Basic Log Amplifier Using Diode

Basic Log Amplifier Using Diode

Basic Log Amplifier Using Diode circuit diagram of basic log amplifier using a diode is shown in the Fig. 2.69.

Basic Log Amplifier Using Diode

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

Basic Log Amplifier Using Diode

As the op-amp input current is zero,

Basic Log Amplifier Using Diode

Now If is the current through diode and voltage across diode is

Basic Log Amplifier Using Diode

Hence using equation (10) we can write,

Basic Log Amplifier Using Diode

As I R is constant d.c.voltage, let us denote it as Vref

 

Basic Log Amplifier Using Diode

Where             Vref = Io R

Thus the output voltage V° is a function of logarithm of the input voltage Vm The circuit 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,

Basic Log Amplifier Using Diode

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