EL2176C/EL2276C
70 MHz/1 mA Current Mode Feedback Amp w/Disable
actually allows the EL2176C/EL2276C to main-
b
less of closed-loop gain. However, as closed-loop
gain is increased, bandwidth decreases slightly
while stability increases.
Applications Information Ð Contd.
back and gain resistors further exacerbates the
problem by further lowering the pole frequency.
tain about the same 3 dB bandwidth, regard-
The EL2176C/EL2276C have been specially de-
signed to reduce power dissipation in the feed-
back network by using large 1.0 kX feedback and
gain resistors. With the high bandwidths of these
amplifiers, these large resistor values would nor-
mally cause stability problems when combined
with parasitic capacitance, but by internally can-
celing the effects of a nominal amount of parasit-
ic capacitance, the EL2176C/EL2276C remain
very stable. For less experienced users, this fea-
ture makes the EL2176C/EL2276C much more
forgiving, and therefore easier to use than other
products not incorporating this proprietary cir-
cuitry.
Since the loop stability is improving with higher
closed-loop gains, it becomes possible to reduce
the value of R below the specified 1.0 kX and
F
still retain stability, resulting in only a slight loss
of bandwidth with increased closed-loop gain.
Supply Voltage Range and Single-
Supply Operation
The EL2176C/EL2276C have been designed to
operate with supply voltages having a span of
greater than 3V, and less than 12V. In practical
terms, this means that the EL2176C/EL2276C
g
will operate on dual supplies ranging from 1.5V
to 6V. With a single-supply, the EL2176C will
g
The experienced user with a large amount of PC
board layout experience may find in rare cases
that the EL2176C/EL2276 C have less band-
width than expected. In this case, the inverting
input may have less parasitic capacitance than
expected by the internal compensation circuitry
of the EL2176C/EL2276C. The reduction of feed-
back resistor values (or the addition of a very
small amount of external capacitance at the in-
verting input, e.g., 0.5 pF) will increase band-
width as desired. Please see the curves for Fre-
a
a
operate from 3V to 12V.
As supply voltages continue to decrease, it be-
comes necessary to provide input and output
voltage ranges that can get as close as possible to
the supply voltages. The EL2176C/EL2276C
have an input voltage range that extends to with-
in 1V of either supply. So, for example, on a sin-
a
gle 5V supply, the EL2176C/EL2276C have an
input range which spans from 1V to 4V. The out-
put range of the EL2176C /EL2276C is also quite
large, extending to within 1V of the supply rail.
quency Response for Various R and R , and
G
F
Frequency Response for Various C
.
b
IN
g
On a 5V supply, the output is therefore capable
of swinging from
b
a
4V to
4V. Single-supply
Feedback Resistor Values
The EL2176C/EL2276C have been designed and
output range is even larger because of the in-
creased negative swing due to the external pull-
a
down resistor to ground. On a single 5V sup-
ply, output voltage range is about 0.3V to 4V.
a
a
2 with R
e
specified at gains of
1.0 kX. This value of feedback resistor gives
e a
1 and
F
b
70 MHz of 3 dB bandwidth at A
about 1.5 dB of peaking, and 60 MHz of 3 dB
1 with
V
b
Video Performance
e a
peaking. Since the EL2176C/EL2276C are cur-
bandwidth at A
2 with about 0.5 dB of
V
For good video performance, an amplifier is re-
quired to maintain the same output impedance
and the same frequency response as DC levels are
changed at the output. This is especially difficult
when driving a standard video load of 150X, be-
cause of the change in output current with DC
level. Until the EL2176C/EL2276C, good Differ-
ential Gain could only be achieved by running
high idle currents through the output transistors
(to reduce variations in output impedance).
These currents were typically in excess of the
rent-feedback amplifiers, it is also possible to
change the value of R to get more bandwidth.
F
As seen in the curve of Frequency Response For
Various R and R , bandwidth and peaking can
G
F
be easily modified by varying the value of the
feedback resistor.
Because the EL2176C is a current-feedback am-
plifier, the gain-bandwidth product is not a con-
stant for different closed-loop gains. This feature
11
ELANTEC [ ELANTEC SEMICONDUCTOR ]
