Clapp oscillator circuit is a slight modification of the Colpitt oscillator circuit configuration. The feedback circuit in the case of Clapp oscillator uses an extra capacitor (refer to figure below) in series with the coil. The function of C3 is to minimize the effect of junction capacitance on the operating frequency.
Clapp oscillator
The operating frequency (f) of a Clapp oscillator is given by
If C3 is chosen to be much smaller than either C1 or C2, then expression for frequency (f) simplifies to
C1 and C2 provide the required phase shift for the regenerative feedback.
Clapp oscillator is preferred over Colpitt oscillator for designing variable frequency oscillators as the tuning element (C3 in Clapp oscillator) adjustment does not alter the attenuation factor in Clapp oscillator. The attenuation factor is decided by C1 and C2. In the case of Colpitt oscillator, any attempt to vary the frequency by varying either C1 or C2 might cause cessation of oscillations over a portion of desired frequency range.