Hello, I am an aerospace engineering student, trying to get a better understanding in the field of electrical engineering.

I have a problem understanding how to choose the best topology for an amplifier.

For example the question in problem1. It was asked if this topology is the best current-to-voltage amplifier. Although the right answer is marked yellow, I don't really care about the answer itself but more on how they got the answer.

As I see it now it would have picked answer c, because it seems that if I increase the value of Rs, more of the current will go trough R1 (knowing that the current into the amplifier = 0 and using Kirchoff's current law at the node above Rs). But it seems that this reasoning is wrong?

Also if I compare problem1 and problem2, the output type seems the same (Rl with a voltage across it). However in problem1 the transfer does not depend on the load resistance, but in problem2 it does.

Can anyone help me to understand how to pick the right topology or have a reference with a good explanation?

 

Yes - I agree that both answers (in yellow) are correct (for my opinion).

@1) Because the opamp is considered to be ideal the voltage between both input nodes disappears and any resistor across the input has no influence.
@2) Because of unity feedback the voltage across R1 is identical to Vs.
Hence, we have Iout=Vs/R1 and Vout=-Iout*RL.

Thus: Vout=-Vs*RL/R1.

Edit: For further investigations it is perhaps helpful to know that the 2nd figure represents a "Four-Terminal Floating Nullor" (FTFN) with feedback.

 

As I see it now it would have picked answer c, because it seems that if I increase the value of Rs, more of the current will go trough R1 (knowing that the current into the amplifier = 0 and using Kirchoff's current law at the node above Rs). But it seems that this reasoning is wrong?

For the first problem ...

Be careful, in the given configuration, the OA has negative feedback, so (assuming correct power supply bias), both input terminals will have the same voltage.

As the + pin is attached to the reference, the negative will have the same voltage, which means that the voltage drop in Rs is 0 !

So, whatever the value of other elements, no current will flow through Rs, and so the final Voltage/Current amplification does not depend on Rs.

Therefore, the answer c) IS NOT CORRECT.

 

Thanks LvW and Efron, I think I'm getting the hang of it now .

So for the second amplifier I can say that it is not dependent on the source resistance, since there is no current trough Rs and thus no voltage drop ? And that this amplifier is a voltage-to-current amplifier ?

 

For the first problem ...

Be careful, in the given configuration, the OA has negative feedback, so (assuming correct power supply bias), both input terminals will have the same voltage.

As the + pin is attached to the reference, the negative will have the same voltage, which means that the voltage drop in Rs is 0 !

So, whatever the value of other elements, no current will flow through Rs, and so the final Voltage/Current amplification does not depend on Rs.

Therefore, the answer c) IS NOT CORRECT.

Are you looking at op amp circuit or nullor circuit?

 

Are you looking at op amp circuit or nullor circuit?

Op amp circuit

 

Thanks LvW and Efron, I think I'm getting the hang of it now .
So for the second amplifier I can say that it is not dependent on the source resistance, since there is no current trough Rs and thus no voltage drop ? And that this amplifier is a voltage-to-current amplifier ?

* Yes, no current through Rs and, hence, no voltage drop (nullator allows no current).
* No - it is wired as a voltage-out amplifier because the output signal is defined as Vout (developped across Rload).