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参数资料
| 型号： | LM2202N/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频／视频放大 |
| 英文描述： | 1 CHANNEL, VIDEO AMPLIFIER, PDIP20 |
| 封装： | N20A |
| 文件页数： | 14/17页 |
| 文件大小： | 416K |
| 代理商： | LM2202N/NOPB |
Circuit Description (Continued)
VIDEO AMPLIFIER SECTION (Output Stage)
A simplified schematic of LM2202’s video amplifier output
stage is shown in
Figure 4. The output stage is the second
gain stage. Ideally the gain of the second gain stage would
be A
V2 = R21/R18 = 16. Because of the output stage’s
low open loop gain, the gain is approximately A
V2 = 10.
Thus the maximum gain of the video amplifier is A
V = AV1 x
A
V2 = 20. Transistors Q23 and Q24 provide a push-pull drive
to the load. The output voltage can swing from 0.2V to 10V.
CONTRAST CONTROL SECTION
A simplified schematic of LM2202’s contrast control section
is shown in
Figure 5. A 0V to 4V DC voltage is applied at the
contrast input (pin 8). Transistors Q29, Q30 and Q34 buffer
and level shift the contrast voltage to the base of Q36. The
voltage at the emitter of Q36 equals the contrast voltage
(V
cont) and the current through Q36’s collector is given by
I
C36 = Vcont/R28.
Transistor Q36’s collector current is used to unbalance the
current through the differential pair comprised of Q38 and
Q40. Q40’s base is internally biased at 5.3V and made avail-
able at pin 20. Pin 20 is externally connected to pin 1 through
a 100
resistor (see Figure 2 and Figure 3). The base of
Q38 (pin 3) is externally connected to pin 2 through a 100
resistor (see
Figure 2 and Figure 3). With V
cont = 2V, the dif-
ferential pair (Q38, Q40) is balanced and the voltage at pins
1 and 2 is 5.3V. Under this condition, Q8’s collector current is
equally split between Q9 and Q10 (see
Figure 3) and the
amplifier’s gain is half the maximum gain. If contrast voltage
at pin 8 is greater than 2V then Q36’s collector current in-
creases, thus pulling Q38’s collector node lower and conse-
quently moving Q38’s base below 5.3V. With pin 2 at a lower
voltage than pin 1, current through Q10 (see
Figure 3) in-
creases and the amplifier’s gain increases. With V
cont = 4V,
the amplifier’s gain is maximum.
If the contrast voltage at pin 8 is less than 2V then Q36’s col-
lector current decreases and Q38’s base is pulled above
5.3V. With pin 2 voltage greater than pin 1 voltage, less cur-
rent flows through Q10 (see
Figure 3), consequently the am-
plifier’s gain decreases. With V
cont = 0V, the amplifier’s gain
is minimum (i.e., maximum attenuation).
DS012591-9
FIGURE 3. Simplified Schematic of the LM2202 Video Amplifier Input Stage
www.national.com
6
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