EL4451C
Wideband Variable-Gain Amplifier, Gain of 2
360X or less. Alternatively, a small capacitor
across R can be used to create more of a fre-
Applications Information
F
The EL4451 is a complete two-quadrant multipli-
er/gain control with 70 MHz bandwidth. It has
three sets of inputs; a differential signal input
quency-compensated divider. The value of the ca-
pacitor should scale with the parasitic capaci-
tance at the FB input. It is also practical to place
small capacitors across both the feedback and the
gain resistors (whose values maintain the desired
gain) to swamp out parasitics. For instance, two
10pF capacitors across equal divider resistors for
a maximum gain of 4 will dominate parasitic ef-
fects and allow a higher divider resistance.
V
IN
, a differential gain-controlling input V
,
GAIN
and another differential input which is used to
complete a feedback loop with the output. Here is
a typical connection:
The REF pin can be used as the output’s ground
reference, for DC offsetting of the output, or it
can be used to sum in another signal.
Gain-Control Characteristics
The quantity V
in the above equations is
2, even though the exter-
GAIN
s
s
bounded as 0
V
GAIN
4451-2
nally applied voltages exceed this range. Actual-
ly, the gain transfer function around 0 and 2V is
‘‘soft’’; that is, the gain does not clip abruptly
The gain of the feedback divider is
R
G
e
H
.
below the 0%-V voltage nor above the
GAIN
a
R
G
R
F
100%-V
applied to V
level. An overdrive of 0.3V must be
GAIN
The transfer function of the part is
a
to obtain truly 0% or 100%.
GAIN
e
c
a
b
b
c
b
b
(V
GAIN
a
))
V
A
O
(((V
IN
)
(V
)) ((V
GAIN
)
OUT
IN
Because the 0%- or 100%- V levels cannot
GAIN
b
(V
REF
V
)).
FB
be precisely determined, they are extrapolated
from two points measured inside the slope of the
gain transfer curve. Generally, an applied V
V
is connected to V
OUT
c
V
through a feedback
. A is the open-
OUT O
FB
network, so V
e
H
FB
GAIN
loop gain of the amplifier, and is approximately
600. The large value of A drives
b
range of 0.5V to 2.5V will assure the full nu-
merical span of 0
a
O
s
s
V
GAIN
2.
a
b
b
c
a
b
b
a
b
((V
IN
)
(V
IN
)) ((V
GAIN
)
(V
GAIN
))
(V
V
FB
)
REF
x
0.
The gain control has a small-signal bandwidth
equal to the V channel bandwidth, and over-
load recovery resolves in about 20 nsec.
Rearranging and substituting for V
a
IN
FB
e
a
b
b
c
b
a
V
(((V
IN
)
(V
)) ((V
GAIN
)
(V
))
V
)/H,
REF
OUT
IN
GAIN
or
V
Input Connections
e
c
a
(V
IN
V
V
)/H
REF
OUT
GAIN
The input transistors can be driven from resistive
and capacitive sources, but are capable of oscilla-
tion when presented with an inductive input. It
takes about 80nH of series inductance to make
the inputs actually oscillate, equivalent to four
Thus the output is equal to the difference of the
’s times the difference of V and offset
V
IN
by V
GAIN’S
, all gained up by the feedback divider
REF
ratio. The EL4451 is stable for a direct connec-
tion between V and FB, and the divider may
OUT
inches of unshielded wiring or 6 of unterminat-
×
be used for higher output gain, although with the
traditional loss of bandwidth.
ed input transmission line. The oscillation has a
characteristic frequency of 500 MHz. Often plac-
ing one’s finger (via a metal probe) or an oscillo-
scope probe on the input will kill the oscillation.
Normal high-frequency construction obviates
any such problems, where the input source is rea-
sonably close to the input. If this is not possible,
one can insert series resistors of around 51X to
de-Q the inputs.
It is important to keep the feedback divider’s im-
pedance at the FB terminal low so that stray ca-
pacitance does not diminish the loop’s phase
margin. The pole caused by the parallel imped-
ance of the feedback resistors and stray capaci-
tance should be at least 150 MHz; typical strays
of 3 pF thus require a feedback impedance of
7
ELANTEC [ ELANTEC SEMICONDUCTOR ]
