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