Transistor Clapp Oscillator

Author: Leonard Krugman

The transistor equivalent of the Clapp oscillator is illustrated in Fig. 6-3 (A). The operating frequency is set by the series resonant circuit in the collector circuit. Feedback is taken from the voltage divider consisting of capacitors C₁ and C₂. The upper frequency limit depends largely on the transistor in use, and can be increased considerably by careful selection of the unit. Upper frequencies as high as 3 mc can be attained using typical junction types in this basic circuit. The numerical values shown are based on the average of a group of Raytheon CK720 transistors. If crystal control is used, the frequency stability is improved, and there is also a considerable increase in the upper frequency limit of the oscillator. One method of achieving crystal control in this circuit is to replace the collector resonant circuit by a crystal.

Fig. 6-3. (A) Transistor Clapp oscillator.