Log amplifier
A log amplifier is an amplifier for which the output voltage Vout is K times the natural log of the input voltage Vin. This can be expressed as,
where Vref is the normalization constant in volts and K is the scale factor.
The logarithm amplifier gives an output voltage which is proportional to the logarithm of applied input voltage To design a logarithm amplifier circuit, high performance op-amps like LM1458, LM771, LM714 are commonly used and a compensated logarithm amplifier may include more than one.
Logarithmic amplifiers are used in many ways, such as:
The reverse saturation current for the diode changes with temperature. In fact, it doubles for every ten degree Celsius rise in temperature. Similarly the emitter saturation current varies significantly from one transistor to another and also with temperature. Hence, it is very difficult to set the reference voltage for the circuit.
The relationship between the input voltage and the output voltage is given by:
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