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How do I go about designing a threshold amplifier?

Greetings everyone!

I was using a comparator as a threshold detector for part of my circuit. Then, I realized that without any sort of feedback, the output will most always be close to the VCC supply levels. I'm trying to envision a threshold amplifier circuit, but my head is clouded.

Pretty much I need something that will output zero if an input signal does not meet the threshold level and output a replica of the input signal with no gain if it surpasses the threshold level. I am trying to come up with this circuit using only the following components: opamps, comparator, resistors, capacitors, etc.

Any tips to get me started?

Thanks!

Reply to
MRW
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So consider this - what you're looking for is basically an amplifier whose output is gated by a comparator looking at the same input signal, no?

Bob M.

Reply to
Bob Myers

What means "output is gated by a comparator"?

Reply to
MRW

You have an ordinary amplifier, with an electrically controlled switch in its output lead, and alongside it, you have a comparator, that determines if the input is above your threshold. When the input is above the threshold, the comparator's output turns the EC switch to "amp output", and when the input is below the threshold, it turns the EC switch to "threshold level".

Hope This Helps! Rich

Reply to
Rich Grise

Hmm.. Thanks, Rich! I think I see what you are saying.

I can get a CMOS SPDT and have one of the inputs connected to the amplifier output and the other input to one of the comparator input connected to the threshold, reference voltage. Then, I can also tie the amplifier output to the other comparator input and tie the comparator output to the logic input of the SPDT. The output of this SPDT would either be amplifier output or threshold output.

Does this sound about right?

I'm a bit concerned with tying the amplifier output to the SPDT input since I've read some application notes suggesting that the switch inputs should be tied normally to high impedance inputs.

Thanks!

Reply to
MRW

have a look in the op-amps chapters of "art of electronics"

Bye. Jasen

Reply to
jasen

As long as you don't exceed the input spec (which will be on the datasheet somewhere), you should be fine.

Cheers! Rich

Reply to
Rich Grise

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