LM1209 datasheet(12/24 Pages) NSC

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Circuit Description (Continued)

transistors Note that the bias voltage passes through three

diode drops (Q5 Q6 and Q7) before setting the voltage

across R9 Q2 Q3 and Q4 also provide three diode drops

to the bias voltage at the base of Q5 temperature compen-

sating for the diode drops of Q5 Q6 and Q7 This insures

that the bias voltage across R9 remains very constant over

temperature providing an accurate bias current for the dif-

ferential transistor pair Q8 and Q9 thus assuring proper op-

eration of the contrast control

Q6 serves as a buffer to the input video signal Its emitter

drives the base of Q7 Thus the video signal modulates the

current flowing through R9 which in turn modulates the cur-

rents through the differential pair formed by Q8 and Q9 The

current flow through Q8 and Q9 is controlled by a DC volt-

age from the Contrast Control circuit This DC voltage is

common to all three channels Increasing the voltage to the

base of Q9 with respect to the base of Q8 increases the

current flow through Q9 A higher current flow through Q9

increases the video gain (contrast) of the LM1208LM1209

Q10 and Q11 also form a differential pair at the collector of

Q9 The operation of this differential pair is similar to Q8 and

Q9 The DC control voltage is from the Drive Control cir-

cuits Each channel has its own drive control circuit In-

creasing the voltage to the base of Q11 increases the video

gain (drive) of the LM1208LM1209 At a 4V control voltage

the gain of the LM1208LM1209 is at maximum with all the

current flowing through Q11 If all the current was flowing

through Q10 the video signal attenuation would be over

40 dB the maximum attenuation Q12 through Q17 are part

of the final section shown in

Figure 8 DC restoration is done

at this stage The clamp comparator

(Figure 11) drives the

clamp cap at pin 8 to a voltage that sets the correct black

level of the video signal This cap is also connected to the

base of Q17 Q17 and Q16 are one half of the darlington

differential pair The clamp cap voltage establishes the cur-

rent flow through R16 Q15 and R15 With the bases of

Q14 and Q15 held to the same voltage the current through

Q15 is mirrored into Q14 and the other half of the differen-

tial pair Q12 and Q13 By this current mirror the voltage at

the collector of Q14 is set to the correct DC value for the

video signal by controlling the voltage drop across R13

completing the DC restoration

TLH11884 13

FIGURE 8 Simplified Schematic of LM1208LM1209 Video Amplifier Input Stage

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