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,

Substituting

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,