opamp feedback question

Hey guys, thanks for your help.


I have two questions relating to opamp feedback.

1) The first question I have is i don't understand the difference between all these topologies.

• Voltage amp - (voltage mixing, voltage sampling) - series-shunt
• Current amp - (current mixing, current sampling) - shunt-series
• Transconductance amp - (voltage mixing, current sampling) - series-series
• Transimpedance amp - (current mixing, voltage sampling) - shunt-shunt

What I can't see is the difference between how you can sample current or voltage from the output of an opamp. As far as I can see, every time you connect a wire to the output of an opamp and feed it back, you're going to have some voltage and current being fed back!!

Could somebody clear this up for me?





2) My second question is that I dont understand how the input impedance of an opamp changes with feedback. I'll give you an example question to show what I mean..



Question

Info:
Rs = 50kΩ, R1 = 10kΩ, RF = 200kΩ and RL = 500Ω.
• Open-loop gain a = 100,000 V/V
• Input resistance ri = 100kΩ
• Output resistance ro = 100Ω

find:
d) input resistance Rinf
e) output resistance Routf.

Click to expand...

What I'm looking for here is how to arrive at the answers d) and e)

Answers:

d) Rinf = 395MegΩ
e) Routf =33.5mΩ

Click to expand...

I thought that the input and output resistance of an opamp always stayed the same!!

bumclouds, for the second part of your first question--yes, you are feeding back a voltage. This is done to reduce the gain of the opamp; Without it, your gain would be too high and your waveforms would be clipped.

For your second question, both your input Z and output Z will NOT stay the same--especially for a non-inverting amplifier. They are both dependent on the feedback fraction (Beta).

For a non-inverting amplifier,

Beta = Rin/(Rin + Rf), where your R1 is Rin.
Rinf = Rinopamp*(1 + Beta*Avol)
Routf = Routopamp/(1 + Beta*Avol)

You might want to try your calculations again.

Resistors are measured in Ohms, not in Watts. Watts is the amount of heat they are allowed to dissipate.
Rs = 50k ohms, R1 = 10k ohms, RF = 200k ohms and RL = 500 ohms.

okay ive fixed it up audioguru thanks. it was a cut-and-paste problem.

silvrstring thanks for your help.

silvrstring said:

For a non-inverting amplifier,

Beta = Rin/(Rin + Rf), where your R1 is Rin.
Rinf = Rinopamp*(1 + Beta*Avol)
Routf = Routopamp/(1 + Beta*Avol)

Click to expand...

So do these always apply to non-inverting amplifier? According to my lecture notes there is a different equation for Rinf and Routf for each of series-series, series-shunt, etc.

Ill show you what I have.






Are these all true and correct? The formula for Rinf that you gave me agrees with the voltage amp (series-shunt) one. But when i use that, my answer comes out as 476MegΩ, and the answer given to me by the lecturer is 395MegΩ.

Oh, and there's this one too.



I also cannot for the life of me see the connection between these diagrams and real life feedback opamps. There's two lines coming off the end of the amplifier and through the box labelled "feed network" and back to the voltage source again, but in a negative-feedback opamp scenario, there is only one feedback resistor with one wire. not two!! can you help me see what i need to see?

Here is how you draw the circuit in your first post,
in the formal series-shunt connection: