The Colpitts Oscillator

  
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A typical Colpitts oscillator design. This circuit is very similar in operation to the Hartley oscillator described in Oscillators Module 2. 1 but the Colpitts LC tank circuit consists of a single inductor and two capacitors. The capacitors form in effect, a single `tapped` capacitor instead of the tapped inductor used in the Hartl
The Colpitts Oscillator - schematic

ey. The values of the two capacitors (connected in series) are chosen so their total capacitance in series(CTOT), is given by: This gives the total capacitance necessary for the tank circuit to achieve parallel resonance at the required frequency. The frequency of oscillation is given by the same formula as for the Hartley oscillator: The individual values of C2 and C3 are chosen so that the ratio of the values produces the necessary proportion of feedback signal. However, the ratio of voltages across two capacitors in series is in inverse proportion to the ratio of the values, i. e. the smaller capacitor has the larger signal voltage across it. The main advantage of the Colpitts arrangement, is that the single inductor in the tuned circuit removes the effect of any mutual inductance between two coils where the alternating magnetic field built up around one inductor induces a current into the their inductor. This would affect the total inductance of the coils and so changes the resonant frequency of the tuned circuit. The circuit in Fig. 2. 3. 2 is the Colpitts equivalent of the Common Emitter Hartley Oscillator described in Oscillators Module 2. 1 (Fig. 2. 1. 4). It uses a common emitter amplifier, and as the tuned (tank) circuit tapping point in this configuration is connected to ground, the tank circuit produces anti-phase waves at top and bottom of L2, which ensures the correct phase relationships for positive feed back...



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