VCA810_16 Datasheet, PDF (22/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier

VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
Application Information (continued)
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OPA657
20kW
C_F
-V_B
20W
VCA810
20W ^VC
V_C
0
ADC
and DSP
t
-2V
Time-Gain Compensated Control Voltage
Figure 35. Typical Range-Finding Application
The control voltage V_C varies the amplifier gain for a basic signal-processing requirement: compensation for
distance attenuation effects, sometimes called time-gain compensation (TGC). Time-gain compensation
increases the amplifier gain as the signal moves through the air to compensate for signal attenuation. For this
purpose, a ramp signal applied to the VCA810 gain control input linearly increases the dB gain of the VCA810
with time.
9.1.3 Wide-Range AGC Amplifier
The voltage-controlled gain feature of the VCA810 makes this amplifier ideal for precision AGC applications with
control ranges as large as 60 dB. The AGC circuit of Figure 36 adds an operational amplifier and diode for
amplitude detection, a hold capacitor to store the control voltage and resistors R₁ through R₃ that determine
attack and release times. Resistor R₄ and capacitor C_C phase-compensate the AGC feedback loop. The
operational amplifier compares the positive peaks of output V_O with a DC reference voltage, V_R. Whenever a V_O
peak exceeds V_R, the OPA820 output swings positive, forward-biasing the diode and charging the holding
capacitor. This charge drives the capacitor voltage in a positive direction, reducing the amplifier gain. R₃ and the
C_H largely determine the attack time of this AGC correction. Between gain corrections, resistor R₁ charges the
capacitor in a negative direction, increasing the amplifier gain. R₁, R₂, and C_H determine the release time of this
action. Resistor R₂ forms a voltage divider with R₁, limiting the maximum negative voltage developed on C_H. This
limit prevents input overload of the VCA810 gain control circuit.
Figure 37 shows the AGC response for the values shown in Figure 36.
V_IN
2mV to 2V
RSSI 100kHz
Port
VCA810
V_{C R3}
1kW HP5082
R₁
50kW
R₂
50kW
C_H
0.1mF
OPA820
C_C
47pF
V_O
V_{OUT PEAK} = V_R
R₄
100W
V_R
0.1 VDC
V-
Figure 36. 60-dB Input Range AGC
22
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