Transistor Phase Shift Oscillator:

At the low frequencies (around 100 kHz or less), resistors and capacitors are usually employed to determine the frequency of oscillation. Figure 1 shows transistorized phase shift oscillator circuit employing RC network. If phase shift through the common emitter amplifier is 180°, then the oscillation may take place at the frequency where the RC network produces an additional phase shift of 180°.

Since the transistor is used as an active element, the output across R of feedback network is shunted by the relatively low input resistance of transistor, because input diode is a forward biased diode

Figure 1

Hence,  instead of employing  the voltage series  feedback,  voltage shunt  feedback  is used  for the transistor phase shift oscillator. The load resistance RL  is also connected by the coupling capacitor. The equivalent circuit using h-parameter is shown in the figure 2.

Figure 2

For the circuit, load resistance R_L  may be lumped with the R_C  and effective load resistance becomes R'_L(= R_C  || R_L). The 2 h-parameters of the CE transistor amplifier, hoe  and hre  can be neglected.

The capacitor C provides some impedance at frequency of oscillation and, thus, it is kept as it is, while coupling capacitor behaves like AC short. The input resistance of transistor is R_i≈h_ie. Thus the resistance R₃ is selected so that R=R₃+R_i=R₃+h_ie. This choice makes 3 R _C selections alike and simplifies calculation. The effect of biasing resistor R₁, R₂, & RE_on the circuit operation can be neglected.

As this is a voltage shunt feedback, hence instead of finding V_R  /V_O, we should find current gain of the feedback loop.

The simplified equivalent circuit is shown in the figure 3.

Figure 3

Applying KVL,

Since I₃ and I_b must be in phase to satisfy the Barkhausen criterion, hence

Initially I₃ > I_b also, therefore, for the oscillation to start,

Therefore, the 2 conditions should be satisfied for the oscillation to begin and sustain.  

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