIC CONNECTIONS
AND PINOUT CONFIGURATION
1
1
RQ
For trouble-free operation of the OPA660, several basic
components on the power supply, as well as those compo-
nents which define function, are necessary. See Figure 6.
The triple configuration of the supply decoupling capacitors
at pins 4 and 7 guarantees a low-impedance supply up to
1GHz and supplies the IC during large-signal high-fre-
quency operation. The voltage supply is ±5V, resulting in a
maximum rated output of ±4V. As already mentioned, the
external resistor RQ between pin 1 and –5V adjusts the
quiescent current. A resistance value of 250Ω results in a
quiescent current of 20mA. Process variations can cause this
current to vary between 16mA and 26mA. The product data
sheet illustrates the exact relation between RQ and IQ. As in
discrete HF (high-frequency) transistors, a low-impedance
resistor damps oscillation that might arise at the inputs. The
circuit consists of the pin capacitances and inductances of
the bond wires. The resonant frequency is between 750MHz
–VCC
FIGURE 4. PTAT Power Supply.
IC = AI • gm • VBE
±VCC
C
RC
CC
4.2pF
IIN = gm • VBE
25kΩ
B
2.1pF
1
VBE
1MΩ
gm
±VCC
1
gm
~ 8Ω at IQ = 20mA
E
FIGURE 5. Equivalent Circuit.
RQ = 250Ω
1
2
3
4
8
7
6
5
C
+VCC
E
RG = 80 to 250Ω
Out
+1
B
470pF
2.2µF
10nF
–VCC
In
RG = 80 to 250Ω
2.2µF
470pF
10nF
FIGURE 6. OPA660 Basic Connection
3
and 950MHz, depending upon the package type and layout,
and is outside the operating range. The damping resistance
is between 50Ω and 500Ω, depending upon the application.
BASIC CIRCUITS WITH THE OPA660
Listed below are the basic circuits possible with the Dia-
mond Transistor:
• Emitter Circuit
• Base Circuit
• Common Emitter
• Common Emitter with Doubled Output Current
• Current-Feedback Amplifier
• Direct-Feedback Amplifier
TEST CONFIGURATION
FOR DETERMINING THE DYNAMIC FEATURES
OF THE DIAMOND TRANSISTOR
Figure 7 shows the test configuration to determine the
dynamic features of the Diamond Transistor. The entire test
system functions as a 50Ω transmission system to avoid
reflections from the input resistances. Various signal genera-
tors and indicators can be used depending upon the measure-
ment task. The layout of the demo boards used here is
designed for minimum line length and stray capacitance and
uses the three-level combination of supply decoupling ca-
pacitors and 50Ω HF-connectors. Burr-Brown offers these
demo boards to support design engineers during the test
phase.
As already mentioned, the signal transmission of the Dia-
mond Transistor is the inverse of that of the bipolar transis-
tor. The emitter circuit functions in non-inverting mode and
the base circuit in inverting mode.
The emitter-collector connection enables the user to increase
the output current of the emitter follower. Since both cur-
rents flow in the same direction and the current loop factor
AI of the OPA660 equals 1, the output current doubles to
±30mA and the output resistance halves. See Figure 9.
In many applications, the high-impedance collector output is
a disadvantage. One possible solution to this problem is to
insert the complementary emitter follower between the col-
lector and the output. The emitter follower then ensures that
the load resistance of the collector pin is high and that the
output of the circuit can drive low-impedance loads. See
Figure 10.
FUNCTION DIAGRAMS
The diagrams introduce two important characteristics that
help engineers to understand how the Diamond Transistor
functions as a voltage-controlled current source with adjust-
able quiescent current.
Figure 8a shows the transfer curve IO/VBE with the quiescent
current as a parameter. The transconductance increases with
increasing quiescent current.
The inverting base circuit has a low-impedance input. This
current input has clear advantages in amplifying outputs of
sensors which deliver currents instead of voltages.
Figure 8b illustrates the transconductance dependency upon
the input voltage. It is clear from this diagram that the
transconductance of the Diamond Transistor remains more
stable over the whole input voltage range than that of a
bipolar transistor.
Common Collector
Emitter Follower
with doubled
Current Output
R2
VIN
VIN
R1
50Ω
VOUT
VOUT
50Ω
RE
RE
50Ω
R4
R3
Network
Analyzer
Generator
FIGURE 9. Emitter-Collection Connection.
Common Base
RC
Common Emitter
FIGURE 7. Test Circuit OTA.
