Gas station without pumps

2015 May 28

Power amps working

Filed under: Circuits course — gasstationwithoutpumps @ 21:34
Tags: , , , ,

The power-amp lab went quite smoothly today—just about everybody got a class-D power amp working and a couple of groups wired their sup to a headphone plug and played music from their cell phones.  They were impressed at how loud the amplifiers were (though they were only producing about 4W of music, and their speakers could go to 10W or 15W, depending which model they got).

The biggest problems I saw in helping students debug was careless wiring—often because students had not bothered to make complete schematics including pin numbers and every power connection.  Connecting power up incorrectly was common.  Another common problem for students who did multi-sheet schematics (not recommended for such a small circuit), was having discrepancies at the boundaries between the sheets (like leaving out the high-pass filter needed for recentering the output of the pre-amplifier at the same voltage as the triangle wave).  A number of students got confused between the lowest power rail and ground (especially since the LM2903 data sheet calls the low power voltage to it GND, even though in the student schematics it was connected to the lowest power rail.

Despite my rather awkward lectures on the class-D power-amp, I think that the building and testing went smoother this year—perhaps because I tried having them do a crude-approximation-and-test approach to sizing the pull-up resistors.  I think that few groups ended up using the resistor sizes they started with.  Most had to make the nFET gate’s pull-up larger and the pFET gate’s pull-up smaller, to make the transistors turn off fast enough.  I’ll have to see if I can come up with some design guidance that will make the initial estimates closer, without complicating the design process.

I have to correct something I said yesterday in Last power-amp lecture—I did include a current-vs-voltage graph for the LM2903 comparator!  I need to rewrite the prelab to have them use that figure to compute their open-collector pull-up resistors: choose a desired low output voltage, figure out the current at that voltage, then use the voltage drop across the pull-up resistor to size the resistor.

I tried using the digital scope in lab today to get Miller plateau pictures without slowing down the transitions, and I recorded a few with the lower power rail at –6V, the upper power rail at +6V and a 330Ω pull-up resistor to 0V on the open-collector output:

Rising edge for the gate voltage with a 330Ω pullup to 0V from –6v.

Rising edge for the gate voltage with a 330Ω pullup to 0V from –6v.

Falling edge for the gate voltage with no load on the FET, from 0v down to –6v.

Falling edge for the gate voltage with no load on the FET, from 0v down to –6v.

With an 8Ω loudspeaker as a load, turning off the nFET (gate voltage in blue) causes a large inductive spike on the drain (yellow).

With an 8Ω loudspeaker as a load, turning off the nFET (gate voltage in blue) causes a large inductive spike on the drain (yellow).

Here is a detail of turning off the nFET, showing both the Miller plateau on the gate voltage (blue) and a large inductive spike on the drain (yellow).

Here is a detail of turning off the nFET, showing both the Miller plateau on the gate voltage (blue) and a large inductive spike on the drain (yellow).

The on-transition of the nFET shows the Miller plateau clearly in the gate voltage (blue), with a smooth transition in the drain voltage (yellow).

The on-transition of the nFET shows the Miller plateau clearly in the gate voltage (blue), with a smooth transition in the drain voltage (yellow).

Unfortunately, these tiny little images were all the scope recorded, and they are too low quality as images to put in the book.  I’ll have to do it again sometime, with the scope downloading the data for me to plot properly.  Unfortunately, the download format is a 3.8Mbyte CSV file, which takes a long time for the scope to download to a flash drive (slow USB 1 speeds, I fear). I did not have the patience to do that today, together with writing scripts to ignore the meta data and plot just the real data.  I saved one file, which I’ll use for script writing, and some time later go back and record the transitions again.

 

2 Comments »

  1. […] the waveforms: see Last power-amp lecture for  Bitscope recording of slowed-down transitions and Power amps working for Tektronix images of full-speed transitions). (I did a cursory check tonight, and it looks like […]

    Pingback by Last day of class for Spring 2015 | Gas station without pumps — 2015 June 5 @ 23:31 | Reply

  2. […] was rather unsatisfied with this approach, as I really want to show the full-speed transitions. In Power amps working, I showed some Tektronix plots, but their little screen images are terrible (as bad as the Bitscope […]

    Pingback by Bitscope jitter and nFET Miller plateau | Gas station without pumps — 2015 June 8 @ 00:00 | Reply


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