AB-184 [ETC]
AB-184 - DRIVING VIDEO OUTPUT STAGES WITH MONOLITHIC INTEGRATED AMPLIFIERS ; AB - 184 - 驾驶视频输出级，带有单片集成放大器\n
| 型号： | AB-184 |
| 厂家： | 
ETC |
| 描述： | AB-184 - DRIVING VIDEO OUTPUT STAGES WITH MONOLITHIC INTEGRATED AMPLIFIERS
|
| 文件： | 总7页 (文件大小：89K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
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AP P LICATION BULLETIN
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Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132
DRIVING VIDEO OUTPUT STAGES
WITH MONOLITHIC INTEGRATED AMPLIFIERS
By Christian Henn, Burr-Brown International GmbH
Increasingly powerful computers and the rapidly expanding
process produces the correct color on the screen. For this
reason, displaying the color white at the maximum ampli-
tude is the toughest job for the video amplifier in graphic
monitors.
use of picture processing and CAD/CAM systems in almost
all industry branches have combined to generate a greater
and greater demand for higher resolution graphic monitors.
Controlling the video output stages of these graphic moni-
tors is a key to producing such high resolutions. Until
recently, only highly complex, expensive systems have been
available to drive hybrid video output stages. But using the
monolithic amplifiers OPA623 and OPA2662 from Burr-
Brown, new methods are possible that make complicated
Table I summarizes the various timing requirements
necessary to produce the most commonly used graphic
formats. The TH ACTIVE can be calculated by multiplying the
horizontal cycle time by 0.8, and it includes time for the
electron ray to return from the right side to the left side of the
screen during the horizontal retrace time. When calculating
solutions a thing of the past. The OPA623 allows rise (tRISE
)
t
RISE and tFALL, it was assumed that each was one third of the
pixel time. The –3dB bandwidth (f–3dB) is dependent upon
the rise time and can be calculated as 0.35/tRISE
and fall (tFALL) times of 3ns and 2.3ns, respectively, at the
output, while the OPA2662 is even more impressive at tRISE
= 2.4ns and tFALL = 2.15ns. With this kind of performance,
the OPA623 and OPA2662 can be used in graphic systems
with resolutions of 1600 x 1200 pixel and more.
.
The video signal levels at the interface between the video
card and monitor are standardized at +0.7Vp for the video
signal and –0.3Vp for the synchronization pulse. A high-
resolution cathode ray tube (CRT) functions with bias volt-
ages between +65V and +75V and a modulation voltage of
HIGH-RESOLUTION PICTURE
PROCESSING SYSTEMS: AN OVERVIEW
The various standard resolutions range from the commonly
used VGA standard with 640 x 400 pixels to the super VGA
with 800 x 600 pixels to CAD/CAM and radar systems with
over 1600 x 1200 pixels. But while radar and computer
tomography systems generally use high-resolution 1600 x
1200 color graphic monitors, monochrome displays with
2k x 2k resolution and 500MHz bandwidth are now in
development. To achieve such high resolutions, the moni-
tors use horizontal deflection frequencies for electron rays
between 64kHz and 96kHz, as well as data rates between
100Mbit/s and 250Mbit/s, which are read out from the video
RAM card. Raising the vertical deflection frequency to more
than 70Hz causes the horizontal frequency and data rate to
increase while the resolution remains the same. Controlling
the pixels by the video controller adds to the demands on the
video amplifier, and significantly increases the power con-
sumption during video signal processing. Instead of a con-
tinuous video signal, the video card produces pulse se-
quences that return to zero between every two pulses. The
amplitude of each pulse is equal to the luminance of the
respective color (R, G, B). An additive optical mixing
+0.7V
Graphic Card
0V
TH ACTIVE
–0.3V
TH
+65V
50Vp-p Contrastmax
Cathode Voltage
+15V
0V
FIGURE 1. Pulse Sequences from a Signal Graphic Cathode.