(B) TRANSCONDUCTANCE
vs INPUT VOLTAGE
RC
(A) DT CHARACTERISTIC
10
200
100
VOUT
VOUT
VIN
0
gm
mA
V
IO
mA
RE
RE
–10
0
VIN
–40
0mV
VBE
40
–40
0mV
VBE
40
FIGURE 10. Emitter Follower.
FIGURE 8. OTA Transfer Characteristics.
4
Figures 11 through 13 show the frequency response attained
with a gain of 3.85 and the pulse response achieved with an
input pulse rise time of 1.3ns of the open-loop amplifier
illustrated in the diagram below. We call this open-loop
amplifier a “straight forward amplifier.”
VOUT
f–3dB
±100mV
±300mV
±700mV
±1.4V
351MHz
374MHz
435MHz
460MHz
443MHz
±2.5V
With a quiescent current of 20mA and the applied compo-
nent values, the –3dB bandwidth of the open-loop amplifier
is between 350MHz at ±100mV and 460MHz at ±1.4V,
depending upon the output voltage. See Table III.
TABLE III. –3dB Bandwidth vs Output Voltage.
180Ω
VOUT
The rise/fall time at the output is 1.4ns, and the maximum
overshoot is under 10% and settles to less than 1% after 5ns.
The settling time at 0.1%/10-bit is 25ns.
56Ω
8
100Ω
3
VIN
2
CURRENT-FEEDBACK AMPLIFIER
75Ω
51Ω
Advantages:
• Fewer transistor stages (signal delay time).
• Shorter signal delay time = larger bandwidth.
• Small-signal bandwidth independent of gain compensa-
tion of the frequency response possible with feedback
resistance instead of capacitance.
5.6pF
FIGURE 11. Straight Forward Amplifier.
OPA660 OTA
• Complementary-symmetric circuit technique improves
large-signal performance.
20
2.5Vpo
15
Disadvantages:
1.4Vpo
10
• Low-impedance inverting input.
• Asymmetric differential inputs.
• Low common-mode rejection ratio.
• Relatively high input offset voltage.
0.7Vpo
0.3Vpo
5
0
–5
–10
–15
–20
–25
–30
0.1Vpo
ADVANTAGES A CF-AMPLIFIER
HAS WITH THE OPA660
The –3dB bandwidth stays constant over the entire modula-
tion range up to ±2.5V and gains up to 12. Quiescent current
control guarantees an excellent bandwidth /quiescent current
ratio. See Figure 16.
100k
1M
10M
100M
1G
Frequency (Hz)
IQ = 20mA, R1 = 100, R4 = 51, R2 = 180,
3 = 56, R4p = 75, C4p = 5.6pF
R
RQ varies the quiescent current to produce the necessary
bandwidth. Feedback resistances can optimize frequency
response over a broad range. This configuration also pro-
vides excellent pulse behavior, even up to large pulse ampli-
tudes. Burr-Brown offers the Current-Feedback Amplifier
completely assembled as a small demo board under the part
number DEM-OPA660-2GC.
FIGURE 12. Straight-Forward Amplifier Frequency
Response.
OTA PULSE RESPONSE
C1
R3
47Ω
R2
5
6
DB
VOUT
0V
8
2
R1
3
VIN
R4
R5
Output Voltage = 5Vp-p
FIGURE 13. Pulse Behavior of a Straight-Forward Amplifier
with Compensation.
FIGURE 14. Current-Feedback Amplifier.
5
DIRECT-FEEDBACK AMPLIFIER
OPA660 CURRENT FEEDBACK
20
15
Another interesting basic circuit with the OPA660 is the so-
called Direct-Feedback Amplifier, Figure 15. The idea of
using voltage feedback from the collector to the emitter for
Current-Conveyor structures was suggested for the first time
a few years ago, and even in test configurations with simple
Current-Conveyor structures, this design demonstrated ex-
cellent RF features. We named this structure the Direct-
Feedback Amplifier, due to its short feedback loop across
the complementary current mirror. As shown in detail with
the Current-Feedback Amplifier, the open-loop gain of the
Direct-Feedback Amplifier varies according to the closed-
loop gain. This relation causes the product of the open-loop
gain, VO, and feedback factor, kO, to stay constant, while the
bandwidth also remains independent of the adjusted total
gain. The currents at the emitter and collector always flow in
the same direction. The current from the collector across R3
causes an additional voltage drop in XE and counteracts the
base-emitter voltage. The reduced voltage difference, how-
ever, causes reduced current flow at the emitter and across
the current mirror at the collector. It functions like double
feedback and is adjusted by the ratio between R3 and XE. The
Diamond Buffer decouples the high-impedance output.
