Electrical – Odd op-amp circuit with a potentiometer


Im supposed to calculate the gain and impedance (in & out) of the inverting amplifier below. I also need to simulate the circuit with Ltspice using different \$V_s\$ values.

inverting op-amp

Im aware that the gain is calculated as \$\cfrac{V_o}{V_s}=-\cfrac{R_2}{R_1}\$. The problem with calculations and simulating is the circuit inside the red ellipse. I suppose the voltages +15, and -15 are for the amp (usage voltages).
But how to take that part into account calculating voltage \$V_s\$. Im not quite sure how the potentiometer in this case (or in overall) works. Also if possible, is there some equivalent circuit for understanding and simulating on Ltspice?

Any help will be appreciated.

Best Answer

M.P..... Now - after some answers and some related comments - I think, it is clear to you that the resistive chain (at left) does NOT influence the ratio (gain) Vo/Vs.

However, if you want - in addition - to find the value of Vs as a function of the potentiometer position, you simply have the problem of a loaded voltage divider, which is relatively simple to solve. The load is simply the 10k resistor because in this caculation, you can assume the inverting opamp input to be grounded.

Simulation: I am not familiar with LTSpice, however in the PSpice program you can do the following (perhaps something similar works in LTSpice):

  • Replace the pot by two resistors Rx and Ry.

  • Do not allocate any values, but write instead {Rx} and for Ry write {5k-Rx}

(the form of the brackets is important in PSpice).

  • In addition, declare Rx as a variable parameter (in PSpice using the .param command).

  • Now, you can do a DC sweep, however, not for a DC source but for the quantity you have declared as a parameter (in your case: Rx). Sweep Rx from zero to 5k.