Gas station without pumps

2013 July 18

Improved rectifier with Schottky diodes

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 22:32
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In the Improved rectifier post, I gave the following circuit for an inverting rectifier and showed traces of its performance using diode-connected S9018 NPN transistors as diodes:

Only one of D1 and D2 can be conducting.

Only one of D1 and D2 can be conducting.

With a constant amplitude triangle wave input (about 2.6v peak-to-peak),  the circuit had some pretty serious glitches:

frequency positive glitch negative glitch
3kHz 40mV 40mv
10kHz 80mV 80mv
20kHz 120mV 100mv
30kHz 160mV 140mv
40kHz 200mV 180mv
50kHz 220mV 210mv
60kHz 250mV 260mv
70kHz 260mV 300mv

I claimed that I could reduce the glitches  by replacing the NPN transistors with 1N5817 Schottky diodes.  The diodes arrived today, and I tried them out with the same 10kΩ resistors and 30kHz triangle wave as before:

With the 1N5817 Schottky diodes, the glitches at 30kHz are much reduced—only about 68mV of overshoot.

(click to embiggen) With the 1N5817 Schottky diodes, the glitches at 30kHz are much reduced—only about 68mV of overshoot when turning off, which is half of the glitch with the S9018 NPN transistors as diodes.

I noticed that there was a bit of phase shift for the 30kHz signal, as well as the small overshoot. I tried adding capacitors in parallel with the resistors to improve the performance at 30 kHz (both to correct the phase shift and to keep the gain at -1).

This circuit works well up to 30kHz, and is still somewhat functional at 100kHz, though the "corners" have gotten soft enough that the clipping to the threshold voltage is no longer very precise at 80kHz.

This circuit works well up to 30kHz, and is still somewhat functional at 100kHz

C2 seems to adjust the overshoot, and C1 then needs to be set to fix the phase and gain.  I had the best results at 30kHz with C1=330pF and C2=220pF:

With capacitors in parallel with the feedback resistors, the phase shift is mostly corrected and there is less than 20mV of overshoot—the turn-on and turn-off corners are softened somewhat.

(click to embiggen) With capacitors in parallel with the feedback resistors, the phase shift is mostly corrected and there is less than 20mV of overshoot—the turn-on and turn-off corners are softened somewhat.

Unfortunately, there is no easy way in the BitScope software to set the offset of the traces precisely. You can do a lot of range changing and clicking the left or right sides of buttons (and start all over if you accidentally hit the middle of the button), but the offset is only displayed to 2 decimal points, but can be adjusted somewhat finer, making it hard to guess exactly what it is set to. As result, I’ve not been able to measure the overshoot or undershoot when it is less than 10mV—I’m never sure exactly what I’m measuring with respect to, and visually similar settings result in ±10mV in the estimate. In any event, the errors in this version of the improved rectifier are at least 5× better than in the version with the S9018 diode-connected transistors.

The circuit works well throughout the audio range, and can be pushed to 100kHz, though the “corners” have gotten soft enough that clipping to the threshold voltage is no longer very precise at (about 60mV off @ 80kHz—undershoot, not overshoot). At 100kHz, the output signal is still pretty good, but there is about an 85mV error in the threshold, and the corners are so rounded that the output almost looks like a sine wave:

Waveform at 100kHZ (sine wave input), showing the soft corners at that frequency.  The output doe snot get down to the threshold voltage, but only to about 85mV above threshold.

(click to embiggen) Waveform at 100kHZ (sine wave input), showing the soft corners at that frequency. The output does not get down to the threshold voltage, but only to about 85mV above threshold.

I can get better performance at 100kHz with smaller capacitors (100pF and 220pF, instead of 220pF and 330pF), but at the cost of some overshoot at 20kHz and 30kHz.  I suspect that the right values for the capacitors depend heavily on what op amp is used (especially its slew rate), but since I only have MCP6002 (and the equivalent MCP6004) op amps, I’ve not tested this suspicion.

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1 Comment »

  1. […] a log amplifier to convert to a dB scale. We were planning to use a Schottky diode, rather than a precision rectifier for the peak […]

    Pingback by More on loudness circuits | Gas station without pumps — 2014 February 8 @ 21:07 | Reply


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