VCA810_16 Datasheet, PDF (16/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier

VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
8 Detailed Description
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8.1 Overview
The VCA810 is a high gain adjust range, wideband, voltage amplifier with a voltage-controlled gain, as shown in
Functional Block Diagram. The circuit’s basic voltage amplifier responds to the control of an internal gain-control
amplifier. At its input, the voltage amplifier presents the high impedance of a differential stage, permitting flexible
input impedance matching. To preserve termination options, no internal circuitry connects to the input bases of
this differential stage. For this reason, the user must provide DC paths for the input base currents from a signal
source, either through a grounded termination resistor or by a direct connection to ground. The differential input
stage also permits rejection of common-mode signals. At its output, the voltage amplifier presents a low
impedance, simplifying impedance matching. An open-loop design produces wide bandwidth at all gain settings.
A ground-referenced differential to single-ended conversion at the output retains the low output offset voltage.
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 operational amplifier connection. The
control voltage pin is referred to ground as shown in Functional Block Diagram. The control voltage V_C varies the
amplifier gain according to the exponential relationship:
G = 10 _(V/V)
-2 (V_C + 1)
(1)
This translates to the log gain relationship:
G_(dB) = –40 × (V_C + 1)dB
(2)
Thus, G_(dB) varies linearly over the specified −40 dB to 40 dB range as V_C varies from 0 V to −2 V. Optionally,
making V_C slightly positive (≥ 0.15 V) effectively disables the amplifier, giving greater than 80 dB of signal path
attenuation at low frequencies.
Internally, the gain-control circuit varies the amplifier gain by varying the transconductance, g_m, of a bipolar
transistor using the transistor bias current. Varying the bias currents of differential stages varies g_m to control the
voltage gain of the VCA810. A g_m-based gain adjust normally suffers poor thermal stability. The VCA810
includes circuitry to minimize this effect.
8.2 Functional Block Diagram
1
V+
8
V-
2
+5V
6
VCA810
Gain
Adjust
+
X1
5 ^VOUT
3
V_C
0 ® -2V
-40dB ® +40dB Gain
7
-5V
8.3 Feature Description
8.3.1 Input and Output Range
The VCA810’s 80 dB gain range allows the user to handle an exceptionally wide range of input signal levels. If
the input and output voltage range specifications are exceeded, however, signal distortion and amplifier overdrive
will occur. Figure 11 shows the maximum input and output voltage range. This chart plots input and output
voltages versus gain in dB.
16
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