EL5397C - Preliminary
Triple 200MHz Fixed Gain Amplifier
Applications Information
particularly for the SO package, should be avoided if
possible. Sockets add parasitic inductance and capaci-
tance which will result in additional peaking and
overshoot.
Product Description
The EL5397C is a current-feedback operational ampli-
fier that offers a wide -3dB bandwidth of 300MHz and a
low supply current of 4mA per amplifier. The EL5397C
works with supply voltages ranging from a single 5V to
10V and they are also capable of swinging to within 1V
of either supply on the output. Because of their current-
feedback topology, the EL5397C does not have the nor-
mal gain-bandwidth product associated with voltage-
feedback operational amplifiers. Instead, its -3dB band-
width to remain relatively constant as closed-loop gain is
increased. This combination of high bandwidth and low
power, together with aggressive pricing make the
EL5397C the ideal choice for many low-power/high-
bandwidth applications such as portable, handheld, or
battery-powered equipment.
Capacitance at the Inverting Input
Any manufacturer’s high-speed voltage- or current-
feedback amplifier can be affected by stray capacitance
at the inverting input. For inverting gains, this parasitic
capacitance has little effect because the inverting input is
a virtual ground, but for non-inverting gains, this capac-
itance (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 response. The use of large-value feedback and gain
resistors exacerbates the problem by further lowering
the pole frequency (increasing the possibility of
oscillation.)
For varying bandwidth needs, consider the EL5191C
with 1GHz on a 9mA supply current or the EL5192C
with 600MHz on a 6mA supply current. Versions
include single, dual, and triple amp packages with 5-pin
SOT23, 16-pin QSOP, and 8-pin or 16-pin SO outlines.
The EL5397C has been optimized with a 475W feedback
resistor. With the high bandwidth of these amplifiers,
these resistor values might cause stability problems
when combined with parasitic capacitance, thus ground
plane is not recommended around the inverting input pin
of the amplifier.
Power Supply Bypassing and Printed Circuit
Board Layout
As with any high frequency device, good printed circuit
board layout is necessary for optimum performance.
Low impedance ground plane construction is essential.
Surface mount components are recommended, but if
leaded components are used, lead lengths should be as
short as possible. The power supply pins must be well
bypassed to reduce the risk of oscillation. The combina-
tion of a 4.7µF tantalum capacitor in parallel with a
0.01µF capacitor has been shown to work well when
placed at each supply pin.
Feedback Resistor Values
The EL5397C has been designed and specified at a gain
of +2 with RF approximately 500W. This value of feed-
back resistor gives 200MHz of -3dB bandwidth at AV=2
with 2dB of peaking. With AV=-2, an RF of approxi-
mately 500W gives 175MHz of bandwidth with 0.2dB of
peaking. Since the EL5397C is a current-feedback
amplifier, it is also possible to change the value of RF to
get more bandwidth. As seen in the curve of Frequency
Response for Various RF and RG, bandwidth and peak-
ing can be easily modified by varying the value of the
feedback resistor.
For good AC performance, parasitic capacitance should
be kept to a minimum, especially at the inverting input.
(See the Capacitance at the Inverting Input section) Even
when ground plane construction is used, it should be
removed from the area near the inverting input to mini-
mize any stray capacitance at that node. Carbon or
Metal-Film resistors are acceptable with the Metal-Film
resistors giving slightly less peaking and bandwidth
because of additional series inductance. Use of sockets,
Because the EL5397C is a current-feedback amplifier,
its gain-bandwidth product is not a constant for different
closed-loop gains. This feature actually allows the
EL5397C to maintain about the same -3dB bandwidth.
As gain is increased, bandwidth decreases slightly while
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