EL4451C
Wideband Variable-Gain Amplifier Gain of 2
Applications Information
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
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
360X or less Alternatively a small capacitor
across R
F
can be used to create more of a fre-
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
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
GAIN
in the above equations is
bounded as 0
s
V
GAIN
s
2 even though the exter-
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
below the 0%-V
GAIN
voltage nor above the
100%-V
GAIN
level An overdrive of 0 3V must be
applied to V
GAIN
to obtain truly 0% or 100%
Because the 0%- or 100%- V
GAIN
levels cannot
be precisely determined they are extrapolated
from two points measured inside the slope of the
gain transfer curve Generally an applied V
GAIN
range of
b
0 5V to
a
2 5V will assure the full nu-
merical span of 0
s
V
GAIN
s
2
The gain control has a small-signal bandwidth
equal to the V
IN
channel bandwidth and over-
load recovery resolves in about 20 nsec
4451-2
The gain of the feedback divider is
H
e
R
G
R
G
a
R
F
The transfer function of the part is
V
OUT
e
A
O
c
(((V
IN
a
)
b
(V
IN
b
))
c
((V
GAIN
a
)
b
(V
GAIN
b
))
a
(V
REF
b
V
FB
))
V
FB
is connected to V
OUT
through a feedback
network so V
FB
e
H
c
V
OUT
A
O
is the open-
loop gain of the amplifier and is approximately
600 The large value of A
O
drives
((V
IN
a
)
b
(V
IN
b
))
c
((V
GAIN
a
)
b
(V
GAIN
b
))
a
(V
REF
b
V
FB
)
x
0
Rearranging and substituting for V
FB
V
OUT
e
(((V
IN
a
)
b
(V
IN
b
))
c
((V
GAIN
a
)
b
(V
GAIN
))
a
V
REF
) H
or
V
OUT
e
(V
IN
c
V
GAIN
a
V
REF
) H
Input Connections
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
inches of unshielded wiring or 6 of unterminat-
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
7
Thus the output is equal to the difference of the
V
IN
’s times the difference of V
GAIN’S
and offset
by V
REF
all gained up by the feedback divider
ratio The EL4451 is stable for a direct connec-
tion between V
OUT
and FB and the divider may
be used for higher output gain although with the
traditional loss of bandwidth
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
ELANTEC [ ELANTEC SEMICONDUCTOR ]
