An Amplitude Stabilized Oscillator Circuit
Stabilized Colpitts Oscillator
Often when you are designing a radio receiver or transmitter circuit an oscillator with constant output would enable uniform circuit behavior over a wide frequency range. Unfortunately most schemes to stabilize the output of an oscillator are rather complex. There is however a simple way of providing constant output over a wide range of frequencies and supply voltage variation.
Figure 1 shows an amplitude stabilized Colpitts oscillator. Q1 forms the basis of the conventional Colpitts circuit. The addition of the components R1, C1 and Q2 create a feedback control loop that keeps the output amplitude constant.
As the level of oscillation increases the average current through the collector of Q1 increases. The voltage across R1 increases and C1 filters out the RF component. As a result Q2 starts to conduct more thereby increasing the voltage at R2. This causes less current to flow through Q1 and completes the feedback control loop. The constraint on the current flowing through Q1 stabilizes the output amplitude of the oscillator at about 1 volt peak to peak. The output voltage is stable to within about 20% over a wide tuning range. This make the circuit very suitable for use with direct conversion receivers with diode mixers and also for conventional super heterodyne circuits.
The output amplitude can be controlled by adjusting R1 over a reasonable range for example 1k to 10k. At a DC level the circuit is similar to the Complementary Feedback Pair (CFP) circuit (see figure 2).
The CFP circuit is basically an emitter follower circuit with an added PNP transistor doing the heavy current lifting. The output impedance is lower than with the basic emitter follower and the distortion level is lower too. The resistor R3 is sometimes necessary to prevent parasitic oscillations that can occur if the PNP transistor has a higher high frequency response than the NPN transistor.
The addition of 3 extra components to the basic Colpitts oscillator circuit allows you to create radio receiver and transmitter circuits with much more consistent behavior over a wide range of frequencies. The same modification can also be used with the very similar Clapp oscillator circuit.