Mumbai University > Electronics Engineering > Sem 4 > Discrete Electronic Circuits

Marks: 5M

Year: May 2015, Dec 2015, May 2016

Figure 1 Block Diagram of Positive feedback

• Figure 1 shows the diagrammatic view of Barkhausen’s criteria. In this Vs is the input signal which is zero, Vin is the noise voltage which act as input voltage and Vf is the feedback voltage, Vo is the output voltage.
• Oscillator is a circuit which converts dc signal into ac signal. It consist of voltage amplifier (small signal amplifier) and positive feedback network.
• The circuit with amplifier and positive feedback can produce sustained oscillations of desired frequency that satisfy barkhausen’s criteria.

Barkhausen’s Criteria :

In order to produce sustain oscillation following criteria should be satisfied :-

1. The total phase shift around close loop should be 360º or 0º i.e the total phase shift afforded by the circuit as signal proceeds from input terminal to output terminal and to the feedback network and back to input terminal is 360º or 0º.
2. Oscillations will be developed only if the product of magnitude of amplifer gain i.e. A and feedback factor of the feedback network i.e. β is unity. Hence for sustained oscillation |Aβ|=1.

                    |Aβ|>1
                                                       |Aβ|=1