log amplifier
TRANSCRIPT
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.