2.5Vpo
1.4Vpo
10
5
0.7Vpo
0.3Vpo
0
–5
–10
–15
–20
0.1Vpo
–25
–30
100k
1M
10M
100M
1G
Frequency (Hz)
I
Q = 20mA, R1 = 47Ω, R2 = 56Ω, R4 = 200Ω, R5 = 22Ω, Gain = 10
FIGURE 16. Current-Feedback Amplifier Frequency
Response.
OPA660 DIRECT FEEDBACK
20
2.5Vpo
15
1.4Vpo
10
5
0
1.7Vpo
0.3Vpo
Burr-Brown offers the Direct-Feedback Amplifier completely
assembled as a small demo board under the part number
DEM-OPA660-3GC.
–5
–10
–15
–20
Figures 17 and 18 show the excellent test results with the
Direct-Feedback Amplifier.
0.1Vpo
Using a quiescent current of 20mA and the given component
values and compensation at the emitter, it is possible to
attain 330MHz at ±100mV and max 550MHz at ±1.4V
bandwidth. The frequency response curve is extremely flat
and shows peaking of 1dB only with output signals of
±2.5V. The voltage gain G is 3. The pulse diagrams shown
here for small-signal modulation illustrate the excellent
pulse response. There is no difference in pulse response
between 300mVp-p and 5Vp-p.
–25
–30
100k
1M
10M
Frequency (Hz)
100M
1G
R1 = 100Ω, R2 = 120Ω, R3 = 390Ω, R4 = 200Ω,
R6 = 68Ω, IQ = 20mA, Rp = 80Ω, Cp = 6.4p
FIGURE 17. Direct-Feedback Amplifier Frequency
Response.
The calculated slew rate is 2500V/µs during 5Vp-p signals.
Previously, this slew rate could only be achieved using
hybrid circuits with a quiescent current between 20mA and
500mA and a voltage supply of ±15V for ±2.5V signals. See
Table IV.
Gain = 3, tr = tf = 2ns, VI = 100mVp–p
FUNCTIONAL CIRCUITS WITH THE OPA660
120Ω
5
6
DB
VOUT
8
2
100Ω
3
R3
390Ω
VIN
XE
82
100Ω
6.4pF
FIGURE 18. Pulse Behavior of the Direct-Feedback
Amplifier.
FIGURE 15. Direct-Feedback Amplifier.
6
VIDEO RECORD AMPLIFIER
VOUT
f–3dB
±100mV
±300mV
±700mV
±1.4V
331MHz
362MHz
520MHz
552MHz
490MHz
A good way to see the advantages of current control over
voltage control is to compare them when driving magnetic
heads in video technology. Analog recording requires high
linearity, while digital recording demands sharp edges and
low phase distortion, since the zero crossing point contains
the relevant information.
±2.5V
TABLE IV. –3dB Bandwidth vs Output Voltage.
A special recording amplifier is necessary to drive the
rotating video heads. This amplifier, Figure 19, delivers the
current to magnetize the tape. The recording current can be
between 1mA and 60mA, depending upon the amplitude,
type of recording, and type of tape used. The current flowing
through the video heads must be independent of the fre-
quency and load. Current source control can deliver current
through the load up to the rated output limit, independent of
the voltage drop. In addition, the recording current is di-
rectly proportional to the magnetic field intensity and flux
density. The record drive amplifier for digital signals shown
here functions in a bridge configuration, in which the invert-
ing and non-inverting digital data streams control the signal
differentially. Bridge operation, and thus a doubled voltage
range, is necessary because the voltage drop across the load
inductance exceeds the voltage range of the Diamond Tran-
sistor at the 30MHz recording rate and maximum record
current. The common emitter resistor allows simple adjust-
ment of the transconductance.
• Driver Circuits for Diodes Capacitive/Inductive Loads
• Operational Amplifiers
• Line Drivers
• Integrators/Rectifier Circuits
• Receiver Amplifier for Pin Diodes
• Active Filters
The new circuit technology really comes into full use,
however, in applications in which the current is the actual
signal. Such applications include active filters with Current-
Conveyor structures, control of LED and laser diodes, as
well as control of tuning coils, driver transformers, and
magnetic heads for analog and digital video recording.
+VCC +VCC OUT
RQ
–VCC
In previous amplifiers, relays separated the replay and record
amplifiers when switching from recording to replay. Using
the OPA660 or the OPA2662, which contains two Diamond
Transistors with high-current output stages, the IQ (OPA660)
or EN inputs (OPA2662) can switch the record drive ampli-
fier into high-impedance mode. The gate in front of the
output stage stops the digital data stream. In high-impedance
mode, the output stage requires very little current.
