Logarithmic Amplifier

 
  • AMPLIFIER
  • LOGARITHMIC
  • VOLTAGE SOURCE
  • BJT
  • OPAMP
  • DIODE
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Overview

The basic device which is responsible for helping us use an op amp to obtain a logarithm or antilogarithm amplifier is a diode, semiconductor diode. It is a known fact that base emitter voltage for example VBE of a silicon transistor is logarithmically related to the collector current. The inverting input terminal is at virtual ground as the other terminal is connected to ground. Thus the input current Vs/R is same as the current flowing through the diode (Is). We know that
Is=?Io(e?^(Vf/?kT)-1)
If you consider the exponent term, compared to 1 it is much larger and so it can be approximated to
Is=?Ioe?^(Vf/?kT)
Simplifying further we will get
Vo=?(kT/e)ln?(Vs/IoR)
From this it is also found that for each decade increase in diode current (equal to input current) results in an increase of the VBE by 60 mV

Circuit info

Author

Created on: 07 Nov 2013
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