Transistor Colpitts Oscillator

Author: Leonard Krugman

The transistor Colpitts oscillator is similar to the Clapp type except that the resonant load is a parallel arrangement in the collector circuit. Thus the circuit becomes voltage, rather than current, controlled. The feedback is again taken from a point between the two series capacitors connecting the collector to ground. The upper frequency limit for this oscillator is in the same range as that of the current-controlled Clapp arrangement. The typical values illustrated in Figure 6-3 (B) are again based on the average of a small group of Raytheon CK720 transistors.

Fig. 6-3. (B) Transistor Calpitts oscillator.

In both cases, the parallel combination C_bRb provides the necessary emitter bias. This arrangement provides some degree of amplitude stability similar to the control provided by bypassed cathode or grid leak resistors in vacuum-tube oscillator circuits.

In servicing transistor oscillators, the emitter bias measured at the base end of the C_BR_B combination is a useful indication of the signal amplitude. In addition, the variation of the emitter bias over the frequency range indicates the relative uniformity of the signal output. Special care is necessary during these measurements to avoid affecting circuit operation. A vacuum-tube voltmeter may be used without causing additional loading. The use of a high resistance meter also minimizes that oscillator loading due to the stray reactance of the measuring probe. While direct current measurement is better, it requires disturbing the circuit wiring.