Harmonic Oscillator:

One feedback type harmonic oscillator circuit is shown in figure 1.

                                Figure 1                                                                                Figure  2

The DC equivalent circuit is shown in the figure 2. The DC operating point is set by selecting V_CC, R_B and R_E. The AC equivalent circuit is also shown in the figure 3.

Figure 3

The transformer provides 180° phase shift to guarantee the positive feedback so that at the required frequency of the oscillation, total phase shift from vin to vx is made equal to 0° and magnitudes are made equal by selecting turns ratio properly. R_E also controls and stabilizes the gain through negative feedback. C₂ and transformers equivalent inductance make up a resonant circuit which determines frequency of oscillation.

C₁  is used to block DC through low DC resistance of the transformer primary. C₁ has negligible reactance at frequency of oscillation, thus it is not part of the frequency determining network, and the same is applied to C₂.

In this circuit, there is active device suitably biased to provide the necessary gain. Since the active device produces loop phase shift 180º (from base to collector), a transformer in the feedback loop provides an additional 180º to yield to a loop phase shift of 0º. The feedback factor is equivalent to the transformer's turns ratio. There is a turned circuit also, to determine the frequency of oscillation.

The load is in parallel with C₂  and the transformer. If the load is resistive, which is usually the case, the Q of the tuned circuit and the loop gain are both affected, this must be taken into account when determining the minimum gain required for oscillation. If the load has a capacitive component, then the value of C₂ should be reduced accordingly.

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