Oscillators generate an output signal without an input signal by converting DC power to AC power. They produce periodic waveforms like sine, square, triangle, and sawtooth waves. The Wien bridge oscillator is a two-stage RC coupled amplifier circuit that is stable at its resonant frequency. It uses a feedback circuit of series and parallel RC networks to produce a phase shift. Monostable multivibrators have one stable and one quasi-stable state. They switch to the quasi-stable state when triggered externally and return to the stable state after a set time period determined by resistor-capacitor values in the circuit.

1. Oscillator

2.  Oscillators are electronic circuits
that generate an output signal
without the necessity of an input
signal.
 It produces a periodic waveform
on its output with only the DC
supply voltage as an input.

3.  It converts the dc power from the source
to ac power in the load.
 A rectifier circuit converts ac to dc power,
but an oscillator converts dc signal/power
to its ac equivalent.
 The output voltage can be either
sinusoidal or nonsinusoidal, depending on
the type of oscillator.
 Different types of oscillators produce
various types of outputs including sine
waves, square waves, triangular waves, and
sawtooth waves.

16.  The Wien Bridge Oscillator is so called because the circuit is
based on a frequency-selective form of the Wheatstone bridge
circuit. The Wien Bridge Oscillator is a two-stage RC coupled
amplifier circuit that has good stability at its resonant frequency,
low distortion and is very easy to tune making it a popular circuit as
an audio frequency oscillator but the phase shift of the output signal
is considerably different from the previous phase shift RC
Oscillator.
 The Wien Bridge Oscillator uses a feedback circuit consisting
of a series RC circuit connected with a parallel RC of the same
component values producing a phase delay or phase advance circuit
depending upon the frequency. At the resonant frequency ƒ0 the
phase shift is 0o. Consider the circuit below.

21.  Advantages of Wien-Bridge Oscillator:
 1. The frequency of oscillation can be easily
varied just by changing RC network
 2. High gain due to two-stage amplifier
 3. Stability is high
 Disadvantages of Wien-Bridge Oscillator
 The main disadvantage of the Wien-bridge
oscillator is that a high frequency of
oscillation cannot be generated.

32. Astable Multivibrator: This multivibrator will not be stable in both the states;
it will rapidly get switches from one state to another state. This type of
multivibrator is mainly used as oscillators. Astable multivibrator is also called
as free running multi vibrators there is no limit or time for switching the
states.
Monostable Multivibrator: Monostable is also called one shot multivibrator.
In monostable multivibrator there is one stable state and one astable state.
This multivibrator need a trigger (external signal) to enter into the astable
state and get back to the stable state after some time period. The time period
will be set by the user, the monostable multivibrator is mainly used as timer.
Bi-Stable Multivibrator: This circuit has both the stable states. Valid trigger
will change state; it will be in that state only up to the next trigger. This circuit
is mainly used as flip-flops.

33.  Monostable multivibrator has one stable state and one quasi stable
state (astable state). When an external trigger applied to the circuit,
the multivibrator will jump to quasi stable state from stable state.
After the period of time it will automatically set back to the stable
state, for returning to the stable state multivibrator does not require
any external trigger. The time period to returning to stable state
circuit is always depends on the passive elements in the circuit
(resistor and capacitor values

35. Circuit Operation:
 When there is no external trigger to the circuit the one
transistor will be in saturation state and other will be in cutoff
state. Q1 is in cutoff mode and put at negative potential until
the external trigger to operate, Q2 is in saturation mode.
 Once the external trigger is given to the input Q1 will get turn
on and when the Q1 reaches the saturation the capacitor
which is connected to the collector of Q1 and base of Q2 will
make transistor Q2 to turn off. This is state of turn off Q2
transistor is called astable stable or quasi state.
 When capacitor charges to VCC the Q2 will turn on again and
automatically Q1 is turn off. So the time period for charging
of capacitor through the resistor is directly proportional to
the quasi or astable state of multivibrator when a external
trigger occurred (t=0.69RC).
Uses of Monostable Multivibrator:
 The monostable multivibrators are used as timers, delay
circuits, gated circuits etc.