LM1205N National Semiconductor, LM1205N Datasheet - Page 11

• Manufacturer Part Number: LM1205N
• Description: IC VIDEO AMP 130MHZ RGB 28DIP
• Manufacturer: National Semiconductor

Specifications of LM1205N

• Applications: General Purpose
• Number Of Circuits: 3
• -3db Bandwidth: 130MHz
• Current - Supply: 90mA
• Current - Output / Channel: 28mA
• Voltage - Supply, Single/dual (±): 10.8 V ~ 13.2 V
• Mounting Type: Through Hole
• Package / Case: 28-DIP (0.600", 15.24mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LM1205N

Circuit Description
pensate 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 compensating 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 differential transistor pair Q8
and Q9 thus assuring proper operation of the contrast con-
trol
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 LM1205 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 circuits Each
FIGURE 8 Simplified Schematic of LM1205 Video Amplifier Input Stage
(Continued)
11
(Figure 11) drives the clamp cap at pin 8 to a voltage that
channel has its own drive control circuit Increasing the volt-
age to the base of Q11 increases the video gain (drive) of
the LM1205 R10 and R11 are of the same value but R10 is
common to both Q10 and Q11 If all the current was 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 adjustment 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
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 collector of Q14 is set to the cor-
rect DC value for the video signal by controlling the voltage
drop across R13 completing the DC restoration
TL H 11831– 12