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Activity: The Clapp Oscillator


Oscillators come in many forms. In this lab activity we will explore the Clapp configuration which uses a taped capacitor divider to provide the feedback path and a series LC resonator.


A Clapp oscillator is in effect a series tuned version of the Colpitts oscillator. The Clapp oscillator is much like a Colpitts oscillator with the capacitive voltage divider producing the feedback signal. The addition of a capacitor C3 in series with the inductor L1 results in the difference in the two designs and distinguishes the Clapp Oscillator from the Colpitts and Hartley configurations. 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. The frequency of oscillation can be calculated in the same way as any resonant circuit, using:

Ignoring the transistor capacitive effect between the base and collector, the resonant frequency may be calculated using the total equivalent capacitance (CTOT) given by:

Figure 1 shows a typical Clapp oscillator. The frequency determining series resonant tuned circuit is formed by L1 and CTOT and is used as the collector load impedance of the common base amplifier Q1. A large inductance, L2, provides a DC path for the collector current while presenting a high impedance at the resonate frequency. This gives the amplifier a high gain only at the resonant frequency. This configuration of the Hartley oscillator uses a common base amplifier, the base of Q1is biased to an appropriate DC level by resistor divider R1 and R2 but is connected directly to an AC ground by C4. In the common base mode the output voltage waveform at the collector, and the input signal at the emitter are in phase. This ensures that the fraction of the output signal from the node between C1 and C2, fed back from the tuned collector load to the emitter provides the required positive feedback.

Figure 1, Basic Clapp Oscillator

The combination of C1and C2 also forms a low frequency time constant with the emitter resistor R3 to provide an average DC voltage level proportional to the amplitude of the feedback signal at the emitter of Q1. This provides automatic control of the gain of the amplifier to give the closed loop gain of 1 required by the oscillator. The emitter resistor R3 is not decoupled because the emitter node is used as the common base amplifier input. The base is connected to AC ground by C4, which will provide a very low reactance at the oscillator frequency.

Pre Lab Simulations

Build a simulation schematic of the Clapp oscillator as shown in figure 1. Calculate values for bias resistors R1 and R2 such that with emitter resistor R3set to 500 Ω, the collector current in NPN transistor Q1 is approximately 1 mA. Assume the circuit is powered from a +10V power supply. Be sure to keep the sum of R1 and R2 ( total resistance greater than 10 KΩ) has high as practical to keep the standing current in the resistor divider as low as practical. Remember that C4 provides an AC ground at the base of Q1. Set base decoupling capacitor C4 and output AC coupling capacitor C5 to 0.1uF. Calculate a value for L1 such that the resonate frequency, with C1 set equal to 1nF and C2 set to 1 nF, will be close to 750 KHz. Use a high value for L3 of at least 10 mH. Perform a transient simulation. Save these results to compare with the measurements you take on the actual circuit and to include with your lab report.


ADALM2000 Active Learning Module
Solder-less breadboard, and jumper wire kit
1 - 2N3904 NPN transistor
1 - 1 uH inductor
1 - 10 uH inductor
1 - 100 uH inductor
1 - 10 mH inductor ( L3 )
1 - 1 nF capacitor ( C1 )
1 - 4.7 nF capacitor ( C2 )
2 - 0.1 uF capacitors ( marked 104 )
1 - 470 Ω resistor ( R3)
Other resistor, capacitors and inductors as needed


Build the Clapp Oscillator shown in figure 2 using your solder-less breadboard. Pick standard values from your parts kit for bias resistors R1 and R2 such that with emitter resistor R3set to 470 Ω, the collector current in NPN transistor Q1 is approximately 1 mA. Start with C1 = 1 nF and C2 = 4.7 nF. The frequency of the oscillator can be from around 500 KHz to 2 MHz depending on the values chosen for C1, C2, C3 and L1. Calculate a value for C3 and pick the closest value from your parts kit. This oscillator circuit can produce a sine wave output in excess of 10 Vpp at an approximate frequency set by the value chosen for L1.

Figure 2 Clapp Oscillator

Hardware Setup:

Figure 3 Clapp Oscillator Breadboard circuit

The green squares indicate where to connect the ADALM2000 module AWG, scope channels and power supplies. Be sure to only turn on the power supplies after you double check your wiring.


Having finished construction the Clapp oscillator check that the circuit is oscillating correctly by turning on both the + and - 5 V power supplies and connecting one of the oscilloscope channels to the output terminal. It may be found that the value of R3 is fairly critical, producing either a large distorted waveform or an intermittent low or no output. To find the best value for R3, it could be replaced by a 1 KΩ potentiometer for experimentation to find the value that gives the best wave shape and reliable amplitude.

A plot example using R1=10KΩ, R2=1KΩ, R3=100Ω, L1=100uH, L2=10uH, C1=1nF, C2=4.7nF, C3=10nF is presented in Figure 4.

Figure 4 Clapp Oscillator plot


Measure the peak to peak output voltage of the output. Measure the DC ( average ) level of the output waveform at the collector of Q1 and on the other (output) side of AC coupling capacitor C4. Measure the voltage at the emitter of Q1. Calculate the average emitter current by measuring the voltage across R3. Compare your measurements with your calculations and simulations. Measure the period (time T) of the output waveform and its frequency (1/T). Compare this measured frequency to what you calculated by:
F_R = 1 / {2 pi sqrt(LC)}.

Fill in the table below with the measured frequency for other L1 values for two different values of C3. Use the values in the table as suggested options but try to include as many different values as possible using series and parallel combinations of the inductors supplied in your parts kit. Any of the L1 optional values shown below should give reliable oscillation.

L1 OptionsC1 = 1nF L2 = 4.7nF C1 = 10nF C2 = 1nF
ValueFrequency with C3= ?Frequency with C3= ?
10 uH
20 uH
50 uH
100 uH

Lab Resources:

For Further Reading:

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