More on that inverter

I decided to try moving the ADC, to look at the voltage next to the base of the transistor, rather than next to the collector:

If I choose R1 as 10k and R2 as 1M, and also include the readings from the collector (see the circuit diagram in the previous post), then I get the following graph:

This is all starting to make sense – between my experiments and what I’ve read, I’m starting to get a feeling for what’s going on. Between the base and the emitter, the transistor acts like a diode – a bit like the LED I plotted a graph for – except that the current flowing base-to-emitter also controls the current flowing collector-to-emitter.

Now the challenge is voltage amplification. I can make the ADC voltage go down steeply when the DAC voltage goes up. But what’s the simplest way to make the ADC voltage go up steeply when the DAC voltage goes up. Given that the voltage at the base is going to be higher than the voltage at the emitter, we’re going to have to think about the voltage at the collector. Possibly this entails another transistor, or maybe some clever tricks with resistors. Or I could even stray a bit from the beaten path and try to understand PNP resistors. Absolutely doubtless there’s a nice canned solution somewhere waiting for me to just look up, but figuring it out is more fun.