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
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 V
IN
b
terminal 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 V
IN
a
pin can oscillate in the 200 MHz to
500 MHz realm if presented with a resonant or
inductive source impedance A series 27X to 68X
resistor right on the V
IN
a
pin will suppress such
oscillations without affecting frequency response
b
3 dB bandwidth is inversely proportional to
The greatest frequency response flatness (to
0 1 dB for instance) occurs with R
F
e
300X to
330X Even the moderate peaking caused by low-
er values of R
F
will cause the gain to peak out of
the 0 1 dB window and higher values of R
F
will
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 V
IN
a
to the
output No more than
g
5V can be placed be-
tween V
IN
a
and V
IN
b
in 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 V
a
With
g
5V supplies a CMOS or TTL driver with
pull-up resistor will suffice
g
15V supplies re-
quire a
a
14
a
11V drive span or
a
15
a
10V
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 V
IN
e
0 only a small switching glitch occurs at the
output When V
IN
is some other value the out-
put overshoots by about 0 7V when settling
toward its new enabled value
the value of feedback resistor R
F
The EL2120C
will tolerate values as low as 180X for a maxi-
mum bandwidth of about 140 MHz but peaking
will increase and tolerance to stray capacitance
will reduce At gains greater than 5
b
3 dB band-
width begins to reduce and a smaller R
F
can be
used to maximize frequency response
9
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