The Clapp oscillator uses a single inductor and three capacitors to set its frequency. The Clapp oscillator is often drawn as a
Colpitts oscillator that has an additional capacitor () placed in series with the inductor. The oscillation frequency in Hertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor (
FET), is : f_0 = {1 \over 2\pi} \sqrt{ {1 \over L} \left( {1 \over C_0} + {1 \over C_1} + {1 \over C_2} \right)} \ . The capacitors and are usually much larger than , so the term dominates the other capacitances, and the frequency is near the series resonance of and . Clapp's paper gives an example where and are 40 times larger than ; the change makes the Clapp circuit about 400 times more stable than the Colpitts oscillator for capacitance changes of . Capacitors , and form a voltage divider that determines the amount of feedback voltage applied to the transistor input. Although the Clapp circuit is used as a variable frequency oscillator (
VFO) by making a variable capacitor, Vackář states that the Clapp oscillator "can only be used for operation on fixed frequencies or at the most over narrow bands (max. about 1:1.2)." The problem is that under typical conditions, the Clapp oscillator's loop gain varies as , so wide ranges will overdrive the amplifier. For VFOs, Vackář recommends other circuits. See
Vackář oscillator.
Practical example The schematic shows an example with component values. Instead of
field-effect transistors, other active components such as
bipolar junction transistors or
vacuum tubes, capable of producing gain at the desired frequency, could be used. The
common drain amplifier has a high input impedance and a low output impedance. Therefore the amplifier input, the gate, is connected to the high impedance top of the LC circuit C0, C1, C2, L1 and the amplifier output, the source, is connected to the low impedance tap of the LC circuit. The
grid leak C3 and R1 sets the
operating point automatically through
grid leak bias. A smaller value of C3 gives less
harmonic distortion, but requires a larger load resistor. The supply current for J1 flows through the
radio frequency choke L2 to ground. The oscillator radio frequency current uses C2, because for the oscillator frequency this component has less
reactance. The load resistor RL is part of the simulation, not part of the circuit. == References ==