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LM1209 Datasheet

  • LM1209

  • National Semiconductor [130 MHz/100 MHz RGB Video Amplifier...

  • NSC

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Circuit Description
(Continued)
CONTRAST CONTROL
Figure 9
is a simplified schematic of the Contrast Control
circuit The output of this circuit is common to all three chan-
nels A reference voltage is generated by Z2 Q34 Q35
R30 and R31 Q36 Q39 and Q41 are all current sources
that are controlled by the reference voltage The contrast
signal has a 0V to 4V range with its input at pin 12 R32 is
used for current limiting any voltage surge that may occur at
pin 12 Note that the input stage (Q37 Q38 and Q42) are
all PNP transistors This configuration is necessary for oper-
ation down to near ground At Q44 the input voltage is con-
verted to a current by R33 The input stage will apply the
same voltage across R33 as is applied at the input and with
no temperature variations from the transistors Q37 is con-
nected to a current source (Q36) to keep a constant current
flow through Q37 and a predictable diode voltage for the
base-emitter of Q37 Q40 is connected as a diode and is
biased by the current source Q39 The current through Q40
is mirrored into Q43 giving a current bias for Q42 Again this
is done to give a predictable diode voltage for Q42 Q41 is a
current source for both Q38 and Q42 With the current
through Q42 already established the rest of the current
from Q41 flows through Q38 As one can see the input volt-
age is accurately reflected across R33 with no temperature
coefficients from the input stage of the contrast control cir-
cuit
Pin 1 of the contrast control output is held at a constant
voltage two diode drops below V
CC
To generate this ref-
erence the base of Q51 is held at exactly V
CC
R44 and
R45 form a voltage divider With both Q53 and Q54 con-
nected as diodes the voltage at the junction of R44 and R45
is V
CC
plus one diode drop Q52 is a buffer to this refer-
ence voltage generating exactly V
CC
at its emitter Q51 is
used to drive the bases of Q49 and Q50 to one diode drop
below the reference voltage Q50 is used to further buffer
the reference voltage to the base of Q9 (see
Figure 8
) and
the corresponding transistors in the other channels Q48 is
used to bias the collector of Q49 to V
CC
the same volt-
age as the collector of Q47 when the differential pair is bal-
anced This keeps the characteristics of Q47 and Q49 well
matched Going back to Q44 and R33 these parts set up a
current source that varies the current through R36 With a
2V contrast voltage the differential pair is balanced mean-
ing that the voltage drop across R36 is V
CC
Q45 buffers
the voltage at R36 driving the bases of Q46 and Q47 Q46
further buffers the voltage driving the base of Q8 (see
Fig-
ure 8
) and the corresponding transistors in the other two
channels In the balanced condition the voltage at pin 2 will
also be two diode drops below V
CC
giving a well bal-
anced drive to the differential pair consisting of Q8 and Q9
in the video amplifier input stage With the contrast voltage
set to 0V the voltage at pin 2 will increase by about 400 mV
to 500 mV A 4V contrast voltage decreases the voltage at
pin 2 by about 400 mV to 500 mV from the balanced condi-
tion Reviewing
Figure 8
note that decreasing the voltage at
pin 2 will decrease the current flow through Q8 Thus the
current flow through Q9 increases increasing the gain of
the LM1208 LM1209 So increasing the contrast control
voltage at pin 12 increases the gain of the LM1208
LM1209 The contrast control voltage from Q46 and Q50 is
common to all three channels To minimize crosstalk it is
necessary to add a decoupling capacitor of 0 1
mF
across
R37 and R40 Since this can only be done externally these
two nodes are brought out to pins 1 and 2 The 30X resistor
is added in series with the capacitor for improving stability
To prevent a destructive current surge due to shorting either
pins 1 or 2 to ground R38 was added for current limiting
DRIVE CONTROL
Figure 10
is a simplified schematic of the Drive Control cir-
cuit Each channel has its own drive control circuit This
circuit is almost identical to
Figure 9
the contrast control
circuit It will be easier to cover the differences between the
two circuits instead of going through virtually the same cir-
cuit description Note that the input stage is
exactly
the
same The generation of the reference voltage at the right
hand side of
Figure 10
is slightly different than the circuit in
Figure 9
ln the drive control circuit the reference voltage at
the base of Q72 is to be V
CC
In the contrast control
circuit the reference voltage at the base of Q51 was to be
V
CC
To generate the V
CC
R57 and R58 form a 2 to 1
voltage divider With the two to one ratio it is now necessary
to have three transistors connected as diodes which are
Q74 Q75 and Q76 Q73 is the buffer for this voltage divider
and its emitter is exactly V
CC
with temperature compen-
sation R52 and R53 also differ from their corresponding
resistors in
Figure 9
R36 and R39 The value difference is
so the base of Q66 is also at V
CC
when the input drive
voltage is at 2V R38 in
Figure 9
was needed for current
limiting at the output pins Since each channel has its own
drive control circuit no filtering is required eliminating the
need for external pins With no external pins no current limit-
ing is necessary thus the 1k resistor is not used in the drive
control circuit
13
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