PIXEL/CLOCK
SYSTEM
STANDARDS
RESOLUTIONS
fH
(Hz)
fV
(Hz)
tH
(µs)
tACTIVE
(ns)
TIME/PIXEL
(ns)
FREQUENCY
(Hz)
tRISE/FALL
(ns)
–3dB BW
(Hz)
H x V
VGA
640 x 400
800 x 600
1280 x 1024
1600 x 1200
31.5k
38k
64k
70
70
60
70
31.74
26.31
15.62
13.15
25.39
21.04
12.49
10.52
39.67
26.30
9.75
25M
38M
102M
152M
13.22
8.76
3.25
2.19
26.47M
40M
107M
160M
Super VGA
CAD/CAM
Work Station
76k
6.57
TABLE I. Timing Requirements.
©1993 Burr-Brown Corporation
AN-184
Printed in U.S.A. November, 1993
up to 50Vp-p with high luminance densities between the
cathode and ground. For sufficient contrast, the total gain
between the input and cathode must be between 70 and 166,
depending upon the contrast control method in use. The
cathode is a capacitive load of about 8pF, which rises to at
least 12pF when combined with stray capacitances from the
supply lines, connectors, and required protection circuitry.
VIDEO OUTPUT STAGE
Until a few years ago, the standard circuit for video output
stages was a cascode stage with or without a subsequent
complementary emitter follower. The advantages of this
circuit are that it is easy to design and avoids the Miller
effect (harmful collector-base capacitances) in the amplify-
ing transistor. Inductances in series to the collector resistor
and RC parts parallel to the emitter resistor allow users to
adjust the circuit as required by their particular application.
The disadvantages of the cascode stage are its asymmetrical
transient response and high power dissipation at short rise
and fall times.
Gain
70 to 166
50VMAX
2ns
±300mAp-p
25000V/µs
1%
Output Amplitude
tRISE/tFALL (40V, 12pF)
Driver Current
Slew Rate
Linearity
We conducted several experiments with various configura-
tions to test the ability of the OPA623 and OPA2662 to
control a discrete cascode stage. As shown in Figure 3, a few
of these configurations failed because there are no discrete
cascode transistors effective for this application. The inte-
grated dual current source OPA2662 can produce a charge
current of up to 300mA in the emitter of a transistor like the
BFQ262 at rise times of about 2ns, but internal transistor and
emitter resistances and any package stray capacitances limit
and delay the current conversion from the emitter to the
collector of the BFQ262.
TABLE II. Video Output Stage Requirements for a
1600 x 1280 Graphic System.
VIDEO AMPLIFIER CONCEPT
Since the development of the first monitors, various types of
amplifiers have been designed according to specific require-
ments and applications. The type of amplifier structure
shown in Figure 2 has become the standard for high-grade
monitors.
The amplifier at the front end of the circuit is equipped with
a simple transconductance multiplier to control the signal.
Since this type of multiplier has a small linear modulation
range, it is necessary to reduce the signal in the amplifier
from 0.7Vp to 0.3Vp. The following driver stage amplifies
the signal 8 to 15 times and drives the output stages at
approximately 4Vp-p. The output stage then amplifies the
signal again to 50Vp-p max and provides the necessary
driving power to charge the cathode and stray capacitances.
At the back porch that occurs at the beginning of each line
after the horizontal switch, the control circuit compares the
cathode voltage to an adjustable bias and corrects any
deviations from the bias. Depending on the type of amplifier
structure, the bias point control drives either the input of the
driver amplifier or the output stage. The entire video ampli-
fier then reverses the video signal. A 0V signal at the input,
which appears as a dark spot, is converted at the cathode to
a voltage between +65V and +75V, depending upon the bias
point of the cathode. A +0.7V signal, which corresponds to
maximum luminance, is converted with maximum contrast
control to a 50V modulation hub between the CRT bias point
and ground. Figure 1 illustrates these conversions.
