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I have been battling to find the theoretical calculations to find the expression for the load current of the following Operational Amplifier circuit exercise I have in my "Electrical Engineering, Principles and Applications, Fifth Edition" by Alan R. Hambley? Unfortunately, there is no solution for it available.

I simulated the circuit in LtSpice, I know the input to feedback resistor ratio will result in a gain of 1 at the difference amplifier stage, with that taken into account I can figure out Iload = Vin*(Rload/Rf) and Output impedance is Rload. Download LtSpice Circuit here. - https://gofile.io/d/LukKU0

I'm definitely missing something with my theoretical analysis of the problem and would be really grateful if someone can assist me.

Thanks in advance Stephan

LT Spice Operation Amplifier Circuit

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  • \$\begingroup\$ Which current is the load current (place an arrow on the schematic to show it)? \$\endgroup\$
    – The Photon
    Commented Aug 9, 2020 at 15:01
  • \$\begingroup\$ What I'm trying to figure out is why would the load current be anything other than \$V_{out}/R_{load}\$? \$\endgroup\$
    – The Photon
    Commented Aug 9, 2020 at 15:03
  • \$\begingroup\$ Howland current pump \$\endgroup\$
    – Andy aka
    Commented Aug 9, 2020 at 16:21

1 Answer 1

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Okay, so we know that the voltage at the RHS of R3 is Vout.

We know that (assuming balance) there is a virtual short between RHS of R1 & R2, so the input current is Vin/4K.

So the voltage at U1+ = U1- = Vout + Vin/2, and the voltage at U1 output is Vout + Vin.

The current into RL is (Vout+Vin-Vout)/Rf , or Vin/Rf.

Output impedance is Very High (because Vout, ideally, does not depend on RL).

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