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# Log Function Generator

IP.com Disclosure Number: IPCOM000095797D
Original Publication Date: 1964-Jul-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 41K

IBM

## Related People

Olson, RE: AUTHOR

## Abstract

Circuit 10 functions to generate a voltage that is proportional to the logarithm of an analog input voltage. The output voltage approximates the logarithm function with two straight lines. If more accuracy is required, additional branch stages can be added to more closely approximate the logarithm function.

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Log Function Generator

Circuit 10 functions to generate a voltage that is proportional to the logarithm of an analog input voltage. The output voltage approximates the logarithm function with two straight lines. If more accuracy is required, additional branch stages can be added to more closely approximate the logarithm function.

An analog signal is applied to base of transistor T1. The output is taken from the collector of T1 which is collected to a positive voltage through resistor R3. The collector of T1 is connected to the base of transistor T4 which is an emitter follower for driving external circuits.

If the emitter of T1 is within a voltage range such as between plus 1. 5 and 2 volts, T2 is cut off. Current flowing through resistor R3 is then equal to (Vin - V3)/R1 and the voltage at the base of T4 is equal to 12 >(Vin - V3)R3|/R1. This latter equation describes a straight line.

When the input voltage is within another range such as between plus 1 and
1. 5 volts, the emitter of T1 is more negative than the emitter of T2 and T2 conducts. Under this condition, the current flowing through R3 is equal to (Vin - V3)/R1 - ( 1. 5-Vin)/R2. Voltage at the base of T4 will now be equal to 12 - R3 >(Vin - V3)/R1 - (1. 5-Vin)/R2|. This latter equation describes a second straight line. The two straight lines approximate the logarithm function over the range of interest.

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