Question

The circuit shown in the figure can be used as a ________.

• A. Full wave rectifier
• B. Voltage to frequency converter
• C. Logarithmic amplifier
• D. Frequency to voltage converter

Hint:

Diode in op-amp feedback = log amplifier. Output voltage is proportional to log(input current).

Answer 1

The Correct Option is C

Step 1: Analyze the circuit — it contains an op-amp, a diode in the feedback loop, and a resistor at the input.
Step 2: In such configurations:
The op-amp operates in inverting mode.
The diode’s exponential \( I \text{-} V \) characteristic is used to generate a logarithmic output.
Step 3: Using the diode equation: \[ I = I_s \left(e^{\frac{V_D}{nV_T}} - 1\right) \Rightarrow V_D = nV_T \ln\left(\frac{I}{I_s} + 1\right) \] Step 4: Since input voltage causes a current through the resistor, the voltage across the diode (which is the output voltage of the op-amp) is proportional to the logarithm of input voltage.
Therefore, the circuit behaves like a logarithmic amplifier.