Problem 6.6 - The Ideal Op Amp in a Difference Amplifier Circuit
*
* The circuit for this problem is from textbook Chap. 6,
* Exercise 14.
* NOTE: This is a three-part problem.
* 1. Determine the gain and input resistance for source VS1 with
* source VS2 set equal to zero (but still in the circuit).
* 2. Determine the gain and input resistance for source VS2 with
* source VS1 set equal to zero (but still in the circuit).
* 3. With both sources active, use PROBE to create a graph of the
* output when VS1 is swept through a range of values while
* VS2 remains fixed. Show by calculation at one or two points
* on the curve that the output is the algebraic sum of the
* two outputs from above: Vo = Vo1 + Vo2 = (R2/R1)*(VS2 - VS1),
* where the "difference" is due to the inversion of the signal
* from input VS1.
*
* Check the input resistances again with both sources active. Do the
* resistances stay the same? Would you agree with the statement that
* there is essentially complete separation (no interaction) between
* the two input signals on the input side of the amplifier circuit?
*
* The circuit is a true difference amplifier when the ratios of
* resistances at each input are the same (the resistance values do
* not have to be the same if the ratios are equal). Two "PSpice
* experiments" are suggested:
* 1. Calculate a new pair of resistances for R21 and R22
* such that both the gain and the input resistance are
* equal for each input.
* 2. Can you predict the output voltage equation when the
* resistance ratios are NOT equal? Let R21 remain the
* same and let R22=51k. Hand calculate the gain expression
* for the circuit and re-run Part 3 to verify your answer.
*
* ===> For Part 3, you can plot several expressions to explore the circuit
* output and the graphing function of PROBE:
* You might plot the input voltages on the same graph as the output;
* or you might plot the circuit gain times the input, as 10*VS2 or
* -10*VS1; or you might plot these two expressions and then plot the
* addition of their trace numbers, as, for example, #3+#4. There are
* many small games such as these that can be used to explore PROBE.
*
.OPT NOPAGE
VS1 1 0 DC ??? ; Choose the source voltages for the
VS2 3 0 DC ??? ; solution by superposition.
R11 1 2 3.3K
R12 2 5 33K
R21 3 4 3.3K
R22 4 0 33K
RL 5 0 4.7K
EAMP 5 0 ??? ??? 1e10 ; Enter the control voltage node numbers.
.DC ; Complete the .DC sweep statement.
.PRINT DC ???? ???? ; Complete the print statement so that
; you can determine the gain and the
; input resistance for the source you
; have selected to be active.
.END
Problem 6.6, Part 2. The Ideal Op Amp in a Difference Amplifier Circuit
*
* Interchange the active and zero-volt sources. Determine the
* gain and input resistance for the second source.
*
.OPT NOPAGE
VS1 1 0 DC ??? ; Choose the source voltages for the
VS2 3 0 DC ??? ; solution by superposition.
R11 1 2 3.3K
R12 2 5 33K
R21 3 4 3.3K
R22 4 0 33K
RL 5 0 4.7K
EAMP 5 0 ??? ??? 1e10 ; Enter the control voltage node numbers.
.DC ; Complete the .DC sweep statement.
.PRINT DC ???? ???? ; Complete the print statement so that
; you can determine the gain and the
; input resistance for the source you
; have selected to be active.
.END
Problem 6.6, Part 3. The Ideal Op Amp in a Difference Amplifier Circuit
*
* Let both sources be active, with the values given in the problem
* statement. Determine the overall gain of the circuit and the
* input resistance for each source. Use PROBE to graph the response.
*
.OPT NOPAGE
VS1 1 0 DC ??? ; Set the source voltages for the solution
VS2 3 0 DC ??? ; with both sources active. Note that VS1
; can be set to any value, since it will
; be swept by the .DC sweep command.
R11 1 2 3.3K
R12 2 5 33K
R21 3 4 3.3K
R22 4 0 33K
RL 5 0 4.7K
EAMP 5 0 ??? ??? 1e10 ; Enter the control voltage node numbers.
.DC ; Complete the .DC sweep statement.
.PRINT DC ????? ????? ; Complete the print statement so that
; you can determine the input resistance
; for each source.
.PROBE
.END