Spectral Qualities of Active and Inverse Mode

This test design was aimed to compare the spectral qualities of an amplifier that uses high-gain bipolar junction transistors in both the forward and inverse active modes (where emitter and collector are swapped). The designs themselves came from a designer specializing in guitar pedals. The designs were interesting to me because they used one of my favorite high-gain transistors, the 2N5088. This device can have a β of 600 under some ranges of collector current.

The inverse mode of a BJT is often used to provide switches with very low offset voltage for Vce when saturated. So, the application for this mode is for digital to analog data converters, gain control switching, and and for auto-zero or chopper operational amplifiers.

It was a bit shocking that there was only about 1.7 dB difference in gain when using the transistor normally, or "upside down." Most interesting was the nature of the spectral output between the two modes, given the same level of signal drive. The inverse mode circuit had a subtle, but nonetheless different spectral response.

The engineering notebook pages and select scope fotos follow.

Schematics of both circuits and characterization notes

1 kHz I/O signals for transistor active mode gain of +25.3 dB

1 kHz I/O signals for inverse mode gain of 23.6 dB

DC-20 kHz spectrum for 420mV drive into active mode circuit

DC-20 kHz spectrum for inverse mode circuit with fundamental amplitude matching the active mode circuit. Shows less 3rd order harmonic, and a different harmonic evolution.