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Exponential Current Generator

IP.com Disclosure Number: IPCOM000080336D
Original Publication Date: 1973-Dec-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 45K

IBM

Related People

Henry, DL: AUTHOR

Abstract

This circuit provides a current which is exponentially increasing or exponentially decreasing, depending on the input gate level.

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Exponential Current Generator

This circuit provides a current which is exponentially increasing or exponentially decreasing, depending on the input gate level.

Assume Q4 is off, and capacitor C has a small positive charge (set by an additional clamp circuit, not shown). Charging current into C is the collector current of Q2 which is equal to the collector current of Q1, assuming matched devices. R2 is large and current into it is neglected. Operational amplifier Q5 through Q10 forces the voltage across R1 to be equal to the voltage across C: V(R1) = V(C) Also: V(R1) = I(R1)R1 = I(CQ1)R1 = I(CQ2)R1 = I(c)R1 = V(c). From this it can be shown that: V(c)(t) = ae/t over RIC/.

Thus, the capacitor voltage is exponentially increasing with time "t". R1C is selected for the desired exponential response. "a" is a constant determined by the minimum charge held on C and is set by the additional clamp circuit, not shown. "e" is the natural log base. With Q3 matched to Q2 and Q1, I out = I(c) = V(c) (t) over RI.

I out is thus an exponentially increasing current. R1 may be chosen to set the current at a specific time, such as t = 0.

Assuming C has been charging, and Q4 is now turned on to saturation, the collector current of Q2 will be bypassed through Q4 and diode D1 will become reverse biased. C will now discharge in an exponential decay through R2. R2 is large so that R2C is large in comparison with R1C. The operational amplifier will force the voltage across R1 to follow the...