Build the Colpitts oscillator shown using either breadboard (proto board) or strip board, then test the oscillator’s operation using a multi meter and oscilloscope. The Colpitts Oscillator is a particularly good circuit for producing fairly low distortion sine wave signals in the RF range, 30kHz to 30MHz. Colpitts oscillator working, circuit diagram and theory. Colpitts oscillator frequency equation. Colpitts oscillator using transistor and opamp.

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Colpitts Oscillator Circuit diagram & working. Frequency equation. Colpitts oscillator using opamp

Other components in the circuit are the same as that owcillator in the case of common-emitter CE which is biased using a voltage divider network i. R C is the collector resistor, R E is the emitter resistor which is used to stabilize oxcillator circuit ozcillator the resistors R 1 and R 2 form the voltage divider bias network. Further, the capacitors C i and C o are the input and output decoupling capacitors while the emitter capacitor C E is the bypass capacitor used to bypass the amplified AC signals.

Here, as the power supply is switched ON, the transistor starts to conduct, increasing the collector current I C due to which the capacitors C 1 and C 2 get charged. On acquiring oscillatkr maximum charge feasible, they start to discharge via the inductor L.

During this process, the electrostatic energy stored in the capacitor gets converted into magnetic flux which in turn is stored within the inductor in the form of electromagnetic energy. Next, the inductor starts to discharge which charges the capacitors once again.


Likewise, the cycle continues which gives rise to the oscillations in the tank circuit. Further the figure shows that the output of the amplifier appears across C 1 and thus is in-phase with the tank circuit’s clpitt and makes-up for the energy lost by re-supplying it.

On the other hand, the voltage feedback to the transistor is the one obtained across the capacitor C 2which means the feedback signal is out-of-phase with the voltage at the transistor by o.

This is due to the fact that the voltages developed across the capacitors C 1 and C 2 are opposite in polarity as the point where they join is grounded. Further, this signal is provided with an additional phase-shift of o by the transistor which results in a net phase-shift of o around the loop, satisfying the phase-shift criterion of Barkhausen principle. The frequency of such a Colpitts Oscillator depends on the components in its tank circuit and is given by Where, the C eff is the effective capacitance of the capacitors expressed as.

As a result, these oscillators can be tuned either by varying their inductance or the capacitance. However the variation of L does not yield a smooth variation.

Hence they are usually tuned by varying the capacitances which are generally ganged, due to which a change in any one of them changes both of them. Nevertheless, the process is tedious and requires special large-valued capacitor.

Colpitts oscillator

Colpitt, the Colpitts oscillators are seldom preferred in the applications where in the frequency varies but are more popular as fixed frequency oscillators due to their simple design.


Oscilltor they offer better stability in comparison with the Hartley Oscillators as they are exempted from the mutual inductance effect present in-between the two inductors of the latter case.

Figure 2 shows such a Colpitts oscillator which uses an Op-Amp in inverting configuration in its amplifier section while the tank circuit remains similar to that in the case of Figure 1. This kind of circuit functions almost analogous to that of the one explained earlier. From this, it can be noted that, in this case, the gain of the circuit is less dependent on the circuit elements of the tank circuit. However they can even be used for microwave applications as their capacitors provide low reactance path for the high-frequency signals.

This results in better frequency stability as well as a better sinusoidal output waveform. Moreover, they are also extensively used as surface acoustical wave SAW resonators, sensors and in mobile and communication systems. Figure 1 shows a typical Colpitts oscillator with a tank circuit in which an inductor L is connected in parallel to the serial combination of capacitors C 1 and C 2 shown by the red enclosure.

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