EL4094C
Video Gain Control Fader
Applications Information
The EL4094 is a self-contained and calibrated
fader subsystem When a given channel has
100% gain the circuit behaves as a current-feed-
back amplifier in unity-gain connection As such
video and transfer distortions are very low As
the gain of the input is reduced a 2-quadrant
multiplier is gradually introduced into the signal
path and distortions increase with reducing gain
The input impedance also changes with gain set-
ting from about 1 MX at 100% gain down to
16 kX at zero gain To maximize gain accuracy
and linearity the inputs should be driven from
source impedances of 500X or less
it is necessary to overdrive the gain control input
by 30 mV or more This would set the gain con-
trol voltage range as
b
0 565V to
a
0 565V or
30 mV beyond the maximum guaranteed 0% to
100% range In fact the gain control inputs are
very complex Here is a representation of the ter-
minals
Linearity
The EL4094 is designed to work linearly with
g
2V inputs but lowest distortion occurs at
g
1V
levels and below Errors are closer to those of a
good current-feedback amplifier above 90% gain
Low-frequency linearity is 0 1% or better for
gains 25% to 100% and inputs up to 1V NTSC
differential gain and phase errors are better than
0 3% and 0 3 for the 25% to 100% gain range
These distortions are not strongly affected by
supply voltage nor output loading at least down
to 150X For settling to 0 1% however it is best
to not load the output heavily and to run the
EL4094 on the lowest practical supply voltages
so that thermal effects are minimized
4094 – 12
Representation of Gain Control
Inputs V
G
and V
G
For gain control inputs between
g
0 5V
(
g
90
mA)
the diode bridge is a low impedance
and all of the current into Vg flows back out
through V
g
When gain control inputs exceed
this amount the bridge becomes a high imped-
ance as some of the diodes shut off and the V
g
impedance rises sharply from the nominal 5 5KX
to about 500KX This is the condition of gain
control overdrive The actual circuit produces a
much sharper overdrive characteristic than does
the simple diode bridge of this representation
The gain input has a 20 MHz
b
3 dB bandwidth
and 17 ns risetime for inputs to
g
0 45V When
the gain control voltage exceeds the 0% or 100%
values a 70 ns overdrive recovery transient will
occur when it is brought back to linear range If
quicker gain overdrive response is required the
Force control inputs of the EL4095 can be used
Gain Control Inputs
The gain control inputs are differential and may
be biased at any voltage as long as V
GAIN
is less
than 2 5V below V
a
and 3V above V
b
The dif-
ferential input impedance is 5 5 kX and the com-
mon-mode impedance is more than 500 kX With
zero differential voltage on the gain inputs both
signal inputs have a 50% gain factor Nominal
calibration sets the 100% gain of V
INA
input at
a
0 5V of gain control voltage and 0% at
b
0 5V
of gain control V
INB
’s gain is complementary to
that of V
INA
a
0 5V of gain control sets 0% gain
at V
INB
and
b
0 5V gain control sets 100% V
INB
gain The gain control does not have a complete-
ly abrupt transition at the 0% and 100% points
There is about 10 mV of ‘‘soft’’ transfer at the
gain endpoints To obtain the most accurate
100% gain factor or best attenuation at 0% gain
8
Output Loading
The EL4094 does not work well with heavy ca-
pacitive loads Like all amplifier outputs the out-
put impedance becomes inductive over frequency
resonating with a capacitive load The effective
output inductance of the EL4094 is about 350
nH More than 50 pF will cause excessive fre-
quency response peaking and transient ringing
The problem can be solved by inserting a low-
value resistor in series with the load 22X or
more If a series resistance cannot be used then
adding a 300X or less load resistor to ground or a
‘‘snubber’’ network may help A snubber is a re-
ELANTEC [ ELANTEC SEMICONDUCTOR ]
