參數(shù)資料
型號(hào)： LM2202M
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻／視頻放大
英文描述： 230 MHz Video Amplifier System
中文描述： 1 CHANNEL, VIDEO AMPLIFIER, PDSO20
封裝： M20B
文件頁(yè)數(shù)： 9/17頁(yè)
文件大?。?/td> 416K
代理商： LM2202M
Circuit Description
(Continued)
DRIVE CONTROL SECTION
A simplified schematic of the LM2202’s drive control section
is shown in Figure 6 A 0V to 4V DC voltage is applied at the
drive control input (pin 9). Transistors Q49, Q50 and Q54
buffer and level shift the contrast voltage to the base of Q56.
The voltage at the emitter of Q56 equals the drive voltage,
V
drive
and the current through Q56’s collector is given by
I
C56
= V
drive
/R43.
Transistor Q56’s collector current is used to unbalance the
current through the differential pair comprised of Q58 and
Q60. Q60’s base is internally biased at 7.3V and connected
to the base of Q12 (see Figure 3). Q58’s base is internally
connected to the base of Q11 (see Figure 3). With V
=
2V, the differential pair (Q58, Q60) is balanced and the volt-
age at the bases of Q11 and Q12 is 7.3V. Under this condi-
tion, Q10’s collector current is equally split between Q11 and
Q12 (see Figure 3). If the drive voltage at pin 9 is greater
than 2V then Q56’s collector current increases, thus pulling
Q58’s collector node lower and consequently moving Q58’s
base below 7.3V. With base of Q11 below 7.3V, current
through Q12 (see Figure 3) increases and the amplifier’s
gain increases. With V
= 4V, the amplifier’s gain is maxi-
mum under maximum contrast condition (i.e., V
cont
= 4V).
If the drive voltage at pin 8 is less than 2V then Q56’s collec-
tor current decreases and Q58’s base is pulled above 7.3V.
With base of Q11 greater than 7.3V, less current flows
through Q12 (see Figure 3), consequently the amplifier’s
gain decreases. With V
= 0V, the amplifier’s gain is 6 dB
less than the maximum gain.
CLAMP GATE AND CLAMP COMPARATOR SECTION
Figure 7 and Figure 8 show simplified schematics of the
clamp gate and clamp comparator circuits. The clamp gate
circuit (Figure 7) consists of a PNP input buffer transistor
(Q82), a PNP emitter coupled pair (Q85 and Q86) refer-
enced on one side to 2.1V and an output switch transistor
Q89. When the clamp gate input at pin 14 is high (
>
1.5V)
the Q89 switch is on and shunts the 200 μAcurrent from cur-
rent source Q90 to ground. When pin 14 is low (
<
1.3V) the
Q89 switch is off and the 200 μA current is mirrored by the
current mirror comprised of Q91 and Q75 (see Figure 8).
Consequently the clamp comparator comprised of the differ-
ential pair Q74 and Q77 is enabled. The input of the clamp
comparator is similar to the clamp gate except that an NPN
emitter coupled pair is used to control the current that will
charge or discharge the clamp capacitor externally con-
nected from pin 12 to ground. PNP transistors are used at
the inputs because they offer a number of advantages over
NPNs. PNPs will operate with base voltages at or near
ground and will usually have a greater emitter base break-
down voltage (BVebo). Because the differential input voltage
to the clamp comparator during the video scan period could
be greater than the BVebo of NPN transistors, a resistor
(R63) with a value one half that of R60 or R68 is connected
between the bases of Q71 and Q79. The clamp compara-
tor’s common mode range is from ground to approximately
9V and the maximum differential input voltage is V
CC
.
www.national.com
9
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