RB
ROG
EN1
Data
TTL
EQ
EN2
Playback
Amplifier
RB
REC
DRIVER AMPLIFIER FOR
LED TRANSMISSION DIODES
–VCC –VCC OUT
The advantages of current control also become apparent
when driving light-emitting diodes and laser diodes in ana-
log/digital telecommunications and in test procedures with
modulated laser light. Using the OPA2662, it will be pos-
sible to control laser diodes by a complementary-bipolar
current source; using the OPA660, it is already possible to
control LEDs with ±30mA drive current. Figure 20 shows
the circuit implementation. The quiescent current is 20mA
max when RQ = 220Ω, and the inputs of both emitter
followers, which are not illustrated in the Figure, are grounded
through 220Ω resistances, since these inputs are not neces-
sary in this application. The current mirror consisting of Q1
and Q2 sets the quiescent current for the LED, which can
then be adjusted by PBIAS. Two Diamond Transistors wired
parallel to each other deliver the signal current. Diamond
Transistors can be connected to each other at the collector
output to increase the output current, which has already
increased to ±30mA in this configuration. The diode 4148
protects the transmitter diodes against excessive reverse
voltages.
FIGURE 19. Video Record Amplifier.
+5V
22Ω
+5V
100Ω
22Ω
2
220Ω
220Ω
3
3
Q1
Q2
8
8
VIN
4148
270Ω
LED
2
PBIAS
100Ω
200Ω
–5V
FIGURE 20. Wideband LED Transmitter.
7
inverting operations are possible. The ratio of the feedback
resistances determines the closed-loop gain, and the user can
attain optimum frequency response by adjusting the open-
loop gain externally with ROG. The frequency response of the
differential amplifier is equivalent to that of a 2nd order low-
pass Butterworth filter with gain. Due to the additional delay
time in the control loop caused by the feedback buffer, the
frequency response is poorer than the current feedback by
30%. The OPA622, which was recently introduced, contains
a Diamond Transistor and two buffers. With the output
current capability of ±100mA, this IC can drive several low-
impedance outputs. The output buffer has its own supply
voltage pins to decouple the output stage from differential
stage and to enable external current limitation. Because of
the identical high-impedance inputs, the typical offset volt-
age at the output is ±1mV, and the common-mode rejection
ratio is over 70dB. These values are excellent results for RF
amplifiers.
+VCC
C
B1
R2
VIN
B2
VOUT
ROG
R1
–VCC
FIGURE 21. Voltage-Feedback Amplifier.
OPERATIONAL AMPLIFIER WITH
VOLTAGE FEEDBACK IN DIAMOND STRUCTURE
The disadvantages of the Current-Feedback Amplifier listed
above are unbalanced inputs, low-impedance inverting in-
put, poor common-mode rejection ratio, and size of the input
offset voltage. Now, we would like to present a concept
which integrates the Diamond structure with voltage feed-
back in one circuit. An additional buffer transforms the
current feedback of the Current Feedback Amplifier into
voltage feedback. Figures 21 and 22 illustrate the circuit
diagram and the extended Voltage-Feedback Amplifier. The
feedback buffer is identical to the input section of the
Diamond Transistor and forms one side of the differential
amplifier, while the Diamond Transistor is the other side.
Both buffer outputs are connected to ROG, which determines
the open-loop gain and corresponds to the emitter degenera-
tion resistor of a conventional differential stage.
DRIVER AMPLIFIER FOR
LOW-IMPEDANCE TRANSMISSION LINES
The ability of the Current-Feedback Amplifier to deliver
±15mA output current makes it a good choice as a driver
amplifier for low-impedance (50Ω/75Ω) coaxial transmis-
sion lines. To transmit the pulse free of reflections, the
transmission line must be terminated on both sides by the
characteristic impedance of the line. A resistance in series to
the output resistance of the driver amplifier, Figure 23,
matches the output of the amplifier to the line. The total
resistance of the output and series resistors should be equal
to the characteristic impedance. The output resistance of
operational amplifiers rises with increasing frequency. Thus,
the impedances are no longer matched and reflections arise
due to high-frequency components in the signal. The output
resistance of Current-Feedback Amplifiers rises, for ex-
ample, up to 25Ω at 50MHz.
