VCA810_16 Datasheet, PDF (30/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier

VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
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9.2 Typical Application
A common use of the log amplifier above involves signal compounding. The inverse function, signal expanding,
requires an exponential transfer function. The VCA810 produces this latter response directly, as shown in
Figure 47. DC reference V_R again sets the amplifier input voltage, and the input signal V_IN now drives the gain
control point. Resistors R₁ and R₂ attenuate this drive to prevent overloading the gain control input. Setting these
resistors at the same values as in the preceding log amp produces an exponential amplifier with the inverse
function of the log amplifier.
Testing the circuit given in Figure 47 gives the exponential response shown in Figure 48.
V_R
-10mV
+0.5V
V_I
OPA698
V_L
VCA810
V_C
R₂
330W
( _{VOL = -VR x 10}^-2
R₁V_{IN +1}
R₁ + R₂
R₁
470W
500W
V_IN
-3.4V
500W
Figure 47. Exponential Amplifier
9.2.1 Design Requirements
To build a wide dynamic range wide exponential amplifier we need an amplifier with continuous voltage gain
control, gain range over 40 dB, low noise, and high maximum gain. The VCA810 has ±40 dB of gain range, so it
meets this criteria. It also has continuous voltage gain control and can support up to 100 V/V of voltage gain.
9.2.2 Detailed Design Procedure
An exponential amplifier will have a linear response on a logarithmic scale. The linear in dB gain control of the
VCA810 is ideal for this application. Note that the input to this circuit is the gain control pin. Using the gain
control pin as the input is what gives an exponential gain response. The design involves the use of an OPA698
to provide the proper DC bias voltage to the gain control pin on the VCA810. The OPA698 supply voltage was
chosen based on the input voltage requirement of the VCA810. The reference voltage (VR) is used to set the DC
output voltage. The reference voltage cannot be 0 V, but it must be small so that at maximum gain the amplifier
outputs are not saturated. In Figure 47 design the reference voltage is set to –10 mV.
9.2.3 Application Curve
1
0.1
0.01
0.001
+3.0 +2.5 +2.0 +1.5 +1.0 +0.5
0
Input Voltage (V)
Figure 48. Exponential Amplifier Response
30
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