EEE 224 CIRCUIT THEORY II
EXPERIMENT 4
Wien-bridge Oscillator Using OPAMP
Instructor: Dr. Hüseyin Bilgekul
OBJECTIVE: To investigate the concept of sinusoidal oscillations and the frequency of oscillation using
the wien-bridge oscillator with an OPAMP circuit. The frequency of oscillation is studied as a function of
component values.
DESCRIPTION:
PART 1: Gain of the inverting and noninverting OPAMP amplifier circuit.
a.) Construct the PSPICE Schematic diagram of the inverting amplifier circuit given below. We are
using an operational amplifier of A741 type. Note that OPAMP is an electronic amplifier circuit
behaving as a dependent source. In order for the OPAMP circuit to function, we have to apply it
DC voltage sources having values +15 Volt and –15 Volts.
b.) The inverting amplifier circuit amplifies the input AC voltage V s. As you should know the
relationship between the output voltage V2 and the input Vs is Vo/Vs=- R2/R1.
c.) For the input voltage Vs, use a VAC source having ACMAG and ACPHASE values as shown.
Choose from the simulation SETUP section the AC sweep analysis. Insert START freq.=1k,
END freq=1k and Total Points =1.
d.) What is the magnitude of the voltage gain Vo/Vs of the circuit. Compare with the theoretical value.
Using phase probes VP find the phase shift between the output and the input.
e.) Repeat PART (d) for other values of R2=80k, 100k, 150k, 200k.
f.) Now construct the PSPICE Schematic diagram of the noninverting amplifier circuit given below.
The noninverting amplifier circuit amplifies the input AC voltage Vs. As you should know the
relationship between the output voltage V2 and the input Vs is Vo/Vs=(R1+R2)/R1.
g.) For the input voltage Vs use an VAC source having ACMAG and ACPHASE values as shown .
Choose from the simulation SETUP section the AC sweep analysis. Insert START freq.=1k,
END freq=1k and Total Points =1.
h.) What is the magnitude of the voltage gain Vo/Vs of the circuit. Compare with the theoretical value.
Using phase probes VP find the phase shift between the output and the input.
i.) Repeat PART (g) for other values of R2=80k, 100k, 150k, 200k.
PART 2: Wien-bridge oscillator using OPAMP amplifier.
a.) An oscillator is a circuit that produces AC waveforms when powered by a DC input. The
Wien-bridge oscillator given below is such a circuit.
b.) An oscillator circuit oscillates when the total phase shift from output to input and back to
output is 360 degrees. This condition is satisfied at a certain frequency only. Thus the
frequency of oscillation for the Wien-bridge oscillator is given by f=1/(2R3R4C3C4).
c.) Construct the PSPICE Schematic diagram of the circuit given below. As you see no AC
voltage is applied but the circuit will produce an AC waveform at a frequency determined
by the component values as f=1/(2R3R4C3C4). These component values produces 360
degrees phase shift from output to input and back to the output at the oscillation
frequency.
d.) Choose from the SETUP section the TRANSIENT analysis. Choose Final time=1s,
Print Delay=.5s, Print step=1ms and Step ceiling=1ms.
d.) From the AC waveform plot measure and calculate the frequency. Compare the
frequency obtained with the frequency obtained from f=1/(2R3R4C3C4).
e.) Compare the phases of the waveforms V1, V2 and V3 from the waveform plot.
f.) With R3=R4=R and C3=C4=C, Choose values of R and C to make the circuit oscillate at
f=1/(2RC)=1000/=318.5 Hz. Use these values, simulate the circuit and measure the
frequency from the plot and compare.
20mV
10mV
0V
-10mV
1.0s
V(1)
V(3)
1.1s
V(2)
1.2s
1.3s
Time
1.4s
1.5s