Antilog Amplifier Circuit & Applications

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.

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Antilog amplifier using diode and op-amp

Fig. 1 Antilog amplifier circuit using diode and op-amp

Diode, resistor and op-amp used in the antilog amplifier as shown in figure 1. The input V_iis 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) $\begin{equation*} I=\frac{0-V_{o}}{R}=-\frac{V_{o}}{R} \end{equation*}$

and diode current $I_{d}$

(2) $\begin{equation*} I_{d}=I_{o}e^{\frac{V_{d}}{\eta V_{T}}} \end{equation*}$

where $V_d$

= forward bias voltage across diode D

Apply KCL at node P

$I_{d} = 0 + I$

(3) $\begin{equation*} I = I_{d} \end{equation*}$

and

$V_{d} = V_{i}-0$

$V_{d} = V_{i}$

Therefore, equation (2) becomes

(4) $\begin{equation*} I_{d}=I_{o}e^{\frac{V_{i}}{\eta V_{T}}} \end{equation*}$

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

$-\frac{V_{o}}{R}=I_{o}e^{\frac{V_{i}}{\eta V_{T}}}$

Therefore, we have

$\boxed{V_{o}=-I_{o}Re^{\frac{V_{i}}{\eta V_{T}}}}$

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

Antilog amplifier circuit using transistor and op amp — Fig. 3 Antilog amplifier circuit using transistor and op-amp

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_iis 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.