EL2120C
100 MHz Current Feedback Amplifier
Applications Information
The EL2120C represents the third generation of
current-feedback amplifier design. It is designed
to provide good high-frequency performance over
wide supply voltage, load impedance, gain, tem-
perature, and manufacturing lot variations. It is
a well-behaved amplifier in spite of its 100 MHz
bandwidth, but a few precautions should be tak-
en to obtain maximum performance.
The power supply pins must be well bypassed.
0.01 mF ceramic capacitors are adequate, but lead
length should be kept below (/4×and a ground
plane is recommended. Bypassing with 4.7 mF
tantalum capacitors can improve settling charac-
teristics, and smaller capacitors in parallel will
not be needed. The lead length of sockets general-
ly deteriorates the amplifier’s frequency response
by exaggerating peaking and increasing ringing
in response to transients. Short sockets cause lit-
tle degradation.
Load capacitance also increases ringing and
peaking. Capacitance greater than 35 pF should
be isolated with a series resistor. Capacitance at
the VINbterminal has a similar effect, and
should be kept below 5 pF. Often, the inductance
of the leads of a load capacitance will be self-reso-
nant at frequencies from 40 MHz to 200 MHz
and can cause oscillations. A resonant load can be
de-Q’ed with a small series or parallel resistor. A
‘‘snubber’’ can sometimes be used to reduce reso-
nances. This is a resistor and capacitor in series
connected from output to ground. Values of 68X
and 33 pF are typical. Increasing the feedback
resistor can also improve frequency flatness.
The VINapin can oscillate in the 200 MHz to
500 MHz realm if presented with a resonant or
inductive source impedance. A series 27Xto 68X
resistor right on the VINapin will suppress such
oscillations without affecting frequency response.
b3 dB bandwidth is inversely proportional to
the value of feedback resistor RF. The EL2120C
will tolerate values as low as 180Xfor a maxi-
mum bandwidth of about 140 MHz, but peaking
will increase and tolerance to stray capacitance
will reduce. At gains greater than 5, b3 dB band-
width begins to reduce, and a smaller RFcan be
used to maximize frequency response.
The greatest frequency response flatness (to
0.1 dB, for instance) occurs with RFe300Xto
330X. Even the moderate peaking caused by low-
er values of RFwill cause the gain to peak out of
the 0.1 dB window, and higher values of RFwill
cause an overcompensated response where the
gain falls below the 0.1 dB level. Parasitic capaci-
tances will generally degrade the frequency flat-
ness.
The EL2120C should not output a continuous
current above 50 mA, as stated in the ABSO-
LUTE MAXIMUM RATINGS table. The out-
put current limit is set to 120 mA at a die temper-
ature of 25§C and reduces to 85 mA at a die tem-
perature of 150§C. This large current is needed to
slew load capacitance and drive low impedance
loads with low distortion but cannot be support-
ed continuously. Furthermore, package dissipa-
tion capabilities cannot be met under short-cir-
cuit conditions. Current limit should not occur
longer than a few seconds.
The output disable function of the EL2120C is
optimized for video performance. While in dis-
able mode, the feedthrough of the circuit can be
modeled as a 0.2 pF capacitor from VINato the
output. No more than g5V can be placed be-
tween VINaand VINbin disable mode, but this
is compatible with common video signal levels.
In disabled state the output can withstand about
1000 V/ms slew rate signals impressed on it with-
out the output transistors turning on.
The /Disable pin logic level is referred to Va.
With g5V supplies, a CMOS or TTL driver with
pull-up resistor will suffice. g15V supplies re-
quire a a14/a11V drive span, or a15/a10V
nominally. Open-collector TTL with a tapped
pull-up resistor can provide these spans. The im-
pedance of the divider should be 1k or less for
optimum enable/disable speed.
The EL2120C enables in 50 ns or less. When VIN
e0, only a small switching glitch occurs at the
output. When VIN is some other value, the out-
put overshoots by about 0.7V when settling
toward its new enabled value.
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