The output of this differential stage is the collector of the
Diamond Transistor, which is driven in quasi open-loop
mode due to the output buffer. Both inverting and non-
+VCC
+VEE
ROG
VIN
VOUT
–VCC
–VEE
R1
R2
FIGURE 22. Extended Voltage-Feedback Amplifier.
8
DIFFERENTIAL OUTPUT
RO
The circuit in Figure 24 is well suited to applications with
larger dynamic ranges, which require a differential output to
drive triax lines. A signal amplitude of ±5V is provided to
drive a load which is not grounded. The load could be the
input resistance of an RF device in an EMC contaminated
environment. Resistances in series to each amplifier output
match the output to the line. These resistances are selected
at somewhat less than half of the characteristic impedance.
While the rise/fall time and bandwidth do not change, the
slew rate doubles.
ZO
50Ω
56Ω
6
DB
5
8
2
47Ω
VIN
3
RIN
50Ω
200Ω
RIN
200Ω
FIGURE 23. 50Ω Driver Amplifier.
200Ω
MONOCHROMATIC MATRIX OR
B/W HARDCOPY OUTPUT AMPLIFIER
100Ω
~ZO /2
~ZO /2
The inverting amplifier in Figure 25 amplifies the three
input voltages, which correspond to the luminance section of
the RGB color signal. Different feedback resistances weight
the voltages differently, resulting in an output voltage con-
sisting of 30% of the red, 59% of the green, and 11% of the
blue section of the input voltage. The way in which the
signal is weighted corresponds to the transformation equa-
tion for converting RGB pictures into B/W pictures. The
output signal is the black/white replay. It might drive a
monochrome control monitor or an analog printer (hardcopy
output).
ZO
47Ω
RL
RO
47Ω
100Ω
200Ω
200Ω
FIGURE 24. Balanced Driver.
OPERATIONAL
TRANSCONDUCTANCE AMPLIFIER (OTA)
6
VLUMINANCE
DB
5
8
2
The Diamond Transistor and Diamond Buffer form a differ-
ential amplifier with two symmetric high-impedance inputs
with current output. This amplifier is also known as the
Operational Transconductance Amplifier, Figure 26. In this
application, RE sets the open-loop gain. The bipolar current
output can be connected to a discrete cascode transistor,
which enables wideband and high voltage outputs.
3
665Ω
340Ω
200Ω
VRED
VGREEN
VBLUE
1820Ω
NANOSECOND INTEGRATOR
One very interesting application using the OPA660 in physi-
cal measurement technology is a non-feedback ns-integrator,
Figures27and28,whichcanprocesspulseswithanamplitude
of ±2.5V, have a rise/fall time of as little as 2ns, and pulse
widthofmorethan8ns. Thevoltage-controlledcurrentsource
charges the integration capacitor linearly according to the
following equation:
FIGURE 25. Monochrome Amplifier.
+15V
1kΩ
VOUT
VC = VBE • gm • t/C
+VB
BFQ262
VC = Voltage At Pin 8
VBE = Base-Emitter Voltage
gm = Transconductance
8
t
C
= Time
= Integration Capacitance
3
+VIN
2
The output voltage is the time integral of the input voltage.
It can be calculated from the following equation:
RE
6
T
–VIN
DB
VO = Output Voltage
= Integration Time
C = Integration Capacitance
5
gm
C
IOUT = gm • (VIN+ – VIN–
)
VO
=
V
dt
BE
∫
T
O
FIGURE 26. Operational Transconductance Amplifier.
9
R3
200Ω
Sample
&
Hold
10
6
ADC
DB
5
R2
780Ω
8
C1
27pF
Preamp
–5V
3
DT
R5
R6
R1
50Ω
2
620Ω
R7
50kΩ
820Ω
+5V
C2
1µF
RQC
470Ω
470pF 470pF
+5V
R8 1kΩ Offset
–5V
10nF
10nF
Trigger
Circuit
2.2µF 2.2µF
1nF
Pin 7
Pin 4
Pin 1
FIGURE 27. Nanosecond Integrator.
Channel 1
Input
2V/DIV
Channel 2
Output
500mV/DIV
10ns/DIV
200mV/DIV
200ns/DIV
Trigger
FIGURE 28. Integrator Performance.
