# Comparator with diodes

Discussion in 'Homework Help' started by jstrike21, Mar 25, 2010.

1. ### jstrike21 Thread Starter Member

Sep 24, 2009
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I'm very stumped on this question i think its just the jumbled mess of components that is doing it but can someone help me out in obtaining the transfer characteristics of this circuit?

I am supposed to assume the diodes are ideal as shown in the bottom of the picture.

Right now im thinking that Vin has no effect on this circuit because the virtual short from the op amp will ground it....

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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This circuit will never work because of positive feedback.
So in this circuit you don't have virtual short.

3. ### jstrike21 Thread Starter Member

Sep 24, 2009
104
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I'm pretty sure it works or else it wouldn't be on my assignment

i think

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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To make this circuit to work you must swap the inputs of a opamp.
Non-invent input must go to GND, and feedback to inventing input.

5. ### jstrike21 Thread Starter Member

Sep 24, 2009
104
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Anyone else have any input? I'm pretty sure this circuit works... It's just like a noninverting comparator

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
1,115
To find threshold voltage you just need assume that opamp is in saturation, its doesn't mater if its is positive saturation or negative sat.
And then you need to find the value of Vin for which voltage on non-inverting input is equal 0V.
And this Vin is equal threshold voltage.
And if I don't made any mistake the right answer look like this

$Vth=\frac{R1}{R2}*V{sat}-\frac{R1}{R2+R3}*V{xx}$

The positive and the negative threshold voltage will be symmetrical.
The Vin must drop below - Vth to get the output to switch to negative saturation.
And Vin must be +Vth to get positive sat.

Last edited: Mar 26, 2010