Further tests were done using an output stage manufactured
on a hybrid process, and these tests were successful. Figure
6 shows the schematic of the output stage, which is available
from Philips under the part number CR3425. Using the test
configuration shown in Figure 7, it was possible to check the
performance of the hybrid circuit by itself. The pulse genera-
tor HP8130A drives the output stages via a terminated 50Ω
line with rise and fall times of 0.7ns each and a signal
amplitude of 4Vp. The output stage is supplied from 80V,
and 60mA quiescent current flows when no signal is being
applied. The rise and fall times measured at a 50V signal hub
and 12pF load capacitance are impressively low at 2.15ns.
Figure 8 shows the pulse responses at 10ns/div and 2ns/div.
AN ALTERNATIVE METHOD
OF DRIVING THE OUTPUT STAGE
With the hybrid circuit CR3425, a cost-effective, high-
performance circuit is now available for high-resolution
graphic monitors that effectively controls the output of the
video output stage. Now, however, the problem is control-
ling the input of the video output stage. What we need is a
CK
Input
50Vp-p
Preamplifier
Driver Amplifier
Output Stage
CL
1
75Ω
Contrast
DC-Restoration
Bias
Sync Separation
FIGURE 2. Video Output Stage Requirements for a 1600 x 1280 Graphic System.
2
+80V
50Vp-p
12pF
RC
BFQ262
BFQ252
FIGURE 4. Cathode Voltage Control.
12pF
BFQ262
14
VB
+80V
+65V
+40V
+15V
50 Vp-p
11
10
–2V
7
2
OPA2662
VIN
15
4V
2V
0V
10nF
10Ω
300Ω
3.7kΩ
VIN
220Ω
220Ω
20pF
100pF
8.2mA
12pF
FIGURE 5. Basic Configuration of the Driver Circuit.
FIGURE 3. Video Output Stage.
driver amplifier that takes the pulse after contrast control
and amplifies it with no edge slopes, as well as controlling
the complex input resistance in the output stage with a slew
rate of over 1500V/µs for positive and negative signal
transitions. The hybrid driver amplifiers currently on the
market function only with NPN transistors in class A opera-
tion. Nonfeedback amplifiers are relatively low-cost but
have high power consumption and, more importantly, can
hardly produce the 1280 x 1024 resolution required for
positive signal edges.
+80V
Out
The Current-Feedback Amplifier OPA623 and the Dual
Current Source OPA2662, two monolithic ICs manufactured
on a complementary bipolar process, offer reasonably priced,
effective alternatives. These new ICs differ both in perfor-
mance and in manufacturing costs. They are not, however,
limited to video output stage control. The problem of con-
trolling an input or load resistance is a much more general
dilemma present in a wide variety of applications. The real
trick is to find amplifiers that can operate stably with
complex loads, have low power consumption, and are ca-
pable of charging load capacitances with high currents in as
little time as possible. In these categories as well, the
OPA623 and OPA2662 prove themselves extremely viable
options.
In
FIGURE 6. Internal Structure of the Video Output Stage
CR3425.
from 0.8Vp to 4Vp and drives the complex input resistance
of the CR3425 output stage. Figure 10 shows the pulse
response at the OPA623 output, and Figure 11 that at the
output of the video output stage. The rise and fall times of
the OPA623 are 1.85ns and 1.95ns, respectively. Thus the
OPA623 can drive complex loads of 24Ω + 287Ω || 50pF at
an output voltage of 4Vp and slew rate of about 1700V/µs
(ca. 4Vp • 0.8ns/1.9ns). Using the OPA623, the output of the
video output stage CR3425 can charge the 12pF load capaci-
tor with 40V in 3ns and discharge 40V in 2.3ns. In contrast
DRIVER AMPLIFIER USING THE OPA623
The Wide-Band Current-Feedback Amplifier, OPA623, is
available in 8-pin DIL and SO packages and delivers up to
±70mA output current at a supply voltage of ±5V and low
quiescent current of 4mA. Figure 9 shows the driver circuit
using the OPA623. The OPA623 amplifies the video signal
3
to direct control, control using the OPA623 results in an
edge slope of 0.85ns for the rising edge and 0.15ns for the
falling edge.
