LM1205 datasheet(12/24 Pages) NSC

12 / 24 page

Circuit Description (Continued)

and R4 is used to compensate for beta variations of the

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 LM1205LM1207

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 LM1205LM1207 R10 and R11 are of

the same value but R10 is common to both Q10 and Q11 If

all the current is flowing through Q10 the video amplitude

would only be half of the maximum gain (all current flowing

through Q11) This gives the drive control a total gain ad-

justment range of 6 dB Since the drive control is only used

to balance the color of each channel a small adjustment

range is desirable 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 current 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 differential pair

Q12 and Q13 By this current mirror the voltage at the col-

lector of Q14 is set to the correct DC value for the video

signal by controlling the voltage drop across R13 complet-

ing the DC restoration

TLH11881 – 16

FIGURE 8 Simplified Schematic of LM1205LM1207 Video Amplifier Input Stage

http

www nationalcom

LM1205 Datasheet(PDF) 12 Page - National Semiconductor (TI)

LM1205 Datasheet(HTML) 12 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL