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 Forum index » DIY Hardware and Software
emitter follower to multiple comparators
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bearblock



Joined: Sep 27, 2006
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PostPosted: Sat Aug 31, 2013 11:12 am    Post subject: emitter follower to multiple comparators Reply with quote  Mark this post and the followings unread

Hi, I've been playing around with the attached circuit and having trouble squashing one bug in particular. Any help much appreciated.

The idea is to compare 2 analogue signals to reference voltages, and also each other, thus generating 3 digital signals. It kind of works but I get unwanted oscillation on the '1 > 2' line when both my input signals go low sometimes. Shouldn't the 1M resistors in the positive feedback loop stop that happening (hysteresis)?

Cheers


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gdavis



Joined: Feb 27, 2013
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PostPosted: Sat Aug 31, 2013 1:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

you should have another resistor in series with the emitter and IN+

Actually, you'll need to make another change also if you truly want 1>2 since with hysteresis 1 has to be < 2 by the threshold amount before it will switch. Not sure what the best way to accomplish that is.

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frijitz



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PostPosted: Sat Aug 31, 2013 6:32 pm    Post subject: Re: emitter follower to multiple comparators Reply with quote  Mark this post and the followings unread

bearblock wrote:
Hi, I've been playing around with the attached circuit and having trouble squashing one bug in particular. Any help much appreciated.

Those comparator chips can be touchy to work with. Try running your circuit with a TL074 and see whether it isn't good enough for what you are doing.

Ian
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JingleJoe



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PostPosted: Sun Sep 01, 2013 12:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've experimented with LM339 comparators extensively, if I remember rightly, my solution to oscillation was a little cap (like 1n) on the input. Not in series, going from the input to ground like a little low pass filter. However, I just had a voltage divider and compared the other input to that, I had my cap on my voltage divider but with two signals you might want to include a small resistor before the cap to stop it acting as a low impedence path- my thinkign is that it might overload some signal sources.

P.S. forget the hysteresis, it never did anything for me either.

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bearblock



Joined: Sep 27, 2006
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PostPosted: Sun Sep 01, 2013 12:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks guys

gdavis wrote:
you should have another resistor in series with the emitter and IN+


Yep this has fixed it Smile i put a 10k in there and it's behaving.
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