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# Amplitude Measurement Circuits

IP.com Disclosure Number: IPCOM000082696D
Original Publication Date: 1975-Jan-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 43K

IBM

## Related People

Allen, JW: AUTHOR

## Abstract

An amplitude measurement circuit which is insensitive to power supply variations and nearly independent of component tolerances is shown. Transistor Q3 along with resistor R3 forms a current source, the magnitude of the current, i1, being determined by R2, Q4 and R3. Similarly, transistor Q7 and resistor R5 form a second current source with R4, Q8 and R5 determining the magnitude of current i2. Normally i2 is designed to be less than i1.

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Amplitude Measurement Circuits

An amplitude measurement circuit which is insensitive to power supply variations and nearly independent of component tolerances is shown. Transistor Q3 along with resistor R3 forms a current source, the magnitude of the current, i1, being determined by R2, Q4 and R3. Similarly, transistor Q7 and resistor R5 form a second current source with R4, Q8 and R5 determining the magnitude of current i2. Normally i2 is designed to be less than i1.

Transistors Q1 and Q2 are connected together to form a simple comparator. When the voltage at A is greater than that at B, transistor Q2 is cutoff and current i1 flows through transistor Q1. The collector current of Q1, by flowing through diode Q5 and resistor R1, creates a voltage at the base of transistor Q6 which causes a current nearly equal to i1 to flow out of the collector of Q6. When conducting, Q6 fulfills the current requirements of current source i2 and the remaining current, ic=i1-i2, flows into the capacitor C charging it positive.

When the voltage at B is more positive than that at A, current i1 flows from Vc through transistor Q2. Transistor Q1 is cutoff; thus, no current flows from transistor Q6 and the capacitor C is discharged by current source i2, ic=-i2. Transistors Q9 and Q10 form a buffer.

By adjusting the magnitudes of current sources i1 and i2, the circuit of Fig. 1 can be used to measure either the positive or the negative peak amplitude of the AC input at point A.