linear integrated circuits

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log and antilog amplifiers

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Logarthmic and exponential amplifiers using OP-AMP : 

Logarthmic and exponential amplifiers using OP-AMP Batch 11

Ideal Op-Amp : 

Ideal Op-Amp Op-amp equivalent circuit. The two inputs are 1 and  2. A differential voltage between them causes current flow through the differential resistance Rd. The differential voltage is multiplied by A, the gain of the op amp, to generate the output-voltage source. Any current flowing to the output terminal vo must pass through the output resistance Ro.

Uses of log & antilog amplifiers: : 

Multiply variable Divide variable Raise variable to a power Compress large dynamic range into small ones To have direct db display on digital voltmeter Uses of log & antilog amplifiers:

Logarthmic amplifiers : 

Logarthmic amplifiers

Slide 5: 

and IC >> IS

Slide 6: 

a grounded base transistor in feedback path collector at virtual ground & base also grounded, so transistor voltage current relationship is that of diode Where k=boltzmann constant T=absolute temperature This is the emitter saturation current

Slide 7: 

Taking natural log on both sides, we get Also from figure, ,

Slide 8: 

Where the output voltage is thus proportional to the logarithm of input voltage. Circuit gives only natural log(ln), we can fing log10 by proper scaling,

Slide 9: 

Problem in circuit- emitter saturation current caries from transistor to transistor with temperature Thus , stable Vref cannot be obtained

Log-amp with saturation current and temperature compensation : 

Log-amp with saturation current and temperature compensation

Slide 11: 

Input to one log-amp ,a reference voltage to another log-amp Two transistors in same silicon wafer-provides a close match of saturation current- ensures good thermal tracking

Slide 12: 

subtractor Non inverting amp

Slide 13: 

Reference level now set with single external voltage source but still depend on temperature T This is compensated by last op- amp stage- produces a non inverting gain of Now the output voltage is, RTC is a temperature – sensitive resistance with a positive coefficient of temperature ( sensitor) so that the slope of the equation becomes constant as the temperature changes

Log-amp using two op-amps : 

Log-amp using two op-amps

Antilog amplifier : 

Antilog amplifier

Slide 16: 

Input Vi for antilog amp is fed into temperature compensating voltage divider R2 and RTC and then to base of Q2 Output Vo is fed back to the inverting input of A1 through R1

Slide 17: 

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

Slide 20: 

Hence an increase of input by one volt causes the output to decrese by a decade. The 755 log / antilog amplifier IC chip is available as functional mode – require some external components to be connected to it

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