VCA810_12 Datasheet, PDF (14/30 Pages) Texas Instruments – High Gain Adjust Range, Wideband, VARIABLE GAIN AMPLIFIER

VCA810
SBOS275F – JUNE 2003 – REVISED DECEMBER 2010
LOW-DRIFT WIDEBAND LOG AMP
The VCA810 can be used to provide a 2.5MHz
(–3dB) log amp with low offset voltage and low gain
drift. The exponential gain-control characteristic of the
VCA810 permits simple generation of a
temperature-compensated logarithmic response.
Enclosing the exponential function in an op-amp
feedback path inverts this function, producing the log
response. Figure 35 shows the practical
implementation of this technique. A dc reference
voltage, V_R, sets the VCA810 inverting input voltage.
This configuration makes the amplifier output voltage
V_OA
=
−GV_R,
where
G
=
10^{-2 (VC}
+
1)
.
V_R
-10mV
V_OA = -GV_R
VCA810
V_C
R₁
470W
( ) ^VOL ^{= -}
1 + ^R1
R₂
1 + 0.5 Log(-V_IN/V_R)
R₂
330W
R₃
V_OL
OPA820
100W
V_IN
C_C
50pF
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produces log-ratio operation. Either way, the log
term’s argument constrains the polarities of V_R and
V_IN. These two voltages must be of opposite polarities
to ensure a positive argument. This polarity
combination results when V_R connects to the
inverting input of the VCA810. Alternately, switching
V_R to the amplifier noninverting input removes the
minus sign of the log term argument. Then, both
voltages must be of the same polarity in order to
produce a positive argument. In either case, the
positive polarity requirement of the argument restricts
V_IN to a unipolar range. Figure 36 illustrates these
constraints.
5
4
3
2
1
0
I
-1
-2
-3
-4
-5
0.001
0.01
II
0.1
1
V_IN/V_R Voltage Ratio
III
10
100
Figure 35. Temperature-Compensated Log
Response
A second input voltage also influences V_OA through
control of gain G. The feedback op amp forces V_OA to
equal the input voltage V_IN connected at the op amp
inverting input. Any difference between these two
signals drops across R₃, producing a feedback
current that charges C_C. The resulting change in V_OL
adjusts the gain of the VCA810 to change V_OA.
At equilibrium:
V_OA = V_IN = -V_R · 10^{-2(VC + 1)}
(1)
The op amp forces this equality by supplying the gain
control voltage, ^{VC =}
R₁ · V_OL
R₁ + R_{2 .}
Combining the last two expressions and solving for
V_OL yields the circuit’s logarithmic response:
( ( ^VOL ^{= -}
1
+
R₂
R₁
·
1 + 0.5·log
-
V_IN
V_R
(2)
An examination of this result illustrates several circuit
characteristics. First, the argument of the log term,
−V_IN/V_R, reveals an option and a constraint. In
Figure 35, V_R represents a dc reference voltage.
Optionally, making this voltage a second signal
Figure 36. Test Result for LOG Amp for V_R =
−100mV
The above V_OL expression reflects a circuit gain
introduced by the presence of R₁ and R₂. This feature
adds a convenient scaling control to the circuit.
However, a practical matter sets a minimum level for
this gain. The voltage divider formed by R₁ and R₂
attenuates the voltage supplied to the V_C terminal by
the op amp. This attenuation must be great enough to
prevent any possibility of an overload voltage at the
V_C terminal. Such an overload saturates the VCA810
gain-control circuitry, reducing the amplifier’s gain.
For the feedback connection of Figure 35, this
overload condition permits a circuit latch. To prevent
this, choose R₁ and R₂ to ensure that the op amp
cannot possibly deliver a more negative input than
−2.5V to the V_C terminal.
Figure 36 exhibits three zones of operation described
below:
Zone I: V_C > 0V. The VCA810 is operating in full
attenuation (−80dB). The noninverting input of the
OPA820 will see ∼0V. V_OL is going to be the
integration of the input signal.
Zone II: −2V < V_C < 0V. The VCA810 is in its normal
operating mode, creating the log relationship in
Equation 2.
14
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