base appears in low-impedance form at the emitter and
produces a current flow toward ground via the emitter
resistor. This current is then reflected by a factor of 3 to the
collector. As shown in Figure 12, it’s easy to connect two
current sources in parallel, which produces driving power of
±150mA. A compensation network connected to the emit-
ters provides even more current during the charge phase.
Figure 13 shows the excellent test results using this configu-
ration. At the output of the CR3425, the design produces rise
and fall times of 2.4ns and 2.15ns, respectively, with cath-
ode voltage variation of 50Vp-p. This variation is the maxi-
DRIVER AMPLIFIER USING THE OPA2662
The second test used the Dual Diamond Transistor OPA2662
to drive the video output stage. This new wide-band IC
contains two voltage-controlled current sources
(transconductance amplifiers) in a 16-pin package. Each
current source delivers or pulls up to ±75mA at its high-
impedance collector. The voltage at the high-impedance
+80V; 60mA
tr = 0.7ns
tf = 0.7ns
tr = 2.15ns
tf = 2.15ns
4Vp
50pF
O
50Ω
50Vp-p
1
9
CR3425
12pF
50Ω
287Ω
FIGURE 7. Driver Circuit Using a Pulse Generator.
∆t = 2.15µs
0
2
4
6
8
10
Time (ns)
12
14
16
18
20
0
10
20
30
40
50
60
70
80
90
100
Time (ns)
FIGURE 8. Test Circuit Response.
tr = 1.85ns
tf = 1.95ns
tr = 0.7ns
tf = 0.7ns
0.8Vp
+80V; 60mA
tr = 3.0ns
tf = 2.3ns
+5V
4Vp
50pF
O
50Ω
150Ω
3
2
7
OPA623
4
24Ω
50Vp-p
6
1
9
CR3425
50Ω
12pF
287Ω
–5V
470Ω
120Ω
6.8pF
FIGURE 9. Driver Circuit Using the OPA623.
4
mum possible cathode modulation, during which most
picture tubes are already in overdrive. Reducing the maxi-
mum output voltage lowers the rise and fall times to less
than 2ns, making it possible to process video pulses of 6ns.
The shorter the pulse, the more important it is to achieve
sufficient cathode voltage, since high resolutions are accom-
panied by high horizontal deflection frequencies so that the
turnaround time of the electron ray at the phosphor point
becomes shorter and shorter. The rise time of a phosphor
point is the time until it converts to the electron charge into
a visible light (R, G, or B).
CONCLUSION
Only in the last few years has it become possible to use
integrated amplifiers in video signal processing with high-
resolution monitors. New developments in circuit technol-
ogy and IC manufacturing processes, as well as the rapidly
increasing demand for low-cost displays, have combined to
accelerate advances in video design. Today, integrated RBG
video amplifiers are already available with a bandwidth of
100MHz. In addition to amplification and contrast control,
these amplifiers offer additional functions such as clamping,
blanking, and sync separation, and they can also drive the
output stage.
In comparison to direct control of the output stage by a
generator, when controlled by the OPA2662, the rising edge
has a small additional edge slope of 0.25ns and the falling
edge is driven exactly as fast as with direct control. Consid-
ering that most signal generators are quite expensive, this
comparison speaks quite well for the OPA2662.
Both driver circuit configurations shown here allow video
output stage control that is less integrated but also more
powerful, and the configurations achieve a level of perfor-
mance previously possible only with complex, large, and
∆t = 1.85µs
0
10
20
30
40
50
60
70
80
90
100
0
2
4
6
8
10
12
14
16
18
20
Time (ns)
Time (ns)
FIGURE 10. OPA623 Output.
