Transistor Phase Shift Oscillator
The circuit arrangement of a phase-shift oscillator using N-P-N transistor in CE configuration is shown in figure. As usual, the voltage divider R1-R2 provides dc emitter base bias, RE and CE combination provides temperature stability and prevent ac signal degeneration and collector resistor RC controls the collector voltage. The oscillator output voltage is capacitively coupled to the load by Cc.
In case of a transistor phase shift oscillator, the output of the feedback network is loaded appreciably by the relatively small input resistance (hie) of the transistor. Hence, instead of employing voltage series feedback (as used in case of FET phase shift oscillator), voltage shunt feedback is used for a transistor phase shift oscillator, as shown in figure.In this circuit, the feedback signal is coupled through the feedback resistor R’ in series with the amplifier stage input resistance h^. The value of R’ should be such that when added with amplifier stage input resistance hie, it is equal to R i.e., R’ + hie = R.
Operation of Circuit
The circuit is set into oscillations by any random or variation caused in the base current, that may be either due to noise inherent in the transistor or minor variation in voltage of dc power supply. This variation in base current is amplified in collector circuit. The output of the amplifier is supplied to an R-C feedback network. The R-C network produces a phase shift of 180° between output and input voltages. Since CE amplifier produces a phase reversal of the input signal, total phase shift becomes 360° or 0° which is essential for regeneration or for sustained oscillations] The output of this network is thus in the same phase as the originally assumed input to the amplifier and is applied to the base terminal of the transistor. Thus sustained variation in collector current between saturation and cut-off values are obtained. R-C phase shift network is the frequency determining network, as already explained in case of FET phase-shift oscillator.
The phase shift oscillator is well suited to the range of frequencies from several hertz to several hundred kilohertz (20Hz to 200 kHz), and so includes the audio frequency range (upto 20 kHz). For generating different audio-frequencies, variable air capacitors are employed as circuit elements in the phase-shift network. It is possible to vary the frequency in the range of about 1 : 10 because the range of capacitors can be varied in the ratio of 10 : 1 (typically from 40 p F to 450 p F). For variations of frequency over a large range, the three capacitors are usually ganged so as to vary the capacitance of the three capacitors simultaneously. Such a variation keeps the input impedance to the phase-shift network constant and also keeps constant the magnitudes of β and αβ. Thus the amplitude of oscillations will remain unaffected as the frequency is adjusted. The phase-shift oscillator is operated in class A so as to keep distortion to the minimum. Frequency range from 20 Hz to 200 Hz, 200 Hz to 2 kHz, 2 kHz to 20 kHz and 20 kHz to 200 kHz can be obtained by using different set of resistors.
Phase-shift oscillators are not suitable for higher frequency operation because at higher frequency, the internal phase shift of the transistor and reduction in hfe cause difficulties in designing the circuit. The frequency of the oscillator cannot be changed easily.