**AntiLog amplifier** or **anti**–**logarithmic amplifier** is an electronic circuit that produces output that is proportional to the anti-logarithm of the applied input. Basically it performs mathematical operation of an anti-logarithm. In this article, we will see the different antilog amplifier circuits, its working and antilog amplifier applications. Basically two circuits are there to perform the anti-logarithmic function. First, using diode and op-amp and second, using BJT and op-amp.

**Contents**show

## Antilog amplifier using diode and op-amp

Diode, resistor and op-amp used in the antilog amplifier as shown in figure 1. The input V_{i }is applied through diode D at the inverting terminal. V_{o} is the output voltage. The non-inverting terminal of the op-amp is connected to the ground. This means that the voltage of the non-inverting terminal is zero volts.

### Analysis

The analysis of the antilog amplifier is shown in figure 2. Since the op-amp is ideal and negative feedback is present, the voltage of the inverting terminal (V_{−}) is equal to the voltage of the non-inverting terminal (V_{+} = 0V), according to the **virtual short concept**.

V_{− }= V_{+} = 0V

The currents entering both terminals of the op-amp are zero since the op-amp is ideal.

Let current I flows through the resistor R.

(1)

and diode current

,(2)

where

= forward bias voltage across diode DApply KCL at node **P**

(3)

and

Therefore, equation (2) becomes

(4)

From equation (1), (3) and (4), we have

Therefore, we have

**Note:** The negative sign in the output signifies that there is a 180° phase difference between output and the applied input.

## Antilog amplifier using diode and transistor

Another circuit comprises of a BJT (PNP) T, resistor (R) and op-amp used as the antilog amplifier as shown in figure 3. The input V_{i }is applied through transistor T at the inverting terminal. V_{o} is the output voltage. The non-inverting terminal of the op-amp is connected to the ground. This means that the voltage of the non-inverting terminal is zero volts.

### Analysis

The analysis of the antilog amplifier is shown in figure 4. Since the op-amp is ideal and negative feedback is present, the voltage of the inverting terminal (V_{−}) is equal to the voltage of the non-inverting terminal (V_{+} = 0V), according to the **virtual short concept**.

V_{− }= V_{+} = 0V

The currents entering both terminals of the op-amp are zero since the op-amp is ideal.

Let current I flows through the resistor R.

(5)

and collector current

,(6)

where

and I_{s} = Reverse saturation current of emitter-base junction

As we know that

since I_{B} ≅ 0 A

Therefore

(7)

Apply KCL at node **P**

(8)

and

Therefore, equation (6) becomes

(9)

From equation (5), (7), (8) and (9), we have

Therefore, we have

**Note:** The negative sign in the output signifies that there is a 180° phase difference between output and the applied input.

## Antilog Amplifier Applications

1. It is used in analog multiplier circuits.