The Colpitts Oscillator

 

The Colpitts oscillator is a discrete LC oscillator that uses a pair of tapped capacitors and an inductor to produce regenerative feedback. A Colpitts oscillator is shown in Figure 18-5. The operating frequency is determined by the tank circuit. By formula:

FIGURE 18-5 Colpitts oscillator.The key to understanding this circuit is knowing how the feedback circuit produces its 180° phase shift (the other 180° is from the inverting action of the CE amplifier). The feedback circuit produces a 180° voltage phase shift as follows:

1. The amplifier output voltage is developed across .
2. The feedback voltage is developed across .
3. As each capacitor causes a 90° phase shift, the voltage at the top of (the output voltage) must be 180° out of phase with the voltage at the bottom of (the feedback voltage).

The first two points are fairly easy to see. is between the collector and ground. This is where the output is measured. is between the transistor base and ground, or in other words, where the input is measured. Point three is explained using the circuit in Figure 18-6.



FIGURE 18-6Figure 18-6 is the equivalent representation of the tank circuit in the Colpitts oscillator. Let’s assume that the inductor is the voltage source and it induces a current in the circuit. With the polarity shown across the inductor, the current causes potentials to be developed across the capacitors with the polarities shown in the figure. Note that the capacitor voltages are 180° out of phase with each other. When the polarity of the inductor voltage reverses, the current reverses, as does the resulting polarity of the voltage across each capacitor (keeping the capacitor voltages 180° out of phase).
The value of the feedback voltage is determined (in part) by theof the circuit. For the Colpitts oscillator,is defined by the ratio of . By formula:

or

The validity of these equations is demonstrated in Example 18.1 of the text.
As with any oscillator, the product of must be slightly greater than 1. As mentioned earlier and . Therefore:

As with any tank circuit, this one will be affected by a load. To avoid loading effects (the circuit loses some efficiency), the output from a Colpitts oscillator is usually transformer-coupled to the load, as shown in Figure 18.14 of the text. Capacitive coupling is also acceptable so long as:


where is the total capacitance in the feedback network