Datasheet

AD539
Rev. B | Page 15 of 20
A 50 MHZ VOLTAGE-CONTROLLED AMPLIFIER
Figure 24 is a circuit for a 50 MHz voltage-controlled amplifier
(VCA) suitable for use in high quality video-speed applications.
The outputs from the two signal channels of the AD539 are
applied to the op amp in a subtracting configuration. This
connection has two main advantages: first, it results in better
rejection of the control voltage, particularly when overdriven
(V
X
< 0 V or V
X
> 3.3 V). Secondly, it provides a choice of either
noninverting or inverting response, using either input, V
Y1
or
V
Y2
, respectively. In this circuit, the output of the op amp equals
V0for
V2
)(
21
>
−
=
X
YYX
OUT
V
VVV
V
Therefore, the gain is unity at V
X
= 2 V. Because V
X
can over-
range to 3.3 V, the maximum gain in this configuration is
about 4.3 dB.
The −3 dB bandwidth of this circuit is over 50 MHz at a full
gain and is not substantially affected at lower gains. When V
X
is
zero (or slightly negative, to override the residual input offset)
there is still a small amount of capacitive feedthrough at high
frequencies; therefore, extreme care is required in laying out the
PC board to minimize this effect. In addition, for small values
of V
X
, the combination of this feedthrough with the multiplier
output can cause a dip in the response where they are out of
phase. Figure 15 shows the ac response from the noninverting
input, with the response from the inverting input, V
Y2
, essentially
identical. Test conditions include V
Y1
= 0.5 V rms for values of
V
X
from 10 mV to 3.16 V; this is with a 75 Ω load on the output.
The feedthrough at V
X
= −10 mV is also shown.
With the VCA driving a 75 Ω load and the transient response of
the signal channel at V
X
= 2 V, V
Y
= V
OUT
= ±1 V is shown in
Figure 16. The rise and fall times are approximately 7 ns.
A more detailed description of this circuit, including differential
gain and phase characteristics, is given in the AN-213 Application
Note, Low Cost, Two Chip Voltage-Controlled Amplifier and
Video Switch, available from Analog Devices.
09679-024
1
2
3
4
16
15
14
13
5 12
6 11
7 10
8 9
AD539
V
X
HF COMP
V
Y1
+V
S
–V
S
V
Y2
INPUT
COMMON
OUTPUT
COMMON
NOTES
1. THOMPSON-CSF BAR. 10 OR SIMILAR SCHOTTKY DIODE
S
HORT DIRECT CONNECTION TO GROUND PLANE.
BASE
COMMON
W1
Z1
CHAN1
OUTPUT
CHAN2
OUTPUT
Z2
W2
C
F
600pF
V
X
75Ω
10Ω
10Ω
D1
V
Y2
IN
–9V
+9V
V
Y2
IN
1µF
1µF
75Ω
75Ω
C
C
3000pF
–9V
10
14
1
9
7
3
+9V
–9V
100kΩ
180Ω
180Ω
470Ω
0.25pF TO
1.5pF
200Ω
GAIN ADJUST
(±4% RANGE)
50kΩ
(OPTIONAL)
OUTPUT OFFSET
0.47µF
V
OUT
0.47µF
+9V
2.7Ω
2.7Ω
C
F
Figure 24. A Wide Bandwidth Voltage-Controlled Amplifier (16-Lead SBDIP and PDIP Shown)