∆t = 2.30µs
0
2
4
6
8
10
12
14
16
18
20
0
10
20
30
40
50
60
70
80
90
100
Time (ns)
Time (ns)
FIGURE 11. Response of the Test Circuit Shown in Figure 6.
5
expensive hybrid circuits. The lower cost, smaller driver
circuit using the OPA623 can be used for 1600 x 1280
resolutions, while the OPA2662 can be used for applications
requiring resolutions of up to 2k x 2k. It should be noted,
however, that at 2k x 2k both the driver circuit and the video
output stage operate at their performance limit. At frequen-
cies over 100MHz, separation of the three color channels in
different video amplifiers is the only effective way to keep
the crosstalk between the channels to less than 30dB. Fi-
nally, a comparison of the two driver circuits demonstrates
the superiority of a high-impedance current source over a
low-impedance voltage source when controlling low-imped-
ance, capacitive loads. Although the OPA623 with 350MHz
appears at first glance a better choice than the OPA2662
with 200MHz, the current-source output and higher drive
capability of the OPA2662 give it an edge in practice.
The next step will be to assemble both the monolithic
integrated driver amplifier and the hybrid video output stage
on the same substrate.
Both driver circuits are available from Burr-Brown as as-
sembled demo boards so that you can test the configurations
for yourself.
REFERENCES
OPA623 Product Data Sheet
Burr-Brown
OPA2662 Product Data Sheet
Burr-Brown
CR3425 Product Data Sheet
Philips
+80V; 60mA
tr = 2.4ns
tf = 2.15ns
50Vp-p
OPA2662
11
tr = 0.7ns
+0.23V
1
9
CR3425
tf = 0.7ns
14
15
–0.23V
12pF
50Ω
150Ω
150Ω
7
2
10
10nF
10Ω
50Ω
220Ω
220Ω
20pF
100pF
FIGURE 12. Driver Circuit Using the OPA2662.
∆t = 2.15µs
0
2
4
6
8
10
12
14
16
18
20
0
10
20
30
40
50
60
70
80
90
100
Time (ns)
Time (ns)
FIGURE 13. Test Circuit Response Curves.
6
+80V
+5V
+5V
C6
C5
47µF
+
10nF
R9
C4
100nF
C8
10nF
3.9kΩ
C7
10nF
L1
10µH
P1
1kΩ
C9
2.2µF
+
Black Level
1
9
Out
50Vp-p
CR3425
D1
4148
R5
CL
12pF
2
3
7
8
240Ω + 47Ω
C2
C3
R2
100nF
150Ω
R6
24Ω
3
7
In
6
0.8Vp-p
OPA623
25pF to 33pF
R1
R8
2
50Ω
100kΩ
4
R4
470Ω
–5V
C10
10nF
R3
120Ω
C1
6.8pF
C11
2.2µF
+
–5V
FIGURE 14. Driver Circuit 1.
80V
C6
C5
47µF
+
10nF
+5V
C7
L1
10nF
R9
10µH
C4
2kΩ
100nF
R0
0
500Ω
1
9
Out
50Vp-p
CR3425
Black Level
CL
12pF
2
3
7
8
11
14
D1
4148
7
2
(1)
(1)
+5V
R3
150Ω
R2
150Ω
10
15
C3
100nF
C9
R5
0
10nF
In
C8
10nF
0.8Vp-p
R1
R8
C10
50Ω
100kΩ
2.2µF
R6
220Ω
R6a
220Ω
R7
10Ω
C2
22pF
1
3
6
4
5
16
–5V
(1)
C1
100pF
OPA2662
RQC
750Ω
C12
10nF
8
9
C11
2.2µF
R6
0
NOTE: (1) The Diamond Transistors of the OPA2662 require appropriate
power supply connections like shown at the right side.
–5V
FIGURE 15. Driver Circuit 2.
7
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