Problem 6.4 - Op-Amp Circuit with Tee Feedback Network
*
* The circuit for this problem is from textbook Chap. 6, Exercise 18.
* In this circuit a large gain may be achieved using relatively
* small values of resistance in the feedback Tee circuit.
*
* NOTE: This is a two-part problem:
* 1. First use a relatively wide range of values for resistance R4 in
* the feedback network. Obtain a graph of voltage gain vs. R4.
* 2. Change the range and step size for resistance R4 for better
* resolution of the gain versus R4 curve in the high gain region.
*
* Use a variable resistance for R4 in each of the parts above.
* Use PROBE to obtain graphs of circuit gain versus R4 for each of
* the program runs. Estimate from the graphs or from the printed
* output the value of resistance R4 that will result in a circuit
* gain of -100. What value of R4 will result in a gain of -50 ?
* Note that the constants -100 and -50 can be entered as "expressions"
* on the PROBE graph, for help in reading the resistance values.
* That is, select "Add_trace" in the PROBE menu. When asked for the
* "variables or expression," simply enter the constant -100. A line
* will be drawn across the screen at the value -100.
* NOTE: you may also use the cursor function to obtain the answers.
*
.OPT NOPAGE
VS 1 0 DC 1 ; Use VS=1 so that Av=V(3)/V(1)=V(3).
EAMP 3 0 ??? ??? 1E10 ; Enter control voltage node numbers.
; Note that a very high gain is used to
; approximate the ideal op amp.
R1 1 2 1K
R2 2 4 5K
R3 4 3 5K
R4 4 0 RMOD 1
RL 3 0 4.7K
.MODEL RMOD RES(R=1)
.DC RES RMOD(R) ??? ??? ??? ; Enter the resistance range and step size
; specified for Part 1 of this problem.
.PROBE V(1) V(3) ; Obtain a PROBE graph of the closed-loop
; voltage gain V(3)/V(1).
.PRINT DC V(1) V(3)
.END
Problem 6.4, Part 2. Op-Amp Circuit with Tee Feedback Network
*
* Change range of values of R4 for better definition of resistance
* value in the high gain region (where gain magnitude > 25).
*
.OPT NOPAGE
VS 1 0 DC 1 ; Use VS=1 so that Av=V(3)/V(1)=V(3).
EAMP 3 0 ??? ??? 1E10 ; Enter control voltage node numbers.
; Note that a very high gain is used to
; approximate the ideal op amp.
R1 1 2 1K
R2 2 4 5K
R3 4 3 5K
R4 4 0 RMOD 1
RL 3 0 4.7K
.MODEL RMOD RES(R=1)
.DC RES RMOD(R) ??? ??? ??? ; Enter the resistance range and step size
; specified for Part 2 of this problem.
.PROBE V(1) V(3) ; Obtain a PROBE graph of the closed-loop
; voltage gain V(3)/V(1).
; Also graph the constants -100 and -50 to
; help in answering questions posed in the
; problem.
; You may also use the cursor function here.
; NOTE: If you enter the expressions -100 and -50, you
; should separate them with commas. PROBE has trouble
; with negative entries, and will subtract the second
; entry from the first (plotting only one variable as
; a result), when you do not use commas.
; That is, if you were to attempt to graph variables
; V(1) and -V(3), you should enter V(1), -V(3).
; Otherwise you will get only one graph of the
; difference between the two waveforms.
.PRINT DC V(1) V(3)
.END