Here three different topology of Colpitts oscillator are shown. There are more variation of the topology but these 3 topology are basic structure. The 3 topology differs in the way the feedback network(the two capacitors and one inductor of a Colpitts oscillator) are connected to the amplifier. Here a single BJT based amplifier is used. The different arrangement of the feedback network of the oscillator and hence topology is based on where the feedback network is connected to the BJT amplifier. The feedback network can be connected either at the output, the input or both the input and output.
So we have the following topology of the oscillator:
1. Feedback network at the output
2. Feedback network at the input
3. Feedback network at the input and output
1. Feedback network at the output
The following shows the circuit diagram of a Colpitts oscillator in which the feedback network is connected at the output.
The feedback network consisting of two capacitor and one inductor is shown inside the violet box. As can be seen the feedback network is connected at the output of the BC547 BJT amplifier. The frequency of oscillator is given by the component value of the two capacitor C1 and C2 and the inductor L1. The formula for the frequency of oscillation is,
\(f = \frac{1}{2*\pi \sqrt{L_1C}}\)
where, \(C=\frac{C_1 C_2}{C_1+C_2}\)
Using L1=1mH, C1=C2=0.1uF the frequency of oscillator is 22.5KHz. This can also be directly calculated using the online Colpitts oscillator calculator.
The capacitors CC1 and CC2 are the coupling capacitors which are used to prevent shifting of the biasing condition of the BJT amplifier. The capacitor CB is the bypass capacitor and is used to prevent voltage drop due to the ac signal that is developed across the emitter resistor RE. At the frequency of interest which is 22.5KHz, the reactance of the CB should be small so that the signal flows into the ground. The coupling capacitor CC2 at the output of the amplifier is not necessarily required. The resistors R1,R2, RE and RC sets the DC bias condition for the BJT amplifier. The values of these biasing resistors, coupling capacitors and the bypass capacitor can be directly calculated using the online BJT amplifier design calculator.
The following shows animation of how the Colpitts oscillator with Feedback network at the output works.
2. Feedback network at the input
Another way of connecting the feedback network in a Colpitts oscillator is to connect the feedback network at the input of the BJT amplifier as shown below.
The feedback network again consisting of two capacitors and one inductor is shown encircled in violet colored box. The frequency of oscillation is again the same as before giving by the equation,
\(f = \frac{1}{2*\pi \sqrt{L_1C}}\)
where, \(C=\frac{C_1 C_2}{C_1+C_2}\)
Again the BJT amplifier is biased using voltage divider biasing method for stability. A detailed tutorial on biasing BJT with voltage divider biasing method is explained in the tutorial How to bias a BJT using voltage divider biasing.
An animation of the Colpitts oscillator with feedback network connected at the input of the amplifier is shown below.
3. Feedback network at the input and output
The third topology of BJT Colpitts oscillator is a structure in which parts of feedback network is connected to the output and input. Such structure of Colpitts oscillator is shown below.
The parts of the feedback network connected at the input and output of the BJT amplifier are shown in violet boxes. As can be seen the capacitor C1 and inductor L1 of the feedback network are connected to the output while the C2 capacitor of the feedback network is connected to the input.
The frequency of oscillation is again the same as before. With the values indicated the frequency of oscillation is 22.5KHz. Also the BJT amplifier design was already explained above.
An animation of how the Colpitts oscillator with feedback network parts connected to the input and output works is shown below.
See the following video demonstration of the different Colpitts oscillator structure works with simulation in Proteus.
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