# Gas station without pumps

## 2013 February 25

### Twentieth day of circuits class

Filed under: Circuits course — gasstationwithoutpumps @ 17:41
Tags: , , , ,

We started today with a do-now problem, asking what the following circuit does:

After 5 minutes, no one had any idea, so I asked questions until we got that In had a voltage of 0V, that the interesting property of In was its current, and that the same current flowed through R1. Then I gave them another 5 minutes. At that point, one or two students had an inkling that the circuit was a current-to-voltage converter, and one student even suggested that $V_{out} = I_{in} R_{1}$, which was almost right. I had draw the current into the circuit from In, so the output voltage was the negative of that. I walked them through the analysis that leads to that conclusion.

Then one student asked the obvious question—the one I was hoping a student would ask—how this differed from just using a pullup resistor as we had done for the electret mic.  So I tried to get them to compute the input impedance $V_{in}/I_{in}$.  After a great deal of difficulty, they came up with 0Ω.  We then compared $V_{in}$ for the op-amp current-to-voltage converter and a pull-down or pull-up resistor.  I think that they see the difference between the op-amp circuit and the single resistor, but I’m not sure they understand when to use one and when to use the other.

I’ll probably see some op-amp current-to-voltage converters in next week’s audio amp lab, though they really aren’t needed with an electret mic (and they’ll probably get the bias voltage wrong, since the FET in the mic has to be kept in saturation, so needs to be at 1v or more). Hmm, with the proper biasing, the op-amp current-to-voltage converter could be used to get a fairly good gain from the mic without needing a DC-blocking capacitor to do level shifting, so maybe it isn’t such a bad idea to try use it.  I’ll try that out in the preamp for the class-D amplifier and see how well it works.

I then returned the lab reports and ranted about how important it was to get schematics exactly right, and how they’d keep redoing the lab reports until they got them right. I also lectured them about using “would” incorrectly (though only a couple of students are still doing that), about doing factoid dumps without connecting the information to the design problem they are addressing (again, just a few students still doing that), and about citations not being sufficient for copied figures or text—there must be explicit “Figure copied from … ” figure credits and explicit quotation marks or block quotes.  Lots of students are still omitting the figure credits.

I also talked about the value of having chunks of the schematic match the blocks of a block diagram, not mixing different blocks randomly, using the tinkering lab schematics (where the hysteresis oscillator board was a block of the block diagram) as an example.

Finally, I talked about the importance of thinking of voltages as being between two points: an FET does not know where “GND” is, and is controlled by VGS, not Vgate. I showed how putting a pFET between a loudspeaker and ground rather than between the positive power rail and the loudspeaker would result in the source voltage being just about the negative of the threshold voltage (so around 2V with our pFETs), and the pFET dissipating a lot of power.

I then took questions about this week’s lab. The first question to come up was about how to choose the Rgain resistor, which I used both as a chance to talk about the instrumentation amp and about gain in general.  I introduced them to gain-bandwidth product, and explained why one might want to use a multi-stage amplifier to get high gain with high bandwidth.  The amplifiers we use are about 1MHz gain-bandwidth product, so we could go up to 100kHz with a gain of 10, but only 1kHz with a gain of 1000.  Of course, for the pressure-sensor lab, we don’t need high frequencies so the gain-bandwidth product is not a limit in this lab.

The students then talked me through a block diagram for the lab, and we discussed (without coming to any conclusion) whether a low-pass filter was needed/useful and whether a second stage amplifier was needed/useful.  I did point out that they had two free screw terminals on the board for talking to the Arduino, so if they wanted to have two different gains or filtered and unfiltered outputs, they could.  There was a little discussion of what corner frequency was appropriate for a low-pass filter, and with a little nudging, I got them to remember last week’s sampling and aliasing lab.

We ran out of time before getting to material I had queued up in case things went faster than I expected:

• common-mode rejection ratio
• looking at the predictions for the tinkering lab as a group
• AC power computation
• microphone current-to-voltage conversion (choosing bias voltage)
• more complicated loudspeaker model
• Zobel networks for compensating loudspeakers

It’s actually a good thing that we didn’t get to the more complicated loudspeaker model, because I was playing around with a non-linear model (the main inductance varying as a power of frequency) and it looks like a better fit with fewer parameters than the linear model I had been using.  I’m considering rewriting the class-D amplifier handout to use that model instead.  Unfortunately, the LC filter design looks like we’ll have more pass-band ripple than I had previously thought, though still not too bad.