Gas station without pumps

2017 January 2

LM2903 open-collector comparator characterization

Filed under: Circuits course — gasstationwithoutpumps @ 18:02
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In Last power-amp lecture, I posted an I-vs-V plot for the LM2903 comparator’s open-collector output, which I had made sometime in 2013, I think:

There are two regions of operation for the open-collector output of the LM2903. In the saturation region, the current goes up slowly with voltage (as about V^0.15, while in the "linear" region, it goes up as about V^1.5). The transition occurs when VOL is about 0.25 V, so we are almost always in the saturation region.

There are two regions of operation for the open-collector output of the LM2903. In the saturation region, the current goes up slowly with voltage (as about V^0.15, while in the “linear” region, it goes up as about V^1.5). The transition occurs when VOL is about 0.25 V, so we are almost always in the saturation region.

I decided to redo the plot using the Analog Discovery~2, as I now include the open-collector curve in the textbook (in an optional section, since we no longer use open-collector comparators). I used a 12V wall-wart and both the function generator and oscilloscope functions. I used the “custom channel” and XY plot features to get the I-vs-V plot on the screen (though I saved the data and replotted with gnuplot, to superimpose different runs). I also averaged 10 sweeps to reduce noise.

R1 was 56Ω for testing high voltages and currents, and R1 was 2.2kΩ for testing low voltages and low currents.

R1 was 56Ω for testing high voltages and currents, and R1 was 2.2kΩ for testing low voltages and low currents.

The triangle-wave generator and the nFET makes a variable load for the comparator, from slightly more than R1 up to about 1MΩ.

Even up to 11V, the LM2903 collector stays below the 20mA maximum current, but I'd want to make sure that there was some current-limiting resistor for any power-supply voltage above 12V.

Even up to 11V, the LM2903 collector stays below the 20mA maximum current, but I’d want to make sure that there was some current-limiting resistor for any power-supply voltage above 12V.

The results with the Analog Discovery 2 are much cleaner than my old results, which were most likely done with an Arduino, which has a very low resolution ADC.

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