10
+5V
Cd8
0.5...2.5p
R1
47kΩ
P2
Rd8
10kΩ
27kΩ
+5V
+5V
–5V
C3
1
R3
100Ω
C3
7
R2
150Ω
ROUT
47Ω
3
2
Ri1
100Ω
RC5
150Ω
4
8
8
DT
BUF600
VOUT
6
5
VIN
DB
0Ω
RQC
220Ω
4
5
Ri2
100Ω
CB
C3
500Ω
P1
–5V
Propagation Delay Time = 6ns
Rise Time = 2.5ns
–5V
D1
D2
HP2711/DMF3068A
INPUT/OUTPUT VOLTAGE OF THE COMPARATOR
200
150
100
50
VOUT = 200mVp-p
fIN = 10MHz
0
–50
Input
–100
–150
–200
Output
5
0
10
15
20
25
30
35
40
45
50
55
60
Time (ns)
FIGURE 29. Comparator (Low Jitter).
11
COMPARATOR
The current source compensates for different voltage drops
across the diodes up to its maximum rated voltage. It is
possible to extend this circuit to a full-wave rectifier by
connecting the second diode, instead to GND, over a resistor
to GND, to rectify the negative half of the input signal.
An interesting and also cost effective circuit solution using
the OPA660 as a low jitter comparator is illustrated in Figure
29. This circuit uses, at the same time, a positive and
negative feedback. The input is connected to the inverting E-
input. The output signal is applied in a direct feedback over
the two antiparallel connected gallium-arsenide diodes back
to the emitter. A second feedback path over the RC combi-
nation to the base, which is a positive feedback, accelerates
the output voltage change when the input voltage crosses the
threshold voltage. The output voltage is limited to the
threshold voltage of the antiparallel diodes. The diagram on
the right side of Figure 29 demonstrates the low jitter
performance of the presented comparator circuit.
CONTROLLING THE GAIN
BY ADJUSTING THE BIAS CURRENT
The transfer curve of the Diamond Transistor demonstrates
that the transconductance varies according to the quiescent
current. The circuit, Figure 31, described here uses this
relation to control the gain. As measurements have shown,
it is possible to produce a gain range of 20dB, but the
minimum quiescent current should not fall short of 1mA.
Quiescent currents smaller than 0.5mA increase the non-
linearities to a value which can no longer be tolerated. A
positive current flowing into the IQ-adjust (pin 1) disables
the OPA660, the output of which goes into high-impedance
state. The switch-on period lasts only a few ns, while the
switch-off time is several µs. The internal capacitances are
discharged at different speeds according to the load. The
possibility of modulating the bias current dynamically has
not yet been investigated. But based on the internal configu-
ration, modulation frequencies up to several kHz should be
possible.
RECTIFIER FOR RF SIGNAL IN THE mV RANGE
Previously, rectifier diodes were included in the feedback
loop of operational amplifier circuits to form ideal diodes for
accurate detection of small signals in the mV range. In this
configuration, the slew rate of the operational amplifier fixes
the maximum frequency which can be rectified. The circuit
in Figure 30 illustrates a new method of rectifying RF
signals. The diodes at the current source output direct the
current either into the load resistance or toward ground. The
output current is zero even during zero crossing, resulting in
a very soft transfer from one diode to the next.
PIN DIODE RECEIVER
Figure 32 illustrates a preamplifier which can recover both
analog and digital signals for a fiber optic receiver. This
preamplifier can amplify weak and noisy signal currents and
convert them into voltage. In this arrangement, the Diamond
Transistor operates in the inverting base configuration, which
functions excellently in this application due to its low-
impedance current input. In the ideal case, the voltage set at
the base by the voltage divider appears at the low-impedance
emitter free of offset errors. The voltage drop above the
1SS83
5
6
VOUT
DB
8
2
220Ω
3
VIN
500Ω
50Ω
25Ω
FIGURE 30. RF Rectifier.
+5V
–5V
4.7kΩ
475Ω
+5V
–5V
2N3906
100Ω
75Ω
5
6
VOUT
DB
1
8
2
2.1kΩ
3
VIN
75Ω
100Ω
25Ω
500Ω
FIGURE 31. Controlled Amplifier.
12
diode is adjusted to zero volts. During exposure to light, the
pin diode functions as a high-impedance current source and
either delivers current to the emitter or removes current. The
resulting voltage difference between the base and emitter
controls the collector current. The current gain error is
dependent both upon the dynamic output resistance of the
pin diode and upon the transconductance of the Diamond
Transistor. It is possible to achieve current gain factors of
200 to 400, depending upon the diode and quiescent current
used. Advantages of this circuit structure include the follow-
ing points:
adjoint network concept. A network is reversible or recipro-
cal when the transfer function does not change even when
the input and output have been exchanged. Most networks,
of course, are nonreciprocal. The networks, Figure 34,
perform interreciprocally when the input and output are
exchanged, while the original network, N, is exchanged for
a new network NΑ. In this case, the transfer function remains
the same, and NA is the adjoint network. It is easy to
construct an adjoint network for any given circuit, and these
networks are the base for circuits in Current-Conveyor
structure. Individual elements can be interchanged accord-
ing to the list in Figure 35. Voltage sources at the input
become short circuits, and the current flowing there be-
comes the output variable. In contrast, the voltage output
becomes the input, which is excitated by a current source.
