EL4421C 22C 41C 42C 43C 44C
Multiplexed-Input Video Amplifiers
Applications Information
Contd
is again
g
3V and the output swing is
g
6V The
EL4443 or EL4444 can be wired for inverting
gain with even more amplitude possible
The output and positive inputs respond to over-
loading amplitudes correctly that is they simply
clamp and remain monotonic with increasing
a
input overdrive A condition exists however
where the
b
input of an active stage is overdriven
by large outputs This occurs mainly in unity-
gain connections and only happens for negative
inputs The overloaded input cannot control the
feedback loop correctly and the output can be-
come non-monotonic A typical scenario has the
circuit running on
g
5V supplies connected for
unity gain and the input is the maximum
g
3V
Negative input extremes can cause the output to
jump from
b
3V to around
b
2 3V This will nev-
er happen if the input is restricted to
g
2 5V
which is the guaranteed maximum input compli-
ance with
g
5V supplies and is not a problem
with greater supply voltages Connecting the
feedback network with a divider will prevent the
overloaded output voltage from being large
enough to overload the
b
input and monotonic
The logic input(s) and ground pin are shifted
2 5V above system ground to correctly bias the
mux-amp Of course all the signal inputs and
output will have to be shifted 2 5V above system
ground to ensure proper signal path biasing
A final caution the ground pin is also connected
to the IC’s substrate and frequency compensation
components The ground pin must be returned to
system ground by a short wire or nearby bypass
capacitor In figure 2 the 22 KX resistors also
serve to isolate the bypassed ground pin from the
a
5V supply noise
Signal Amplitudes
Signal input and output voltages must be be-
tween (V
b
)
a
2 5V and (V
a
)
b
2 5V to ensure
linearity Additionally the differential voltage on
any input stage must be limited to
g
6V to pre-
vent damage In unity-gain connections any in-
put could have
g
3V applied and the output
would be at
g
3V putting us at our 6V differen-
tial limit Higher-gain circuit applications divide
the output voltage and allow for larger outputs
For instance at a gain of
a
2 the maximum input
11
ELANTEC [ ELANTEC SEMICONDUCTOR ]
