LM1202N National Semiconductor, LM1202N Datasheet - Page 7

• Manufacturer Part Number: LM1202N
• Description: IC VIDEO AMP SYST 230MHZ 20DIP
• Manufacturer: National Semiconductor

Specifications of LM1202N

• Applications: General Purpose
• Number Of Circuits: 1
• -3db Bandwidth: 230MHz
• Current - Supply: 48mA
• Current - Output / Channel: 28mA
• Voltage - Supply, Single/dual (±): 8 V ~ 13.2 V
• Mounting Type: Through Hole
• Package / Case: 20-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LM1202N

Circuit Description
VIDEO AMPLIFIER SECTION (Output Stage)
A simplified schematic of LM1202’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
stage’s low open loop gain the gain is approximately
A
is A
a push-pull drive to the load The output voltage can swing
from 0 2V to 10V
CONTRAST CONTROL SECTION
A simplified schematic of LM1202’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
C36
V2
cont
V
e b
e
V2
e
) and the current through Q36’s collector is given by
V
A
e b
cont
10 Thus the maximum gain of the video amplifier
V1 c
R28
R21 R18
A
V2
e
20 Transistors Q23 and Q24 provide
FIGURE 4 Simplified Schematic of LM1202 Video Amplifier Output Stage
e b
16 Because of the output
(Continued)
7
Figure 3 ) increases and the amplifier’s gain increases With
and Q40 Q40’s base is internally biased at 5 3V and made
available at pin 20 Pin 20 is externally connected to pin 1
through a 100
Q38 (pin 3) is externally connected to pin 2 through a 100
resistor (see Figures 2 and 3 ) With V
ential 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
increases thus pulling Q38’s collector node lower and
consequently moving Q38’s base below 5 3V With pin 2
at a lower voltage than pin 1 current through Q10 (see
V
If the contrast voltage at pin 8 is less than 2V then Q36’s
collector current decreases and Q38’s base is pulled above
5 3V With pin 2 voltage greater than pin 1 voltage less
current flows through Q10 (see Figure 3 ) consequently the
amplifier’s gain decreases With V
gain is minimum (i e maximum attenuation)
cont
e
4V the amplifier’s gain is maximum
resistor (see Figures 2 and 3 ) The base of
cont
cont
e
0V the amplifier’s
http
e
2V the differ-
TL H 11440– 5
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