opamp circuit - unfamiliar coupling

[Prototyp_V1.0]

input coupling opamp

I just came over an simple opamp chematic. It's supposed to be an amplivier, but I cannot recognise it. I haven't tried to simulate it (don't have any simulation prog).

The closest I can think of is a difference amplifier, but the resistor from positive input to earth is shorted here.

When the switch is open, it's an inverter. Question is when the switch is closed (not conducting).

 

[FvM]

op amp coupling

It's a +/- 1 switchable amplifier, often used with an analog switch as a modulator or demodulator. Obviously, it works as a conventional inverting amp with the switch closed. If you think about, you'll most likely understand, why it operates as voltage follower with opened switch.

When the switch is open, it's an inverter. Question is when the switch is closed (not conducting).

Usual switch terminology is just opposite, I think.

 

[Prototyp_V1.0]

simple switch with opamp

Thanks. No, I couldn't see.

Well, I suppose that:
at any input voltage input from Uinn, the same voltage will be found on both inputs on the opamp, exept from a small error. Is that correct?

 

[Sadegh.j]

op amp switchable gain

R3 is for offset canceling, that is why R3=R1.

When the switch is closed, the gain is just -R2/R1.

When the switch is closed, the circuit can be a buffer, but you still have to see its application.

Cheers

 

[FvM]

switchable inverter op amp

R3 is for offset canceling, that is why R3=R1.

No. It would have to be 0.5*R1 then. And it doesn't fit in +1 mode. So a low input current OP, typically a FET OP, is suggested for the circuit. BTW, you can also use a double-throw switch and omit R3.

the same voltage will be found on both inputs on the opamp, exept from a small error. Is that correct?

Yes that's a basic property of negative feedback OP circuits, unless the output is saturated. With open switch, this implies cause there's no voltage drop at R3, there is also none at either R1 or R2. So the circuit can't but operate as a +1 buffer.

To my opinion, the circuit is very interesting, if you have any low frequency (up to several kHz)synchronous demodulator or modulator applications with a square wave carrier.

 

[LvW]

coupling on op amp

May I point to another interesting property of the circuit ?
It operates as a buffer with gain=+1 also if R2≠R1 (of course with the switch open).
This has the advantage, that the loop gain can be reduced to a convinient value - for example if the opamp is not compensated for a gain of 1 or 2.
Thus you have a voltage follower with adjustable loop gain according to your stability requirements.

 

[Prototyp_V1.0]

coupling opamp

I was shown a way to find out how to calculate the final amplification. Use the superposition principle: see attached picture.

 

[xulfee]

switching circuit in op amp

with open switch it is working as simple inverting amplifier

 

[LvW]

adjustable gain opamp analog switch

with open switch it is working as simple inverting amplifier

Why do you ignore all former replies ? Are they wrong ?