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Question Bank (2 marks) · Q15

Q.Draw the circuit diagram of a Logarithmic amplifier.

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[!TLDR]

A diode in the feedback path of an inverting op-amp makes Vo∝−ln⁡VinV_o \propto -\ln V_{in}.

The diode's current–voltage law is I=Is(eV/ηVT−1)I = I_s\left(e^{V/\eta V_T} - 1\right). At the virtual-ground node the input current Vin/RV_{in}/R equals the diode current, so Vin/R=Ise−Vo/ηVTV_{in}/R = I_s e^{-V_o/\eta V_T}. Solving for VoV_o gives an output proportional to the natural logarithm of the input, Vo=−ηVTln⁡ ⁣(VinRIs)V_o = -\eta V_T \ln\!\left(\dfrac{V_{in}}{R I_s}\right). Such amplifiers are used …

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