Question

How to provide saturation current and temperature compensation in log-amp?

(a) Applying reference voltage alone to two different log-amps

(b) Applying input and reference voltage to same log-amps

(c) Applying input and reference voltage to separate log-amps

(d) None of the mentioned

The question was posed to me in an online interview.

I would like to ask this question from Log and Antilog Amplifier in section Operational Amplifier Applications of Linear Integrated Circuits

Answer 1

The correct answer is:

(b) Applying input and reference voltage to the same log-amp.

Explanation:

In a logarithmic amplifier (log-amp), the saturation current and temperature compensation are essential for ensuring the stability of the circuit's output with variations in temperature. The saturation current ISI_SIS​ in a diode or transistor varies significantly with temperature, which can affect the logarithmic relationship.

To mitigate these effects:

1. Compensating input and reference voltage in the same log-amp ensures that the temperature-dependent variations in ISI_SIS​ and thermal voltage VTV_TVT​ cancel out because both input and reference signals are processed through the same temperature-sensitive elements.
2. This design uses the differential behavior of the logarithmic function, making the compensation effective and maintaining accurate logarithmic output.