Amplifier Topology

Discussion in 'Homework Help' started by laurens, Feb 18, 2014.

  1. laurens

    Thread Starter New Member

    Feb 9, 2014
    2
    0
    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?
     
  2. LvW

    Active Member

    Jun 13, 2013
    674
    100
    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.
     
    Last edited: Feb 19, 2014
  3. Efron

    Member

    Oct 10, 2010
    81
    15
    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.
     
  4. laurens

    Thread Starter New Member

    Feb 9, 2014
    2
    0
    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 ?
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,374
    494
    Are you looking at op amp circuit or nullor circuit?
     
  6. Efron

    Member

    Oct 10, 2010
    81
    15
    Op amp circuit
     
  7. LvW

    Active Member

    Jun 13, 2013
    674
    100
    * 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).
     
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