ECE 3235 Electronics II
Experiment # 11
Oscillators
1. Wien Bridge Oscillator
The circuit shown in Figure 9.1(a) is a well-known and widely used oscillator circuit.
Figure 9.1(b) is a non-inverting OpAmp circuit used to implement the gain block
(shadowed) of Figure 9.1(a). Wire up the circuit, using nominal values of 9.1(a):
R1=R2=10 K, C1=C2= 0.01 F, and Ra = 10 K. Use a 50 K potentiometer for Rb.
Measure all component values. The power supplies should be set to + 15 Volts.
Figure 9.1 Simple Wien Bridge Oscillator Circuit.
(a) Wien Bridge Oscillator, (b) Amplifier Circuit
With a 1V peak-to-peak sinusoid signal source connected to terminal x, observe the
waveform at node y with an oscilloscope. Adjust Rb so that a nondistorted sinusoid is
present at y. Now vary the frequency of the signal source, and locate the frequency at
which the sinusoid at terminal y is exactly in phase with the one at terminal x. Record its
exact frequency (as measured with your frequency counter). Now adjust Rb until the two
amplitudes are exactly equal. Then remove the signal source and connect terminal x to
terminal y with a conductor. The circuit should oscillate. If it does not, repeat the last
step (with a source connected to x, and y disconnected from x), adjusting Rb so that the
voltage at y is 5% or 10% larger than that at x. Then reconnect x and y. Record the
frequency of oscillation using your frequency counter connected at the OpAmp output.
Sketch the waveforms at Vo and VF, and record their peak-to-peak values. Also,
carefully measure the value of Rb without disturbing its settings. (Simply disconnect one
terminal from the circuit to measure).
2. Automatic Gain Control (AGC)
After performing part one of this experiment, you should be convinced that the output
waveform of the Wien Bridge oscillator is somewhat distorted. This is because the
circuit poles must be placed just a bit to the right of the imaginary axis for oscillation to
build up. The amplitude then builds up until the nonlinearity of the amplifier reduces the
small signal gain sufficiently to move the poles back to the left half plane. This part of
the experiment will be concerned with using an FET in the VCR (voltage controlled
resistance) region in a circuit, which automatically senses the output voltage peak level
and adjusts the gain to just precisely place the poles on the imaginary axis.
Figure 9.2 shows a circuit which you will use to replace the one of Figure 9.1(b), and
hence the gain block of Figure 9.1(a).
Figure 9.2 Automatic Gain Control Circuit.
Wire the circuit, using 50 K potentiometers for Rb and the output-sampling voltage
divider circuit labeled Rx and R-Rx. Let Ra=10 K, RG=100 K, and C=0.1 F. The
FET should be an N channel small signal device. A 2N5486 will function well. It has
the pin configuration shown in Figure 9.3 (when you look at the flat side of the
transistor).
Figure 9.3 Pin-out for 2N5486 N Channel JFET
Wire up the oscillator circuit of Figure 9.1(a) using the circuit of Figure 9.2 for the gain
block labeled A. Adjust the R, Rx potentiometer so that the movable contact is close to
the bottom. Note that you can remove the top and movable connections and measure the
resistance with your DMM in its ohmmeter function. The resistance between the
movable contact and ground should be a low (a few hundred ohms). Then adjust Rb for
oscillation with a minimum amount of distortion. Measure its amplitude and frequency.
Now, slowly change Rx and observe Vout with your scope. You should be able to adjust
Vout to any desired value between 2V or 3V peak-to-peak and approximately 30 V peakto-peak. The output should be an almost undistorted sinusoid of constant frequency.
Measure the peak-to-peak value of Vout, the peak-to-peak value of Vf, and the value of
Rx for 4 or 5 values between the two extremes (minimum and maximum values of Vout).
Now, replace C with a larger capacitor in the 5F to 10F range. Repeat the previous
procedure. With Vout adjusted to approximately 15 Volts peak-to-peak, quickly turn the
Rx potentiometer to decrease Vout. (The idea is to approximate a step change as best you
can.) The output should “ring”, that is the sinusoidal peak-to-peak value should vary
around its steady state value and gradually settle to that value.
