EL2082C
Current-Mode Multiplier
Applications Information Ð Contd.
The output current could be terminated with a 1 kX load resistor to achieve a nominal voltage gain of 1
at the EL2082, but the I
practical total capacitance at I
, load, and stray capacitances would limit bandwidth greatly. The lowest
is about 12 pF, and this gives a 13 MHz bandwidth with a 1 kX
OUT
OUT
load. In the above example a 100X load is used for an upper limit of 130 MHz. The operational
a
amplifier gives a gain of 10 to bring the overall gain to unity. Wider bandwidth yet can be had by
installing C . This is a very small capacitor, typically 1 pf–2 pF, and it bolsters the gain above
IN
100 MHz. Here is a table of results for this circuit used with various amplifiers:
b
Bandwidth
Operational
Amplifier
Power
3 dB
0.1 dB
Supplies
Rf
Rg
C
Bandwidth
Peaking
IN
g
g
g
g
g
g
g
g
g
g
EL2020
EL2020
EL2130
EL2030
EL2090
EL2120
EL2120
EL2070
EL2071
EL2075
5V
620
620
620
620
240
220
220
200
1.5K
620
68
68
68
68
27
24
24
22
240
68
Ð
34 MHz
40 MHz
73 MHz
93 MHz
60 MHz
57 MHz
65 MHz
150 MHz
200 MHz
270 MHz
5.6 MHz
7.4 MHz
11 MHz
12 MHz
10 MHz
10 MHz
11 MHz
30 MHz
30 MHz
30 MHz
0
15V
5V
Ð
Ð
0
1.0 dB
1.3 dB
0.5 dB
0.4 dB
0.3 dB
0.4 dB
0
15V
15V
5V
Ð
Ð
Ð
15V
5V
Ð
2 pF
2 pF
2 pF
5V
5V
1.5 dB
a
b
Maximum bandwidth is maintained over a gain range of 6 to 16 dB; bandwidth drops at lower
gains. If wider gain range with full bandwidth is required, two or more EL2082’s can be cascaded with
the I
of one directly driving the I of the next.
IN
OUT
The EL2082 can also be used with an I
x
V operational circuit:
V
R
F
GAIN
e b
Gain
a
1V
R
95X
#
J
IN
2082–14
Inverting
EL2082
a
Op-Amp
The circuit above gives a negative gain. The main concern of this connection involves the total I
OUT
and stray capacitances at the amplifier’s input. When using traditional op-amps, the pole caused by
these capacitances can make the amplifier less stable and even cause oscillations in amplifiers whose
gain-bandwidth is greater than 5 MHz. A typical cure is to add a capacitor Cf in the 2 pF–10 pF range.
This will reduce overall bandwidth, so a capacitor C can be added to regain frequency response. The
IN
ratio Cf/C is made equal to R /R .
f
IN
IN
8
ELANTEC [ ELANTEC SEMICONDUCTOR ]
