EEVblog #490 - Peak Detector Circuit

Fundamentals Friday.
Designing and measuring basic and precision opamp peak detector circuits.
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[MylesShannon]

I love how he starts off with a simple circuit and talks about how great it is, and then starts talking about how it sucks in some way, and then he gives us a new circuit that fixes that problem, and then it starts all over again. I love this guy!

[pcsczij]

I love the fact you start of so rock basic people understand the workings instantly to then build up the complexity for everyone to follow along rather than just slaping down the state of the art from the get go and tearing it down into understandable sections later down the line (or let alone just going through it front to back).

[martinda]

That was meat and drink to a hungry engineer. Thanks again Dave, hope you are not bored of the complements, you have really inspired me and I cant get enough at the mo.

[gregben]

Seven years later and still a GREAT video. Thank you.

[davidfof]

This is the perfect way of teaching. You go through the conceptual design, the different implementations then you implement the circuit and test it.

[lisakingscott]

I came across this as I was looking for a peak detector to detect the time when the peak occurs rather than the value of the voltage. I've got a microcontroller with A-D doing it, but I wanted a simple circuit to generate a pulse, whenever the voltage starts to reduce.
Just to make things more difficult it needs to be immune from noise, but that really just means a LPF on the input.
It's for triggering a TRIAC on the AC mains peak for switching inductive loads. It should be pretty simple with a comparator some resistors and capacitor, so that in effect one comparator input lags the other and the output changes state whenever the input changes from increasing to reducing and vice versa, or something like that. In fact a Google search finds such a concept was even patented in 1969.

[logitech]

A scope painted a thousand words on trade-offs.  Excellent presentation, Dave.

[andymouse]

Awesome to be able to come back 10 years and get this level of

[lleaves]

Thank you for the video.  I have designed peak detectors for AM waveform demodulation using op-amps to buffer the input and output similarly to what you have done here.  In my applications I have needed the capacitor to discharge in order to follow the envelope of the AM waveform.  I sometimes refer to the peak detector circuits I designed as envelope detectors for this reason.  I did learn a few things from your video that I was not aware of.  I may try to incorporate some of the techniques you presented here in my future designs.  Thanks again!

[hanedoggy]

Man I wish I had these videos 13 or 14 years ago... Would have made college soooo much easier. Sometimes breaking down complex concepts into basic functions is all a bad teacher needs to do to be a good one. Thanks Dave!

[SirMo]

This is one of your best videos Dave. Thank you!

[NickAlexeev]

Note that the diodes reduce the dynamic range. For the first circuit with a single diode, the max input voltage is the OpAmp's max input, minus one diode drop. For the second circuit with two diodes in series, the max input is the OpAmp's max input, minus two diode drops.
These are useful circuits, and this is a good video.

[jonathanknight]

14:15 slew rate (the time it takes for the op amp to swings its output back up to the point where the diode begins to conduct again) and the value of the holding cap are unrelated. The size of the cap only matters when the diode is conducting, and in that case, the charge time is determined by the maximum output current capability of the op amp. They are both important but separate issues.

[DjokovicWu]

Just discovered you channel and love it. I am an EE undergrad at Caltech, and this is just perfect for practical knowledge that we don't get in classes.

[neilalbaugh]

Nice video, Dave. You've done a good job of explaining the peak detector. In practice, your circuit will work nicely with a storage capacitor of 1nF to 10nF if you put a 10 to 50 ohm resistor in series with that capacitor to ground. This cures the instant instability of the op amp caused by the capacitive load on its output. The loop stability is complicated by the diode's varying ON resistance as the current charging the capacitor varies.
Regards, Neil ex-Burr-Brown Corporation

[freon]

Tour d' force. Awesome! There is enough here to keep me playing this video for quite a while!

[BryanByTheSea]

Wish you would bring back Fundamental Fridays. Love these.

[daveb]

i gorgot how much I like this channel. The stupid algorithm didnt pop up any eevblog for over a month and I forgot about it. Why did you tube stop putting eev videos in my que?

[tonydengineering]

I start designing a nonlinear filter. Cut-off frequency must depend on the average amplitude of a signal. Your explanation is very helpful to me!

[maxwellstrange]

Nothing like the aussie accent to put you in the mood to learn about circuits :D