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Log and Antilog Amplifier


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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