Chapter: Linear Integrated Ciruits - Application of Op-Amp

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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.

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.

 

These can be performed continusely with log amps, and also used for direct dB display on a digital Voltmeter and Spectrum analyzer.

 

Log-amp can also be used to compress the dynamic range of a signal.

 

Log Amplifier:

 

The fundamental log amp circuit shown in fig


Fig a. Fundamental log-amp Circuit

 

Where a grounded base transistor is placed in the feedback path. Since the collector is placed in the feedback path. The circuit have one problem.The emitter saturation current Is varies from transistor to transistor and with temperature. Thus a stable reference voltage V ref cannot be obtaine This is eliminated by the circuit given in fig(b) The input is applied to one log-amp, while a reference voltage is applied to one log- amp,while a reference voltage is applied to another log-amp. 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                              IS1=IS2=I

 

Antilog Amplifier

 

The Circuit is shown in fig.The input Vi for the antilog-amp is fed into the temperature compensating voltage divider R2 and RTC and then to the base of Q2 . The output Vo of the antilog- amp is fed back to the inverting input of A1 through the resistor R1. The base to emitter

voltage of transistors Q1 and Q2 can be written as

 

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

or  kT R2 + RTC              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

 






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