Page 47 - Semester-IV-Electronics

 The gain A of the amplifier and the feedback factor β can be complex

quantities.

 If Aβ = (1 + j0), then putting in equation of A , then A becomes infinite.
f
f
 In this situation, V = 0 and V = V , input for the amplifier is totally supplied
i if f
by feedback. The feedback amplifier under this condition acts as an

oscillator.

 Hence, for self-excited oscillations by Aβ = (1 + j0),

 (i) The magnitude of feedback factor should be unity i.e., = .

 (ii) The total phase shift in the amplifier feedback loop should be zero or an

integral multiple of 2π, the feedback must be positive.

 The above condition for self-excitation is called Barkhausen Condition.

 In actual case, in the beginning of oscillations > 1 so that oscillations

build-up rapidly and after that = 1 for maintaining oscillations. In an

oscillator, this arrangement is automatic.

 Due to large amplitude of oscillations, the harmonics are also developed by

nonlinear distortion but by using an oscillatory circuit having a high value of

Q, almost pure oscillations are obtained.
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