**Logarithmic Amplifier using Diode:**

The circuit diagram of basic Logarithmic Amplifier using Diode is shown in the Fig. 2.69.

The diode D is used in the negative feedback path. The node A is grounded hence node B is at virtual ground. Hence V_{B} = 0.

As the op-amp input current is zero,

Now I_{f} is the current through diode and voltage across diode is **V _{B} – V_{o} i.e. -V_{o}.**

Hence using equation (10) we can write,

As I_{o }R is constant d.c. voltage, let us denote it as V_{ref.}

Where

- V
_{ref }= I_{o }R

Thus the output voltage V_{o} is a function of logarithm of the input voltage V_{in}. The Basic Log Amplifier using Diode gives the logarithm to base e i.e., natural logarithm. But the same circuit can be used to find out logarithm values to base 10 by the proper scaling as,