feedback increasing input and output resistance

[bmandl]

Can someone please draw me a circuit with opamp and resistor feedback, that increase input and output resistance of amplifier. In theory this should be transconductance amplifier (series-series) feedback, but how can I inmplement this with opamp? It's a example on a exam, but I can't solve it. It also sais, that gain of such amplifier should be 100. I already tryed with noninverting amplifier, but that kind of feedback reduces output resistance of amplifier.

 

[FvM]

A Widlar OP current source circuit is a popular way to achieve what you want.

 

[bmandl]

The example says: Draw a feedback, which increase amplifier input and output resistance. Draw a block diagram and then draw a practical circuit with operational amplifier. Determine element values, for gain of 100 (write which gain). Calculate output resistance of such amplifier with known parameters of opamp ua741.

This must be some kind of easy way. Series-series feedback with opamp. How would this look like?

 

[Audioguru]

Negative feedback reduces the output impedance of an opamp. It does not increase it.
When negative feedback reduces the gain of an opamp to 100 then the output resistance of 75 ohms is reduced to an impedance of much less than 1 ohm at DC and low frequencies.

 

[FvM]

As said, try a Widlar OP current source. https://www.circuit-fantasia.com/ci...rrent_source/widlar_op-amp_current_source.htm

You'll need an additional buffer to achieve high input impedance.

Negative feedback reduces the output impedance of an opamp. It does not increase it.

Only true for voltage controlled ("parallel") feedback. Current controlled ("series") feedback increases the output impedance. In the Widlar OP current source, the feedback single only depends on output current, output voltage feedback is cancelled by the differential amplifier circuit.

 

[bmandl]

Too much for me. Can't solve it.

 

[FvM]

I have a problem with the unclear question. I don't see how to implement series-series feedback with a grounded load. But it's possible for a floating load. So if this is an option, the solution is quite simple.

 

[bmandl]

Can you draw it, please?

 

[FvM]

 

[bmandl]

But this feedback reduces output resistance of opamp...

 

[FvM]

I asked if a floating load is an option. Viewn from the load, the resistance is increased. You may want to calculate...