2-20
Figure 2-15.—Colpitts oscillator.
Figure 2-16 shows a common-base Colpitts oscillator using a pnp transistor as the amplifying device.
Notice in this version of the Colpitts oscillator that regenerative feedback is obtained from the tank circuit
and applied to the emitter. Base bias is provided by resistor RB and RF. Resistor R
C
is the collector load
resistor. Resistor RE develops the input signal and also acts as the emitter swamping resistor. The tuned
circuit consists of C1 and C2 in parallel with the primary winding of transformer T1. The voltage
developed across C2 is the feedback voltage. Either or both capacitors may be adjusted to control the
frequency. In the common-base configuration there is no phase difference between the signal at the
collector and the emitter signal. Therefore, the phase of the feedback signal does not have to be changed.
When the emitter swings negative, the collector also swings negative and C2 charges negatively at the
junction of C1 and C2. This negative charge across C2 is fed back to the emitter. This increases the
reverse bias on Q1. The collector of Q1 becomes more negative and C2 charges to a negative potential.
This feedback effect continues until the collector of Q1 is unable to become any more negative. At that
time the primary of T1 will act as a source because of normal tank circuit operation. As its field collapses,
the tank potential will reverse and C1 and C2 will begin to discharge. As C2 becomes less negative, the
reverse bias on Q1 decreases and its collector voltage swings in the positive direction. C1 and C2 will
continue to discharge and then charge in a positive direction. This positive-going voltage across C2 will
be fed back to the emitter as regenerative feedback. This will continue until the field around the primary
of T1 collapses. At that time the collector of Q1 will be at a maximum positive value. C1 and C2 will
begin to discharge and the potential at their junction will become less positive. This increases the reverse
bias on Q1 and drives the collector negative, causing C1 and C2 to charge in a negative direction and to
repeat the cycle.
