Analog Solid State Mutual Conductance Measurement
This project was completed for a class in analog electronics. The broad goal of the project was to design and implement an analog system. In the spirit of analog electronics, my project partner and I attempted to build an analog solid-state mutual conductance measurement system for vacuum tubes.
Fundamentally, the project involved powering the tube, generating an input signal, and measuring the ratio of the output signal to the input signal. My main contribution to the project was the analog measurement of the ratio of the output signal to the input signal. Both signals were AC and had to be run through DC-offset removal and peak detection circuits to create two DC values that could be divided.
Dividing DC values using analog solid-state technology is not a trivial task. In the spirit of the analog electronics class, we shunned purpose-built ICs and built our own division circuit. The common “solution” to this problem is an op-amp division circuit that leverages log properties to divide two values. The circuit is seemingly a very elegant solution.
Practically, the circuit is terribly sensitive to component variation because of its reliance on the exponential characteristic of PN junctions. What ensued was a drawn-out series of attempts to tune the circuit, then to create more reliable approximations of the exponential and logarithmic functions. The most interesting and disappointing attempt at the division circuit was the implementation of a Taylor series approximation using a small pile of LM741 op amps.
The following report is comprehensive and includes all of the schematics and graphs pertinent to both the division circuit and the rest of the project.