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Figure 1

enter image description here

In the above Image 1, you see is a differential amplifier . It has a negative feedback as required.

Now take a look at figure 2 below.


Figure 2

enter image description here

Lets take a look at figure 2. Now that I have changed the opamp circuit to a positive feedback,

1) Could it still be called a differential amplifier?

2) What are the functions which could be performed by the circuit above?

3) How different are the output results achieved by it to a differential amplifier?

4) How to derive the equations for this type of a circuit?

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    \$\begingroup\$ Its actually a common building block circuit called a comparator with hysteresis electronics.stackexchange.com/… \$\endgroup\$ – sstobbe Jul 11 '17 at 0:30
  • \$\begingroup\$ @sstobbe So if I say a "comparator with hysteresis = Schmitt Trigger" , is it correct? \$\endgroup\$ – Amy Jul 11 '17 at 0:31
  • \$\begingroup\$ well saying the two are equivalent isn't true, but a schmitt trigger behaves as comparator with hysteresis, only for a schmitt trigger you don't get to choose the reference level (its generally 1/2 supply) \$\endgroup\$ – sstobbe Jul 11 '17 at 0:37
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    \$\begingroup\$ Homework? If so, it should have the [homework] tag. \$\endgroup\$ – Solomon Slow Jul 11 '17 at 2:24
  • \$\begingroup\$ @sstobbe "only for a schmitt trigger you don't get to choose the reference level" I did not get this part? Could you explain this part for me please? \$\endgroup\$ – Amy Jul 11 '17 at 4:56
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Answers to your other questions can be obtained by solving for the last question.

How to derive the equations for this type of a circuit?

A fundamental property of an ideal opamp is that it has infinite gain. In the case of negative feedback that means it's two inputs are at the same differential.

In the case of positive feedback that means the output is either at the highest or lowest output levels. Typically Vcc and Vee.

In this case assuming that vout is at one extreme, holding v2 constant you can find the v1 that will switch vout to the other extreme. You will have two such points, depending the initial state of vout.

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