Simple Logarithmic Amplifier Circuit

This logarithm amplifier circuit provides an output voltage that is equivalent to the logarithm of applied input voltage.

The performance of the logarithmic operational amplifier, the circuit diagram of which is shown in figure 1, is best seen from its input/output characteristic shown in figure 2.

For small input voltages, the amplification is high; when the input voltage rises, the amplification drops off and finally remains almost static for further increases in input voltage.

Some applications of a logarithmic amplifier are: driving a graphic 4 recorder in weather stations, and in remote control systems (for instance, to avoid a too sudden and strong deflection of a servo arm).

When used in conjunction with other equipment, the logarithmic amplifier is very flexible: analogue instruments as well as a row of LEDs can be connected to its output.

How the Circuit Works

Operational amplifiers A1 and A2 form a non-inverting pre-amplifier.

As the input signal of A3 should not under any circumstances be- come negative, the input level of the circuit can be shifted with potentiometer P1 as required.

At the same time, this stage works as a high- impedance input buffer for A3.

As shown, the amplifier accepts inputs up to 8 V. If a higher value ‘ is required, the amplification factors of A1 and A2 can be suitably modified.

The 'logarithmic’ part of the circuit 1 consists of A3 and transistor array IC2: the voltage at pins 4 and 5 of the array is related logarithmically with the output signal of A2.

The output stage of the logarithmic amplifier circuit consists of amplifier A4 which amplifies the inverted signal from A3.

As the amplification factor of this stage can be altered by means of preset potentiometer P2, the output of the circuit can be matched to the load.

To preset P2, connect a multimeter to the output of the circuit and a signal at maximum level to the input: adjust P2 to the required output voltage.