10
EL5196C
Single 400MHz Fixed Gain Amplifier
EL5196C
Applications Information
Product Description
The EL5196C is a current-feedback operational ampli-
fier that offers a wide -3dB bandwidth of 600MHz and a
low supply current of 6mA per amplifier. The EL5196C
works with supply voltages ranging from a single 5V to
10V and they are al so cap able of swin ging to with in 1V
of either supply on the output. Because of their current-
feedb ack topo lo gy , th e EL 519 6C does no t hav e the no r -
mal gain-bandwidth pr oduct associated with voltage-
feedback operational amp lifiers. Instead, its -3dB band-
width to re main relativel y constant as closed-lo op gain is
increased. This combination of high bandwidth and low
power, together with aggressive pricing make the
EL5196C the ideal choice for many low- power/high-
bandwidth applications such as portable, handheld, or
battery-powered equipment.
For varyin g bandwidth needs, consider t he EL5191C
with 1GHz on a 9mA supply current or the EL5193C
with 300MHz on a 4mA supply current. Versions
include single, dual, and triple amp packages with
SOT23-5, 16-pin QSOP and 8-pin or 16-pin SOIC
outlines.
Power Supply Bypass ing and Printe d Circuit
Board Layout
As wit h a ny hi g h fre que ncy d evi ce , go od pri n te d c irc uit
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 t he risk of osci lla tion. Th e combi na-
tion of a 4.7µ F tantalum capacitor in parallel with a
0.01µF capacitor has been shown to work well when
placed at each supply pin.
For good AC performan ce, parasiti c capacitan ce 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,
particularly for the SO package, should be avoided if
possible. Sockets add parasitic ind uctance and capaci-
tance which will result in additional peaking and
overshoot.
Capacitance at the Inverting Input
Any manufacturer’s high-speed voltage- or current-
feedbac k amplifier can b e affected by stra y capacitan ce
at the inverting input. For inverting gains, this parasitic
capacita nce has little effect becau se the invertin g input is
a virtual gr o und, but for non -i nve rtin g gain s, this ca pac-
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 dest abilizing effect as a ze ro in the forward op en-
loop respon se. The use of large-value feedback and gain
resistors exacerbates the problem by further lowering
the pole frequency (increasing the possibility of
oscillation.)
The EL5196C has been optimized with a 375Ω 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 n ot recomm ende d aroun d the inv erting in put pi n
of the amplifier.
Feedback Resistor Values
The EL519 6C ha s bee n des igne d and spec ifi ed at a gain
of +2 with RF approximately 375Ω. This value of feed-
back resistor gives 300MHz of -3dB bandwidth at AV=2
with 2dB of peaking. With AV=-2, an RF of 375Ω gives
275MHz of band width with 1dB of peaki ng. Since the
EL5196C is a current-feedb ack ampli fier, it i s also pos-
sible to change the value of RF to get more bandwidth.
As seen in the curve of Frequency Response for Various
RF and RG, band width an d peaking can be easily mod i-
fied by varying t he value of the feedback resistor.
Because the EL5196C i s a current-feedback amplifier,
its gain-bandwidth product is not a constant for different
closed-loop gains. This feature actually allows the
EL5196C to maintain about t he same -3dB bandwidth.
As gain is increased, bandwidth decreases slightl y while