# Gas station without pumps

## 2013 February 27

### Twenty-first day of circuits class

Filed under: Circuits course — gasstationwithoutpumps @ 20:04
Tags: , , , ,

This post has not been appearing on WordPress.com, so I’m going to try doing binary search on it to find the problem.

I started today’s class with questions about this week’s lab—students had fewer than I expected, so they are either very confident about their designs or so lost they don’t know what to ask. I guess we’ll find out in the lab tomorrow.

I showed them the tiny medical-grade pressure sensor that I had considered using, but ultimately rejected as too fragile, switching to a sturdier sensor with an easier-to-assemble breakout board. I also showed them my assembled prototyping board and talked about twisted-pair cables to minimize inductive pickup, keeping wires short on the board, using color-coded wires for the different types of signals (red for +5v, black for gnd, some other color for Vref, yet another color for signal, … ).

Instrumentation amplifier protoboard with circuit wired for the pressure sensor lab (top left connector to pressure sensor, bottom center connector to Arduino). Red is +5, black is GND, brown is Vref, yellow is low-gain, orange is high gain, and green is the feedback for the second-stage op amp.

I also covered one other topic relevant to instrumentation amps: common-mode rejection ratio.

After talking about tomorrow’s lab, I started on material for next week’s lab:

• AC power computation. I worry that I lost a few people, because I had to give them the $\cos(x)\cos(y) = 0.5(\cos(x+y) + \cos(x-y))$ trig identity and explain complex conjugation. The bottom line (that real power is $Re(\bar{V}\bar{I}^*$) should be useful in Monday’s class when we start looking at how much power is delivered to the loudspeaker at different frequencies with and without an LC filter.
• More complicated loudspeaker model. I did decide to switch from the linear model I had been using to a model with a frequency-dependent inductor, and I updated and released the handout for the power-amp lab today.

We ended class almost precisely where I expected us to, so for once my timing estimates were good.

On Friday we’ll have the quiz—the students wanted the TA to take it before they did, so that they could be assured that it was not as oversized and difficult as the first quiz, but he does not have any time before tomorrow evening, which would be the latest that I could incorporate any feedback. So we’re just going to have to rely on my estimates of how difficult the quiz is (fairly hard, but not as hard as the first one—I’m hoping for a median of 50% this time).

On Monday, I’ll want to talk about

• how class-D amplifiers work. I’m sure it is very strange for a circuits class to cover class-D amplifiers, without ever having covered classes A, B, AB, and C, but that’s the way it worked out for us.
• LC high-pass filters before the loudspeaker.
• Zobel networks for compensating loudspeakers to get resistive behavior.

If it looks like I’ll be running short of time, I’ll cut some of the Zobel network stuff, since I already decided that they would not add lossy compensation networks in their power amplifiers (I didn’t want to buy 10W resistors). Most of them will do LC filters, though, so I’d better cover that carefully, and of course the whole thing about how class-D amplifiers works is rather tricky, so Monday’s lecture will be pretty full.