Zero Gain Amps


"Why would you want zero gain?".

The reason is to help keep things constant as the voltage supply varies and to enable high yields in spite of batch-to-batch differences in IC manufacturing. Inside most ICs there are bias circuits that use zero-gain circuits to set a bias current that is stable with different voltage supplies.

The best source of information I found on this is from A. Pail Brokaw where he writes in Analog Circuit Design, First Edition : Art, Science and Personalities (EDN Series for Design Engineers)
(Edited by non other than Jim Williams).

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Zero Gain Amp

The two key sentences: 1 - "R will equal Re and the voltage drop across it will be about 26mV at the zero gain point."

This value comes from Thermal voltage Vt = kT/q ≈ 26 mv Where k = Boltzmann's Constant, q = Electronic Charge, and T = temp in K

2- "... the voltage appearing at the collector of Q1 is equal to the Vbe of Q when it is operating at the nominal value of I1 minus kT/q."

kT/q varies with temperature, but in my case it doesn't matter because both samples go through the same diode - thus the same temperature.

The Vec should be quite constant in my application even with slight current changes and the circuit is quit simple - changing a diode to a transistor of the same cost and adding a resistor.

I'm getting about 50 ohms for a 2N3904 with current at 1mA

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