Like most oscillator circuits, the Colpitts oscillator exists in several forms, with the most common form being similar to the transistor circuit above. Then to summarise, the Colpitts Oscillator consists of a parallel LC resonator tank circuit whose feedback is achieved by way of a capacitive divider. Of course any op-amp or FET used as the amplifying device must be able to operate at the required high frequencies. Also due to these capacitive reactance properties the FET based Colpitts oscillator can operate at very high frequencies. The advantages of the Colpitts Oscillator over the Hartley oscillators are that the Colpitts oscillator produces a more purer sinusoidal waveform due to the low impedance paths of the capacitors at high frequencies. Resistor R3 provides the required feedback to the LC tank circuit. A minimum gain of 2.9 is required to start oscillations. Note that being an inverting amplifier configuration, the ratio of R2/R1 sets the amplifiers gain. The oscillation frequency for a Colpitts Oscillator is given as: Determine the frequency of oscillations of the circuit, the feedback fraction and draw the circuit. This ratio is called the “feedback fraction” and is given simply as:Ī Colpitts Oscillator circuit having two capacitors of 24nF and 240nF respectively are connected in parallel with an inductor of 10mH. Then the amount of feedback developed by the Colpitts oscillator is based on the capacitance ratio of C1 and C2 and is what governs the the excitation of the oscillator. However, large amounts of feedback may cause the output sine wave to become distorted, while small amounts of feedback may not allow the circuit to oscillate. Therefore, by changing the values of capacitors, C1 and C2 we can adjust the amount of feedback voltage returned to the tank circuit.
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Then the voltage across C1 will be much greater than that across C2.
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We can see that the voltage across C1 is the the same as the oscillators output voltage, Vout and that the voltage across C2 is the oscillators feedback voltage. The amount of feedback depends on the values of C1 and C2.
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The additional 180 o phase shift require for oscillation is achieved by the fact that the two capacitors are connected together in series but in parallel with the inductive coil resulting in overall phase shift of the circuit being zero or 360 o. The configuration of the transistor amplifier is of a Common Emitter Amplifier with the output signal 180 o out of phase with regards to the input signal.
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The frequency of oscillations for a Colpitts oscillator is determined by the resonant frequency of the LC tank circuit and is given as: These two capacitances are generally “ganged” together to provide a constant amount of feedback so that as one is adjusted the other automatically follows.
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The amount of feedback is determined by the ratio of C1 and C2. The required external phase shift is obtained in a similar manner to that in the Hartley oscillator circuit with the required positive feedback obtained for sustained undamped oscillations. A radio-frequency choke (RFC) is used in the collector circuit to provide a high reactance (ideally open circuit) at the frequency of oscillation, ( ƒr ) and a low resistance at DC to help start the oscillations. Resistors, R1 and R2 provide the usual stabilizing DC bias for the transistor in the normal manner while the additional capacitors act as a DC-blocking bypass capacitors. The oscillations across the capacitors are applied to the base-emitter junction and appear in the amplified at the collector output. When the power supply is firstly applied, capacitors C1 and C2 charge up and then discharge through the coil L. The emitter terminal of the transistor is effectively connected to the junction of the two capacitors, C1 and C2 which are connected in series and act as a simple voltage divider.