Experiment 9: The Clapp Oscillator
Purpose and Discussion
The purpose of this simulation is to demonstrate the characteristics and operation of a
Clapp oscillator. The Clapp oscillator is much like a Colpitts oscillator with the
capacitive voltage divider producing the feedback signal. The addition of a capacitor
C in series with the inductor L1 results in the difference in the two designs and is what
makes the Clapp Oscillator unique. As with all oscillators, the Barkhausen criteria
must be adhered to requiring a total gain of one and a phase shift of zero degrees from
input to output.
Ignoring the transistor capacitive effect between the base and collector, the resonant
frequency may be calculated using the equivalent capacitance:
CEQ =
1
1 1
1
+
+
C C1 C 2
But since C is typically much smaller than C1 and C2, the effects of C1 and C2 become
negligible and:
fr =
1
2π LC
As stated above, it is the addition of and the small value of C that creates the Clapp
oscillator’s unique characteristic of not being influenced by stray and transistor
capacitances which would otherwise alter the values of C1 and C2. This results in a
much more stable oscillator whose accuracy is dependable. The range of frequency of
operation is limited in a Clapp oscillator but nevertheless, its reliability makes it a
popular design. C1 and C2 may be adjusted for optimum feedback. The frequency of
oscillation is altered through the adjustment of C.
Parts
DC 10 V Supply
Transistor: BJT 2N4401
Resistors: 20 kΩ, 3.9 kΩ, 1.2 kΩ
Inductor: 2.4 mH, 68 µH
Capacitor: 12 nF, 750 pF, 3.9 nF, 120 pF
Test Equipment
•
•
Oscilloscope
Spectrum Analyzer
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Understanding RF Circuits with Multisim
Formulae
Frequency of Oscillation
fc =
1
2π LC
Equation 9-1
Procedure
Figure 9-1
1. Connect the circuit components illustrated in Figure 9-1.
2. Double-click the Oscilloscope to view its display. Set the time base to 500 ms/Div
and Channel A to 5 V/Div. Select Auto triggering and DC coupling. Set to AC
coupling.
3. Select Simulate/Interactive Simulation Settings, and select Set to Zero for Initial
Conditions. Check maximum time step and set to 3.6 e-008.
4. Start the simulation. The oscillator will require about 20 seconds to stabilize.
Measure the oscillation frequency. Calculate the value of C necessary to achieve
an oscillation frequency of 2 MHz. Change the value of C by double-clicking on
it and run the simulation to verify your results.
5. Compare data with theoretical calculations and complete Table 9-1.
6. Stop the simulation and place a Spectrum Analyzer on the workspace.
7. Connect the output lead of the oscillator to the input of the Spectrum Analyzer.
8. Double-click to open the Spectrum Analyzer window.
The Clapp Oscillator
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9. Press Set Span. Set Start = 1 MHz, End = 4 MHz, Amplitude = LIN and Range =
1V/DIV. Press Enter.
10. Restart the simulation. When the oscillation has stabilized, drag the red marker to
the position of the spectrum line observed. Note the frequency in the lower left
corner of the Spectrum Analyzer window.
fc =
Expected Outcome
Figure 9-2 Oscilloscope Display of Initial Clapp Oscillations
Data for Experiment 9
Measured Value
Calculated Value
fc (step 2)
fc (step 3)
Table 9-1
Additional Challenge
Replace C with a variable capacitor. Highlight C, right-click and choose delete. Select
a variable capacitor from the parts bin and set a value of 120 pF. Highlighting, then
pressing “a” or “A” will alter its capacitance ratio. Determine the upper frequency
limit possible through the varying of C.
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Understanding RF Circuits with Multisim