EL2160C
130 MHz Current Feedback Amplifier
Capacitance at the Inverting Input
Applications Information
Due to the topology of the current feedback am-
plifier, stray capacitance at the inverting input
will affect the AC and transient performance of
the EL2160C when operating in the non-
inverting configuration. The characteristic curve
b
Product Description
The EL2160C is a current mode feedback amplifi-
er that offers wide bandwidth and good video
specifications at a moderately low supply cur-
rent. It is built using Elantec’s proprietary com-
plimentary bipolar process and is offered in in-
dustry standard pin-outs. Due to the current
feedback architecture, the EL2160C closed-loop
3 dB bandwidth is dependent on the value of the
feedback resistor. First the desired bandwidth is
of gain vs. frequency with variations of C
IN
emphasizes this effect. The curve illustrates how
the bandwidth can be extended to beyond
200 MHz with some additional peaking with an
b
additional 2 pF of capacitance at the V
pin
IN
e a
for the case of A
2. Higher values of capac-
itance will be required to obtain similar effects at
V
selected by choosing the feedback resistor, R ,
F
and then the gain is set by picking the gain resis-
higher gains.
tor, R . The curves at the beginning of the Typi-
G
cal Performance Curves section show the effect of
In the inverting gain mode, added capacitance at
the inverting input has little effect since this
point is at a virtual ground and stray capacitance
is therefore not ‘‘seen’’ by the amplifier.
varying both R and R . The 3 dB bandwidth is
G
F
somewhat dependent on the power supply volt-
age. As the supply voltage is decreased, internal
junction capacitances increase, causing a reduc-
tion in closed loop bandwidth. To compensate for
this, smaller values of feedback resistor can be
used at lower supply voltages.
Feedback Resistor Values
The EL2160C has been designed and specified
e
e a
with R
560X for A
2. This value of
F
V
feedback resistor yields extremely flat frequency
response with little to no peaking out to
130 MHz. As is the case with all current feedback
amplifiers, wider bandwidth, at the expense of
slight peaking, can be obtained by reducing the
value of the feedback resistor. Inversely, larger
values of feedback resistor will cause rolloff to
occur at a lower frequency. By reducing R to
F
430X, bandwidth can be extended to 170 MHz
with under 1 dB of peaking. Further reduction of
Power Supply Bypassing and Printed
Circuit Board Layout
As with any high frequency device, good printed
circuit board layout is necessary for optimum
performance. Ground plane construction is high-
ly recommended. Lead lengths should be as short
as possible, below (/4 . The power supply pins
×
must be well bypassed to reduce the risk of oscil-
lation. A 1.0 mF tantalum capacitor in parallel
with a 0.01 mF ceramic capacitor is adequate for
each supply pin.
R
F
to 360X increases the bandwidth to 195 MHz
with about 2.5 dB of peaking. See the curves in
the Typical Performance Curves section which
show 3 dB bandwidth and peaking vs. frequency
for various feedback resistors and various supply
voltages.
For good AC performance, parasitic capacitances
should be kept to a minimum, especially at the
inverting input (see Capacitance at the Inverting
Input section). This implies keeping the ground
plane away from this pin. Carbon resistors are
acceptable, while use of wire-wound resistors
should not be used because of their parasitic in-
ductance. Similarly, capacitors should be low in-
ductance for best performance. Use of sockets,
particularly for the SO package, should be avoid-
ed. Sockets add parasitic inductance and capaci-
tance which will result in peaking and overshoot.
Bandwidth vs Temperature
Whereas many amplifier’s supply current and
consequently 3 dB bandwidth drop off at high
temperature, the EL2160C was designed to have
little supply current variations with temperature.
An immediate benefit from this is that the 3 dB
bandwidth does not drop off drastically with
e
e a
the bandwidth only varies from 150 MHz to
g
temperature. With V
15V and A
2,
S
V
110 MHz over the entire die junction tempera-
k
150 C.
k
ture range of 0 C
§
T
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12
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