Problem 6.3 - Gain Error in a Nonideal Op-Amp Circuit
*
* The circuit for this problem is the same as for Problem 6.2 except
* that a finite input resistance is added between nodes 1 and 2.
*
* Use .DC command to sweep the value of the resistance parameter for
* the input resistance of the op amp from 1E6 to 21E6 in 2E6 steps.
*
* Use PROBE to plot the error expression 100-10*V(3)/V(1), which is
* the percent error between the ideal gain and the actual gain with
* the ideal gain used as the reference.
* Determine from the PROBE graph the (approximate) minimum value of
* input resistance for the gain error to be less than 1%. Note that
* you can select "Add_trace" to draw a line across the graph at the
* 1% level by entering the constant 1 when asked for an expression.
*
* ===> NOTE: You may also use the cursor function in PROBE to determine
* the 1% point (approximately) from the graph. See the description
* of the cursor function given in Problem 5.8 if you have not used
* the cursor function previously.
*
.OPT NOPAGE NUMDGT=5
VS 1 0 DC 1
EAMP 3 0 ??? ??? 2E5 ; Enter the node numbers for the op-amp VCVS
; control voltage. Note that the circuit
; uses a "practical" gain value to represent
; a typical realistic op-amp open loop gain.
RIN 1 2 RMOD 1
R1 2 0 1K
R2 2 3 9K
RL 3 0 4.7K ; Does the load resistance have any effect on
; the overall circuit gain when the input
; impedance is finite? A PSpice experiment
; is suggested to check on your answer.
.MODEL RMOD RES(R=1E6)
.DC RES RMOD(R) 1 21 2 ; You may change the increment from 2 to 1 for
; a smoother graph curve, but at the expense
; of increasing the time for the solution.
.PRINT DC V(1) V(3)
.PROBE ; Use PROBE to graph the gain V(3)/V(1) on one
; graph and the error expression given above
; on a second graph.
.END