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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 C_{3} in series with the inductor L_{1} 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 (C_{TOT}) given by:

Figure 1 shows a typical Clapp oscillator. The frequency determining series resonant tuned circuit is formed by L_{1} and C_{TOT} and is used as the collector load impedance of the common base amplifier Q_{1}. A large inductance, L_{2}, 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 Q_{1}is biased to an appropriate DC level by resistor divider R_{1} and R_{2} but is connected directly to an AC ground by C_{4}. 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 C_{1} and C_{2}, fed back from the tuned collector load to the emitter provides the required positive feedback.

Figure 1, Basic Clapp Oscillator

The combination of C_{1}and C_{2} also forms a low frequency time constant with the emitter resistor R_{3} to provide an average DC voltage level proportional to the amplitude of the feedback signal at the emitter of Q_{1}. 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 R_{3} is not decoupled because the emitter node is used as the common base amplifier input. The base is connected to AC ground by C_{4}, which will provide a very low reactance at the oscillator frequency.

Build a simulation schematic of the Clapp oscillator as shown in figure 1. Calculate values for bias resistors R_{1} and R_{2} such that with emitter resistor R_{3}set to 500 Ω, the collector current in NPN transistor Q_{1} is approximately 1 mA. Assume the circuit is powered from a +10V power supply. Be sure to keep the sum of R_{1} and R_{2} ( 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 C_{4} provides an AC ground at the base of Q_{1}. Set base decoupling capacitor C_{4} and output AC coupling capacitor C_{5} to 0.1uF. Calculate a value for L_{1} such that the resonate frequency, with C_{1} set equal to 1nF and C_{2} set to 1 nF, will be close to 750 KHz. Use a high value for L_{3} 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 ( L_{3} )

1 - 1 nF capacitor ( C_{1} )

1 - 4.7 nF capacitor ( C_{2} )

2 - 0.1 uF capacitors ( marked 104 )

1 - 470 Ω resistor ( R_{3})

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 R_{1} and R_{2} such that with emitter resistor R_{3}set to 470 Ω, the collector current in NPN transistor Q_{1} is approximately 1 mA. Start with C_{1} = 1 nF and C_{2} = 4.7 nF. The frequency of the oscillator can be from around 500 KHz to 2 MHz depending on the values chosen for C_{1}, C_{2}, C_{3} and L_{1}. Calculate a value for C_{3} 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 L_{1}.

Figure 2 Clapp Oscillator

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 R_{3} is fairly critical, producing either a large distorted waveform or an intermittent low or no output. To find the best value for R_{3}, 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 R_{1}=10KΩ, R_{2}=1KΩ, R_{3}=100Ω, L_{1}=100uH, L_{2}=10uH, C_{1}=1nF, C_{2}=4.7nF, C_{3}=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 Q_{1} and on the other (output) side of AC coupling capacitor C_{4}. Measure the voltage at the emitter of Q_{1}. Calculate the average emitter current by measuring the voltage across R_{3}. 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:

.

Fill in the table below with the measured frequency for other L_{1} values for two different values of C_{3}. 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 L_{1} optional values shown below should give reliable oscillation.

L_{1} Options | C_{1} = 1nF L_{2} = 4.7nF | C_{1} = 10nF C_{2} = 1nF |
---|---|---|

Value | Frequency with C_{3}= ? | Frequency with C_{3}= ? |

10 uH | ||

20 uH | ||

50 uH | ||

100 uH |

**Lab Resources:**

- Fritzing files: clapp_osc_bb
- LTspice files: clapp_osc_ltspice

**For Further Reading:**

http://en.wikipedia.org/wiki/Clapp_oscillator

http://en.wikipedia.org/wiki/Barkhausen_stability_criterion

**Return to Lab Activity Table of Contents.**

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