Increase C and/or RG and repeat the above experiments. For some values of RGC time
constants, you might get a steady state, low frequency variation of the peak-to-peak
amplitude.
3. Astable Multivibrator
Figure 9.4 OpAmp Astable Multivibrator.
Construct the circuit depicted in Figure 9.4.
Multivibrator, generating a square-wave output at
Note that the positive and negative levels at the
negative saturation levels of the OpAmp (+Vmax,
levels are not necessarily equal in magnitude.
This is an astable (free-running)
a frequency in the order of 450 Hz.
output are equal to the positive and
-Vmin), respectively; these saturation
Using DC coupling on your oscilloscope, sketch the voltage waveform at the output
showing key voltage levels and times. Measure its frequency with a frequency counter
connected across the 4.7-K load. Additionally, sketch on the same set of axes the voltage
waveforms at points x and y, showing key voltage levels and the time relation between
these two voltages. In this case, also, the input couplings to your oscilloscope should be
on DC to allow you to measure the actual magnitudes of the various voltage levels
involved.
Remove the 0.1-uF capacitor from your circuit. Theoretically, its output should go to
zero (think about it!). To verify this, observe the output with Channel 1 of your
oscilloscope (Channel 2 should be disconnected, at this time); it should be zero. Now,
connect Channel 2 at point x to allow you to observe the voltages (if any) at the inverting
input terminal of the OpAmp and at the output, reality though, your multivibrator will be
operating again, generating a triangular output at a much higher frequency in the order of
130 kHz or so. Measure this frequency. Can you justify this seemingly odd behavior?
Explaining it should illustrate the effect of the input impedance of your oscilloscope at
higher frequencies while reinforcing your understanding of the operation of your astable
multivibrator.
Before taking your circuit apart, label the three resistors R1, R2, and R3 as shown in
Figure 9.4, and then measure their actual resistance as well as the actual value of C for
later use.
4. Cadence Simulation
Simulate the circuit in Figure 9.1, 9.4 in Cadence and compare with your measured
results.
Compound Problems
(A)
Derive the loop gain T(s) for the circuit of Figure 9.1(a) in terms of R1, C1, R2, C2,
and A. Evaluate for s=j, and find the frequency 0 at which
. Then find
the required value of voltage gain A. Now, using your measured values of R1, C1, R2,
and C2, evaluate these values numerically. Compare the theoretical value of fo = o/2
with the experimental value. Do the same with A. Note that the experimental value for
A can be obtained two ways: by taking the ratio of Voutp-p to Vfp-p, as measured, and from
1 + (Rb/Ra), as measured.
(B) Derive the peak output amplitude from the Wien Bridge oscillator with the
automatic gain control as a function of Rx. Assume that the peak detector is ideal, that is
VG=Vout(peak) is a DC voltage (this is equivalent to assuming that the R GC time constant
is very large compared with the period of oscillation). Also assume that the diode is ideal
and recall that RDS = RDS(on)/[1-(VGS/VP)]. Draw a graph of Vop-p versus Rx for both
your theoretical and your experimental results (for the data you obtained with C = 0.1
F).
(C) If your results in point (B) are not so good, try assuming a constant drop across the
diode (experiment with various values between 0.2 and 0.7 volts to get the best fit). Is
your experimental gain block voltage gain (A = Vout-p/Vfp-p) a constant, independent of
Rx? What is its value? Is it close to the theoretical value derived in part (A) above?
(D) Show your calculations for theoretical and experimental values of o and Q (for Q
=2 and Q = 10) and compare the results.
(E) Analyze the circuit of Figure 9.4 for the waveforms at the output, and those at
points x and y. More specifically, derive general expressions (one for each half-cycle of
the output) for the voltage at point x, assuming “large” slew rate and that the magnitudes
of the positive and negative op-amp saturation voltages (+Vmax, -Vmin) are different.
From these equations, derive an expression for the frequency of the output voltage.
Calculate the theoretical value of this frequency, using the output voltage levels observed
and the values of R1, R2, R3, and C measured in part 9.3. Compare this analytical value
with the actual value of the frequency of your astable multivibrator.