EL2421C
EL2421C
Quad, Low Cost, Gain of 2, Video Op Amp
Applications Information
Product Description
The EL2421C operational amplifier is stable at a gain of
1. It is built on Elantec’s proprietary complimentary
bipolar process. This topology allows it to be used in a
variety of applications where current mode amplifiers
are not appropriate because of restrictions placed on the
feedback elements. This product is especially designed
for applications where high bandwidth and good video
performance characteristics are desired but the higher
cost of more flexible and sophisticated products are
prohibitive.
Single 5 Volt Supply Video Cable Driver
These amplifiers may be used as a direct coupled video
cable driver with a gain of 2. With a 75Ω back matching
resistor driving a terminated 75Ω cable the output at the
cable load will be original video level (1V NTSC). The
best operating mode is with direct coupling. The input
signal must be offset to keep the entire signal within the
range of the amplifier. The required offset voltage can
be set with a resistor divider and a bypass capacitor in
the video path (Figure
1).
The input DC offset should be
between.3V and .5V. With R
A
=68K and R
B
=4.7K the
input offset will be .32V. Since these amplifiers require
a DC load at their outputs it is good design practice to
add a 250Ω resistor to ground directly at the amplifier
output. Then if the 75Ω cable termination resistor were
inadvertently removed there would still be an output sig-
nal. The values in figure 1 give an output range of 0V to
2.6V
Output capacitive coupling also has some restrictions.
These amplifiers require a DC load at their outputs. A
75Ω back matching resistor to a cable and a 75Ω load to
ground at the end of the cable provide a 150Ω DC load.
But output capacitive coupling opens this DC path so an
extra pulldown resistor on the amplifier output to ground
is required. Figure 4 shows a 250Ω resistor. Capacitively
coupling the output will require that we shift the output
offset voltage higher than in the direct coupled case.
Using R
A
=43K and R
B
=4.7K will make the quiescent
output offset voltage about 1V. The output dynamic
range will be .6V to 3V.
Power Supplies
The EL2421C is designed to work at a supply voltage
difference of 4.5V to 5.5V. It will work on any combina-
tion of ± supplies. All electrical characteristics are
measured with a 5V supply.
Output Swing vs Load
Please refer to the simplified block diagram. This ampli-
fier provides an NPN pull-up transistor output and a
passive 1250Ω pull-down resistor to the most negative
supply. In a application where the load is connected to
V
S
−
the output voltage can swing to within 200 mV of
V
S
− .
Output Drive Capability
This device does not have short circuit protection. Each
output is capable of than 100 mA into a shorted output.
Care must be used in the design to limit the output cur-
rent with a series resistor.
5
ELANTEC [ ELANTEC SEMICONDUCTOR ]
