EL2360C
Triple 130 MHz Current Feedback Amplifier
Applications Information
Product Description
Capacitance at the Inverting Input
The EL2360C is a triple current feedback ampli-
fier that offers wide bandwidth and good video
specifications at moderately low supply currents.
It is built using Elantec’s proprietary compli-
mentary bipolar process and is offered in both a
16 pin PDIP and SOIC packages. Due to the cur-
rent feedback architecture, the EL2360C clo-
Any manufacturer’s high-speed voltage- or cur-
rent-feedback amplifier can be affected by stray
capacitance at the inverting input. The character-
istic curve of gain vs. frequency with variations
in C
emphasizes this effect. The curve illus-
b
IN
trates how the bandwidth can be extended to be-
yond 200 MHz with some additional peaking
with an additional 2pF of capacitance at the
b
b
sed loop 3 dB bandwidth is dependent on the
value of the feedback resistor. First the desired
bandwidth is selected by choosing the feedback
V
IN
pin. For inverting gains, this parasitic ca-
b
pacitance has little effect because the inverting
input is a virtual ground, but for non-inverting
gains, this capacitance (in conjunction with the
feedback and gain resistors) creates a pole in the
feedback path of the amplifier. This pole, if low
enough in frequency, has the same destabilizing
effect as a zero in the forward open-loop re-
sponse. The use of large value feedback and gain
resistors further exacerbates the problem by fur-
ther lowering the pole frequency.
resistor, R , and then the gain is set by picking a
F
gain resistor, R . The curves at the beginning of
G
the Typical Performance Curves section show the
b
effect of varying both R and R The 3 dB
G.
F
bandwidth is somewhat dependent on the power
supply voltage. As the supply voltage is de-
creased, internal junction capacitances increase,
causing a reduction in the closed loop bandwidth.
To compensate for this, smaller values of feed-
back resistor can be used at lower supply volt-
ages.
Feedback Resistor Values
The EL2360C has been designed and specified at
a
e
Power Supply Bypassing and Printed
Circuit Board Layout
a gain of
2 with R
560X. This value of
F
feedback resistor yields relatively flat frequency
response with little to no peaking out to 130
MHz. Since the EL2360C is a current-feedback
amplifier, it is also possible to change the value
of R to get more bandwidth. As seen in the
F
curve of Frequency Response For Various R
F
and R , bandwidth and peaking can be easily
G
modified by varying the value of the feedback
resistor. For example, by reducing R to 430X,
F
bandwidth can be extended to 170 MHz with un-
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, preferably below (/4’’. The power sup-
ply pins must be well bypassed to reduce the risk
of oscillation. The combination of a 1.0 mF tanta-
lum capacitor in parallel with a 0.01 mF ceramic
capacitor has been shown to work well when
placed at each supply pin.
der 1 dB of peaking. Further reduction of R to
F
360X increases the bandwidth to 195 MHz with
about 2.5 dB of peaking.
For good AC performance, parasitic capacitance
should be kept to a minimum especially at the
inverting input (see the Capacitance at the In-
verting Input section). This implies keeping the
ground plane away from this pin. Carbon or Met-
al-Film resistors are acceptable with the Metal-
Film resistors giving slightly less peaking and
bandwidth because of their additional series in-
ductance. Use of sockets, particularly for the SO
package should be avoided if possible. Sockets
add parasitic inductance and capacitance which
will result in some additional peaking and over-
shoot.
Bandwidth vs Temperature
Whereas many amplifier’s supply current and
b
consequently 3 dB bandwidth drop off at high
temperature, the EL2360C was designed to have
little supply current variation with temperature.
b
An immediate benefit from this is that the
3
dB bandwidth does not drop off drastically with
e a
2,
e
g
temperature. With V
15V and A
S
V
the bandwidth varies only from 150 MHz to 110
MHz over the entire die junction temperature
k
k
150 C.
b
range of 50 C
T
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11
ELANTEC [ ELANTEC SEMICONDUCTOR ]
