Af
772
BME 325
of the op-amp is fed to a three-stage RC network, which provides the needed 180° of
phase shift (at an MEDICAL
attenuation factorELECTRONICS
of 1/29). If the op-amp provides gain (set by rethan
29, a loop
gain greater than unity results and the cirsistors Ri and Rf) of greater
Wien
Bridge
Oscillator
cuit acts as an oscillator [oscillator frequency is given by Eq. (18.33)].
Theory
18.7 WIEN BRIDGE OSCILLATOR
A practical oscillator
uses
an uses
op-amp
and RC
bridge
circuit,
theoscillaoscilla- tor
A practical circuit
oscillator
circuit
an op-amp
and RC
bridge
circuit, with
with the
frequency settor
byfrequency
the R and
Figure 1 shows
basic
version
of version
a Wienofbridge
set C
bycomponents.
the R and C components.
Figure a18.23
shows
a basic
a Wien
bridge
circuit.connection.
Note the basic
bridge connection.
Resistors
R1 and
oscillator circuit.
Note
theoscillator
basic bridge
Resistors
R1 and R2
and capacitors
C1
and
capacitors
C
and
C
form
the
frequency-adjustment
elements,
while
resistors
R
2
1
2
and C2 form the
frequency-adjustment
elements, while resistors R3 and R4 form part of the
and Rop-amp
of the
path.
thec. The
R3 The
4 form part
feedback path.
output
is feedback
connected
asThe
the op-amp
bridge output
input isat connected
points a as
and
bridge input at points a and c. The bridge circuit output at points b and d is the inbridge circuitput
output
at points b and d is the in- put to the op-amp.
to the op-amp.
Figure 18.23
Figure
Wien bridge oscillator circuit
using1op-amp amplifier.
If, in particular, the values are R1 = R2 = R and C1 = C2 = C, the resulting tor frequency is
f=
1
2π RC
Neglecting loading effects of the op-amp input and output impedances, the analysis of the bridge circuit results in
R
R4
R
R2
C
C1
1
!!
# than
!!2 2 will provide sufficient
(18.40)
R3/R4 =2 a ratio of R3 to !
R!43 "
greater
loop gain
for the circuit to oscillate at the frequency calculated.
and
Preliminary Work
1
fo " !!
2$!"
R"
1C"
1R"
2C"
2
(18.41)
If, in particular, the values are R1 " R2 " R and C1 " C2 " C, the resulting oscillator frequency is
1. Using a simulatin software construct the circuit in Figure1. with R=51K and
1 output waveform and frequency of the
C=0.001uF R3=300K R4=100K record
(18.42)
fo "the
!!
2
$
RC
oscillator circuit. Calculate frequency theoritically.
2. Calculate and construct a circuit for 6khz oscillating frequency. Using a simulatin
R3
software
waveform and frequency
and construct the circuit. record !the
! " output
2
(18.43)of the
R4
oscillator circuit.
Thus a ratio of R3 to R4 greater than 2 will provide sufficient loop gain for the circuit to oscillate at the frequency calculated using Eq. (18.42).
Chapter 18 Feedback and Oscillator Circuits
EE 303 - Electronics II Lab Experiments
R1
+15V
270
2
R2
3
+
7
Vout
741
4
6
R3
-15V
C1
10k
10k
fo = 1/2 CR
C = 15nF , R = 10k
R2 = 270 , R1 > 2R2
15n
C2
R4
15n
(a) Wein Bridge Oscillator
Figure 2
Procedure
1. Construct the circuit shown in Figure 2. In each case change the variable resistance
until you get an output on the oscilloscope. This means that your circuit is oscillating.
In each case record the value of the resistance R1 at which oscillation just starts to
R1
appear on the oscilloscope. Check
whether it satisfies the condition of oscillation
+15V
fo = 1/2 6 CR
15n obtained
15n
from your10k
theoretical work.
7
2
C = 15nF, R = 1k
Vout
R
2 = 10k , R1 > 29R2
741
Rto
C
2.C Use the oscilloscope
2 record the output
6 waveform of the oscillator circuit. Adjust R1
+
1k
1k
R maximum
R 1k
for
undistorted3 output
waveform Vo. Record value of R1 for this
4
-15V
undistorted condition.
3. Measure and record the time for one cycle of the waveform.
(b) Phase
shift Oscillator
4. Determine
the frequency
of the waveform.
5. Replace the capacitors with C=10 uF and repeat steps 3-4.
6. Calculate the theoretical frequency using the equation in theory part.
15n
7. Compare the measured and calculated
15n frequency for both capacitors.
15n
C
R
C2
5.1k
2
-
7
Experiment741
List
R2
3
C1
C3
+15V
+
5.1k
6
V
+15V
2
R3
-
4
+
out2
• 741 op-amp
3
-15V
R1
• 2-DC power supplies (±10V)
• Resistors: 2*10K, 270*1
• 15nPotentiometer: 1*10 K
• Capacitors: 3* 10 µF, 3* 10 nF
7
741
4
-15V
Vout1
6
fo = 1/2
C 2 C 3 R 2 R3
C1=C2=C3=15nF, R = 1k
R2=R3=5.1k , R1 > R2
(c) Quadrature Oscillator
References
Figure 1 seventh Edition, Robert L. Boylestad, Louis
Electronic Devices and Circuit Theory,
Nashelsky.
38
Copyright © Electrical Engineering Department, KFUPM.
REPORT
1.
Vout
Vin
Rf = ........
2. T = .......
3. f = 1/T
f = .......
4. T2 = .......
F = .........
5. f (calculated) =....................
f2(calculated) = .................
6. Commands: