Log and antilog
amplifiers are used in applications that require compression of analog input
data, linearization of transducers that have exponential outputs, and analog
multiplication and division. A logarithmic (log) amplifier produces an output
that is proportional to the logarithm of the input, and an antilogarithmic
(antilog) amplifier takes the antilog or inverse log of the input.
Logarithmic
Amplifier
When you place a diode in the feedback loop of an opamp circuit, as shown
in Figure below, it acts as a log amplifier. The output is limited to a maximum
value of approximately -0.7V because the diode’s logarithmic characteristic is
restricted to voltages below 0.7 V.
Since the forward
current trough a diode is given by,
As ID = IF,
The term kT/q is
a constant equal to approximately 25 mV at 25°C. Therefore, the output voltage
can be expressed as
From last equation, you
can see that the output voltage is the negative of a logarithmic function of
the input voltage. The value of the output is controlled by the value of the
input voltage and the value of the resistor R. The other factor, Io
is a constant for a given diode.
Anti-Logarithmic
Amplifier
An antilog amplifier is
formed by connecting a diode at the input side. The output voltage is
determined by the current through the feedback resistor.
As ID = IF and Vin = VD
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