Figure 7 also shows how the contrast control circuit is con-

figured Resistors R23 24 diodes D3 4 and transistor Q13

are used to establish a low impedance zero TC half supply

voltage reference at the base of Q14 The differential ampli-

fier formed by Q15 16 and feedback transistor Q17 along

with resistors R27 28 establish a diferential base voltage

for Q3 and Q4 in

Figure 6 When externally adding or sub-

tracting current from the collector of Q16 a new differential

voltage is generated that reflects the change in the ratio of

currents in Q15 and Q16 To provide voltage control of the

Q16 current resistor R29 is added between the Q16 collec-

tor and pin 12 A capacitor should be added from pin 12 to

ground to prevent noise from the contrast control pot from

entering the IC

Figure 8 is a simplified schematic of the clamp gate and

clamp comparator sections of the LM1203 The clamp gate

circuit consists of a PNP input buffer transistor (Q18) a PNP

emitter coupled pair referenced on one side to 21V (Q19

20) and an output switch (Q21) When the clamp gate input

the Q21 switch is off and the I1 850 mA current source is

mirrored or ‘‘turned around’’ by reference diode D5 and Q26

to provide a 850 mA current source for the clamp compara-

tor(s) The inputs to the comparator are similar to the clamp

gate input except that an NPN emitter coupled pair is used

to control the current which will charge or discharge the

clamp capacitors at pins 5 8 or 10 PNP transistors are

used at the inputs because they offer a number of advan-

tages over NPNs PNPs will operate with base voltages at or

near ground and will usually have a greater reverse emitter

base breakdown 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 (R34) with a value one half that of R33 or R35 is

connected between the bases of Q23 and Q27 This resis-

tor will limit the maximum differential input to Q24 25 to

approximately 350 mV The clamp comparator common

mode range is from ground to approximately 9V and the

TLH9178 – 8

FIGURE 8 Simplified Schematic of LM1203 Clamp Gate and Clamp Comparator Circuits

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