# How do I go about designing a threshold amplifier?

M

#### MRW

Jan 1, 1970
0
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!

B

#### Bob Myers

Jan 1, 1970
0
MRW said:
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.

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.

M

#### MRW

Jan 1, 1970
0
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.

What means "output is gated by a comparator"?

R

#### Rich Grise

Jan 1, 1970
0
What means "output is gated by a comparator"?

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

M

#### MRW

Jan 1, 1970
0
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

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.

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!

J

#### jasen

Jan 1, 1970
0
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?

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

Bye.
Jasen

R

#### Rich Grise

Jan 1, 1970
0
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.

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

Cheers!
Rich

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