Log amplifierFrom Wikipedia, the free encyclopedia

A log amplifier (logarithmic converter) is one 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.

Contents

  [hide] 

1 Basic op-amp diode circuit

2 Transdiode configuration

3 See also

4 External links

[edit]Basic op-amp diode circuit

Basic op-amp diode log converter

The relationship between the input voltage vin and the output voltage vout is given by:

where IS is the saturation current and VT is the thermal voltage.

[edit]Transdiode configuration

A transdiode configuration with a BJT connected in the negative feedback loop.

A necessary condition for successful operation of a log amplifier is that the input voltage, Vin is

always positive. This may be ensured by using a rectifier and filter to condition the input signal

before applying to the log amp input. As Vin is positive, Vout is obliged to be negative (since the op

amp is in the inverting configuration) and is large enough to forward bias the emitter-base junction

of the BJT keeping it in the active mode of operation. Now,

where   is the saturation current of the emitter-base diode and   is

the thermal voltage. Due to the virtual ground at the op amp differential input,

, and

The output voltage is expressed as the natural log of the input voltage.

Both the saturation current   and the thermal voltage   are

temperature dependent, hence, temperature compensating circuits may be

required.