The following equation describes the interreciprocal fea-
tures of the circuit:VOUT/VIN = IOUT/IIN. Resistances and
capacitances remain unchanged. In the final step, the opera-
tional amplifier with infinite input impedance and 0Ω output
impedance is transformed into a current amplifier with 0Ω
input impedance and infinite output impedance. A Diamond
Transistor with the base at ground comes quite close to an
ideal current amplifier. The well-known Sallen-Key low-
pass filter with positive feedback, Figure 36, is an example
of conversion into Current-Conveyor structure. The positive
gain of the operational amplifier becomes a negative second
type of Current Conveyor (CCII), Figure 37. Both arrange-
ments have identical transfer functions and the same level of
sensitivity to deviations. The most recent implementation
of active filters in a Current-Conveyor structure produced a
second-order Bi-Quad filter. The value of the resistance in
the emitter of the Diamond Transistor controls the filter
characteristic.
• The transconductance and speed of the Diamond
Transistor keep the voltage drop across the diode low,
preventing the diode capacitance from increasing with
the modulation.
• A fixed voltage across the diode improves the linearity,
since the sensitivity of the diode varies with diode
voltage.
• The capacitance at the emitter is only 2pF.
• The signal path is short, resulting in a very wide
bandwidth.
ACTIVE FILTERS USING THE OPA660
IN CURRENT CONVEYOR STRUCTURE
One further example of the versatility of the Diamond
Transistor and Buffer is the construction of active filters for
the MHz range. Here, the Current Conveyor structure, Fig-
ure 33, is used with the Diamond Transistor as a Current
Conveyor.
The method of converting RC circuit loops with operational
amplifiers in Current Conveyor structures is based upon the
+5V
47Ω
220Ω
47Ω
5
6
VOUT
DB
8
10kΩ
150Ω
3
RC2
2
10nF
–5V
FIGURE 32. Preamplifier.
IOUT
VOUT
E
B
C
+1
CCII–
R
C
C
VIN
R
R
R
IIN
C/2
C/2
4KQ2/R2C2
s2 + 2/RC[2Q(1– K) + 1]s + 4KQ2/R2C2
VOUT IOUT
=
T(s) =
=
VIN
IIN
FIGURE 33. Current Conveyor.
13
Reciprocal Networks
+
VIN
VOUT
–
IOUT
IIN
N
N
VOUT
IOUT
=
VIN
IIN
Interreciprocal Networks
+
VIN
VOUT
–
IOUT
IIN
N
NA
FIGURE 34. Networks.
TRANSFER FUNCTION
Element
VIN
Adjoint
IOUT
R 2 M
R 3
R1M
R2
S2C1R1M
+ sC1
+ sC 1
+
1
R1
1
1
2
2
1
1
2
2
Signal
Sources
V OUT
VIN
IIN
F( p) =
=
–
+
VOUT
R 2 M
R1M
S2C1C2 R1M
+
1
R
R
1
R3 S
R 2 S
R1S
1
1
2
2
1
1
2
2
Passive
Elements
C
C
FILTER CHARACTERISTICS
Low-pass filter:
High-pass filter:
Bandpass filter:
R2 = R3 = ∞
R1 = R2 = ∞
R1 = R3 = ∞
1
1
3
4
3
4
+
V
–
Controlled
Sources
µV
µI
I
Band rejection filter: R2 = ∞; R1 = R3
All-pass filter:
R1 = R1S, R2 = R2S, R3 = R3S
FIGURE 35. Individual Elements in the Current Conveyor.
R3
R2
VIN
VOUT
C1
C2
R1
R1M
R2M
RB1
RB2
RB3
R1S
R2S
R3S
FIGURE 36. Universal Active Filter.
14
The design of a low-pass filter with a corner frequency of
30MHz results in the following values:
lower curve in the diagram on the right half of Figure 38
shows the behavior of the output impedance vs frequency.
