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

low open loop gain, the gain is approximately A

Thus the maximum gain of the video amplifier is A

A

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

I

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

Ω resistor (see Figure 2 and Figure 3). The base of

Q38 (pin 3) is externally connected to pin 2 through a 100

Ω

resistor (see

Figure 2 and Figure 3). With V

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

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

is minimum (i.e., maximum attenuation).

DS012591-9

FIGURE 3. Simplified Schematic of the LM2202 Video Amplifier Input Stage

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