Problem 6-5 - Practical Op-Amp Circuit and Thevenin Equivalent
*
* NOTE: This is a four-part problem.
* 1. Determine the open circuit (Thevenin) voltage of the op-amp
* circuit. Calculate the gain parameter G to use in the Thevenin
* circuit (Part 4 of this file) to multiply the input voltage VS.
* 2. Determine the output (Thevenin) resistance from a short circuit
* test. Use a zero-volt source in place of the load resistance.
* 3. Connect a variable load at the output of the original circuit.
* Print out the load voltage and current for each resistance.
* 4. Connect the same variable load at the output terminals of the
* Thevenin equivalent circuit. Print out the load voltage and
* current. Compare with the results of Part 3. NOTE: You will
* have to complete the program in Part 4 after an initial program
* run from which you can determine the Thevenin circuit values.
*
* ===> Suggested optional method: Use smaller resistance steps in the .DC
* sweep to obtain a smooth curve. Use a PROBE graph to display the
* results of Parts 3 and 4 for comparison on one graph.
*
.OPT NOPAGE NUMDGT=6
VS 1 0 DC 3 ; Note that the input voltage is NOT equal
; to 1.
EAMP 4 0 ??? ??? 100 ; Low gain value used to emphasize the effect
; of gain and input resistance on the
; equivalent circuit.
; Enter control voltage node numbers.
Ra 1 2 1K
Rb 2 3 10K
*RL 3 0 100 ; RL "commented out" to open the circuit.
Ro 3 4 100
Ri 2 0 10k
.DC V1 ; Enter value and step size. Value should
; agree with specification for V1 above, for
; consistent results. A value other than 1
; volt was chosen so that the calculation of
; the constant G would have some meaning.
.PRINT DC ; Complete the .PRINT statement.
.END
Problem 6.5, Part 2. Practical Op-Amp Circuit and Thevenin Equivalent
*
* It would be impractical to short-out a real op-amp circuit, but
* we can do it in PSpice. Use a zero-volt source at the load
* terminals to measure the short circuit current. Calculate the
* Thevenin resistance to use in the equivalent circuit of Part 4.
*
.OPT NOPAGE NUMDGT=6
VS 1 0 DC 3 ; Note that the input voltage is NOT equal
; to 1.
EAMP 4 0 ??? ??? 100 ; Enter control voltage node numbers.
; Low gain value used to emphasize the effect
; of gain and input resistance on the
; equivalent circuit.
Ra 1 2 1K
Rb 2 3 10K
Ro 3 4 100
Ri 2 0 10k
.DC V1 ; Enter value and step size. Value should
; agree with specification for V1 above, for
; consistent results. A value other than 1
; volt was chosen so that the calculation of
; the constant G would have some meaning.
VSENSE 3 0 DC 0 ; Current sensing "ammeter" added at output.
; Calculate the Thevenin resistance using
; V(open ckt)/I(short ckt) where both of
; the quantities are negative.
.PRINT DC V(1) V(3) ; Complete the .PRINT statement.
.END
Problem 6.5, Part 3. Practical Op-Amp Circuit and Thevenin Equivalent
*
* Substitute a variable resistance model for RL, determine the
* voltage and current at the load for five different values of RL.
* NOTE: Optional method uses more steps and a PROBE graph.
*
.OPT NOPAGE
VS 1 0 DC 1.5 ; Note that the input voltage is NOT equal
; to 1.
EAMP 4 0 ??? ??? 100 ; Low gain value used to emphasize the effect
; of gain and input resistance on the
; equivalent circuit.
; Enter control voltage node numbers.
Ra 1 2 1K
Rb 2 3 10K
RL 3 0 RLOAD 1 ; Resistance model substituted for RL.
Ro 3 4 100
Ri 2 0 10k
; .DC voltage sweep statement removed. Only
; one .DC command per program is allowed.
.MODEL RLOAD RES(R=100)
.DC RES RLOAD(R) ??? ??? ??? ; Enter resistance values and step size.
.PRINT DC V(1) V(3) I(RL)
.END
* Problem 6.5, Part 4. Practical Op-Amp Circuit and Thevenin Equivalent
*
* Use the same variable resistance model as above for RL connected
* to a circuit consisting of the Thevenin equivalent voltage and
* Thevenin (output) resistance of the original op-amp circuit.
* ===> Note that the voltage VS=1.5 volts in Part 3. Use the value of G
* calculated in Part 1 to determine the Thevenin voltage to use in
* this part of the problem so that the results of Part 4 will agree
* with the results of Part 3.
*