For some applications, like the integrator for ns-pulses of
Figure 27, the relatively low output impedance is a real
disadvantage. The fast discharge of the integration capacitor
after (Figure 28) the pulse is over demonstrates this behav-
ior. An easy way to improve the output impedance is a
positive feedback path formed by the resistor divider from
the collector to the base and the GND. The ratio of the two
resistors determines the final output impedance, which can
even be made negative. The capacitor between C and B
supports the improvement vs frequency, which is illustrated
in the diagram of Figure 38. The positive feedback results in
a dynamic increase of the open loop gain, which can be
made higher than 110dB.
R1M = R2M = 91Ω; C1 = C2 = 100pF
R1 = 142Ω; R1S = 161Ω; R2S = 140Ω; R3S = 426Ω
Figure 37 illustrates the frequency response and phase char-
acteristics of the filter. Advantages of active filters in a
Current Conveyor structure:
• The increase in output resistance of operational ampli-
fiers at high frequencies makes it difficult to construct
feedback filter structures (decrease in stop-band attenu-
ation).
• All filter coefficients are represented by resistances,
making it possible to adjust the filter frequency
response without affecting the filter coefficients.
• The capacitors which determine the frequency are
located between the ground and the current source
outputs and are thus grounded on one side. There-
fore, all parasitic capacitances can be viewed as part of
these capacitors, making them easier to comprehend.
DIFFERENTIATOR FOR WEAK AND
DISTURBED DIGITIZED SIGNALS
As it is shown in Figure 39 a RC network can be connected
between the E-output of the OTA and buffer output. The
proposed circuit improves the pulse shape of digitized sig-
nals coming from a magnetic tape or a hard disc drive.
• The features which determine the frequency character-
istics are currents, which charge the integration capaci-
tors. This situation is similar to the transfer characteris-
tic of the Diamond Transistor.
CONTROL LOOP AMPLIFIER
A new type of control loop amplifier for fast and precise
control circuits can be designed with the OPA660. The
circuit of Figure 40 shows a series connection of two voltage
control current sources which have an integral and at higher
frequencies a proportional behavior vs frequency. The con-
trol loop amplifiers show an integrator behavior from DC to
the frequency, represented by the RC time constant of the
network from the C-output to GND. Above this frequency
they operate as an amp with constant gain. The series
connection increases the overall gain to about 110dB and
thus minimizes the control loop deviation. The differential
configuration at the inputs enables one to apply the mea-
sured output signal and the reference voltage to two identical
high-impedance inputs. The output buffer decouples the C-
output of the second OTA in order to insure the AC-
performance and to drive subsequent output stages.
OPTIMATION WITH DIAMOND STRUCTURE
• AGC Amplifier
• DC-Restored Amp
• Analog Multiplexer
• PLL
• Sample/Hold
• Multiplier
• Oscillators
• RF-Instrumentaion Amplifiers
DYNAMIC OUTPUT IMPEDANCE INCREASE
As illustrated in Table II, the output impedance of the OTA
at a quiescent current of ±20mA equals to 25kΩ || 4.2pF. The
7.0
dB
20
dB
180°
P
h
a
s
e
–180°
–10
–60
300k
Frequency (Hz)
50M
1M
Frequency (Hz)
400M
FIGURE 37. Current Conveyor LP.
15
Cd8
4pF
Rd8
RO1
ROUT
=
=
(VO/V8) – 1
1MΩ
26kΩ
ROUT
+5V
7
V(VO)/V(V8) – 1
R3
3
13
R01
C01
1F
14
15Ω
DT
8
2
1E6
1
VO
RO
R02
1Ω
1E9
10k…3GHz
DEC20
Rqc
4
250Ω
–5V
C02
1F
COUT = 1/(2πf10MHz • ROUT 10MHz
ROUT = 1.19MΩ
COUT = 0.50pF
)
02
1E-6
G02
R2
C2
1E9
1F
(PSpice® Simulation)
10M
Rd8, R3
Rd8, R3, Cd8
100k
1k
Without Feedback
10
1k
10k
100k
1M
10M
100M
1G
Frequency (Hz)
FIGURE 38. Transconductance Output Impedance (Dynamic increase with positive feedback).
220Ω
180Ω
5
BUF601
VOUT
8
1
3
5.6pF
180Ω
75Ω
6
5
+1
VIN
2
180Ω
Differentiator
Network
FIGURE 39. Differentiator for Digitized Video Signals.
16
6
8
5
+1
VOUT
3
8
2
180Ω
180Ω
2
10pF
33Ω
10pF
VREF
3
10Ω
10Ω
33Ω
6
5
+1
VIN
FIGURE 40. Control Loop Amplifier.
17
相关型号：
©2020 ICPDF网 联系我们和版权申明