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Discrete comparator

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John Sorensen

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Does anyone have a nice circuit for a discrete comparator? Something single ended, low quiescent current. Adjustable thresholds? You know, something for when it's just not worth it to use an op amp or a 339, with just a few transistors maybe?

tia,
j.
 

ljcox

Well-Known Member
Look at any electronics book or web site for a 2 transistor (bipolar, JFET or MOSFET) Schmitt trigger. Perhaps you could search the web for "Schmitt trigger".

You can design for whatever threshold levels and hystersis you want (within reason).

Len
 

crust

Member
How about a signal conditioner that takes in a pseudo-sinusoid and outputs a square wave at the same frequency. The hysteresis function of the comparator is then used to clean up the noisy signal presumably from the tach output winding of a motor. The tachometer request pops up on this board frequently. It uses nothing more than the comparator and a few passive components.

A while back I posted a simulation of such a circuit being fed with a very noisy signal which as I recall had a couple of significant harmonics. The original plot is the last article in the thread at http://www.electro-tech-online.com/threads/camera-battery-pack-2.4569/. The hysteresis is shown in the green signal.
 

Roff

Well-Known Member
John Sorensen said:
Does anyone have a nice circuit for a discrete comparator? Something single ended, low quiescent current. Adjustable thresholds? You know, something for when it's just not worth it to use an op amp or a 339, with just a few transistors maybe?

tia,
j.
You can make a differential comparator with as few as three transistors (two under some limited conditions), but you need to design it taking into account the input voltages, because the internal currents depend on them. Give me an example of the voltages you want to compare, the supply voltage(s) available, and what you want to drive on the output, and I'll try to design one for you. It won't be as temperature-stable as an IC, and it won't have guaranteed low offset voltages, but I've used a lot of them in my career.
If you want to add a few more trannies, you can improve some of the characteristics considerably - but then you might as well use an IC. In fact, the only reason I can see for using discrete parts is availablity.
 

John Sorensen

New Member
I had already seen the circuit that ljcox referred to, but it didn't have the adjustment range I wanted-- at least, I couldn't work it out to. But after doing more research it seems to have a good comparator you need a current mirror on it and that was just getting too complicated. If it could be done with a Q or 2, then ok. But I think what I'm going to do is start stocking single comparators in a SOT-23-5 package. Like the LMV331.

Thanks for the advice.

j.
 
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