+5V
-5V
V
OUT
V
C
0 2V® -
- ®40dB +40dB Gain
VCA810
X1
2
3
V+
V-
1
6
7
5
8
Gain
Adjust
+
VCA810
V
C
V
I
V
O
+5V
6
7
++
2
R
C
R
T
25W
RL
500W
0.1 Fm
6.8 Fm
3
5
8
1
-5V
0.1 Fm
6.8 Fm
R
S
50W
50W
Source
VCA810
www.ti.com
SBOS275F –JUNE 2003–REVISED DECEMBER 2010
APPLICATION INFORMATION
Thus, G
(dB)
varies linearly over the specified −40dB to
CIRCUIT DESCRIPTION
+40dB range as V
C
varies from 0V to −2V. Optionally,
making V
C
slightly positive (≥ +0.15V) effectively
The VCA810 is a high gain adjust range, wideband,
disables the amplifier, giving greater than 80dB of
voltage amplifier with a voltage-controlled gain, as
signal path attenuation at low frequencies.
shown in Figure 29. The circuit’s basic voltage
amplifier responds to the control of an internal
Internally, the gain-control circuit varies the amplifier
gain-control amplifier. At its input, the voltage
gain by varying the transconductance, g
m
, of a bipolar
amplifier presents the high impedance of a differential
transistor using the transistor bias current. Varying
stage, permitting flexible input impedance matching.
the bias currents of differential stages varies g
m
to
To preserve termination options, no internal circuitry
control the voltage gain of the VCA810. A g
m
-based
connects to the input bases of this differential stage.
gain adjust normally suffers poor thermal stability.
For this reason, the user must provide dc paths for
The VCA810 includes circuitry to minimize this effect.
the input base currents from a signal source, either
through a grounded termination resistor or by a direct
VCA810 OPERATION
connection to ground. The differential input stage also
permits rejection of common-mode signals. At its
Figure 30 shows the circuit configuration used as the
output, the voltage amplifier presents a low
basis of the Electrical Characteristics and Typical
impedance, simplifying impedance matching. An
Characteristics. Voltage swings reported in the
open-loop design produces wide bandwidth at all gain
specifications are taken directly at the input and
settings. A ground-referenced differential to
output pins. For test purposes, the input impedance is
single-ended conversion at the output retains the low
set to 50Ω with a resistance to ground. A 25Ω
output offset voltage.
resistance (R
T
) is included on the V− input to get bias
current cancellation. Proper supply bypassing is
shown in Figure 30, and consists of two capacitors on
each supply pin: one large electrolytic capacitor
(2.2mF to 6.8mF), effective at lower frequencies, and
one small ceramic capacitor (0.1mF) for
high-frequency decoupling. For more information on
decoupling, refer to the Board Layout section.
Figure 29. Block Diagram of the VCA810
A gain control voltage, V
C
, controls the amplifier gain
magnitude through a high-speed control circuit. Gain
polarity can be either inverting or noninverting,
depending upon the amplifier input driven by the input
signal. The gain control circuit presents the high-input
impedance of a noninverting op amp connection. The
control voltage pin is referred to ground as shown in
Figure 30. Variable Gain, Specification and Test
Figure 29. The control voltage V
C
varies the amplifier
Circuit
gain according to the exponential relationship:
Notice that both inverting and noninverting inputs are
connected to ground with a resistor (R
S
and R
T
).
This translates to the log gain relationship:
Matching the dc source impedance looking out of
G
(dB)
= –40 ● (V
C
+ 1)dB.
each input will minimize input offset voltage error.
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