Log and Antilog Amplifier

Log and Antilog Amplifier

There are several applications of log and antilog Amplifiers. Antilog Computation may require functions such as ln x, log x or Sinhx.

With log amps, these can be run Continuously. Additionally Utilised for direct dB display on a digital Voltmeter and Spectrum Analyzer. A log-amp can also be used to Compress a Signal’s dynamic range.

Log Amplifier

The Fundamental log amp circuit shown in fig

Fig a. Fundamental log-amp Circuit

where the feedback path contains a grounded base transistor. The collector is situated in the feedback path, therefore. One issue exists with the circuit. The emitter saturation current Is varies with temperature and from transistor to transistor. As a result, V ref cannot be a stable reference voltage. The circuit shown in fig. eliminates this (b) One log-amp receives the input, one receives a reference voltage, and one receives a reference voltage while another log-amp receives the input. The two transistors are integrated close together in the same silicon wafer. This provides a close match of saturation currents and ensures good thermal tracking. Fig(b) Log-amp with saturation current and temperature compensation

Assume                              I_S1=I_S2=I

Antilog Amplifier

In fig., the Circuit is Depicted. The Temperature-compensating voltage divider R2 and RTC receive the input Vi for the Antilog-amp before passing it on to the base of Q2. Through the Resistor R1, the Antilog-output amp’s Vo is fed back into the Inverting input of A1. Transistors Q1 and Q2’s base to emitter voltage can be Expressed as

Since the base of Q₁ is tied to ground, we get

or  kT R₂ + R_TC              V _ref

Changing the natural log i.e., ln to log10 using eqn(6) we get Hence an increase of input by one volt causes the output to decrease by a decade