01-10-2012, 04:44 PM
Oscillators
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An Electronic device, that generate oscillations (Signals), is called an oscillator. Simply says an oscillator receives DC
energy and converts it into AC energy of desired frequency. The frequency of oscillations depends up on the constants of the
device. Oscillators are extensively used in electronic equipments.
Oscillators can produce sinusoidal or non sinusoidal signals.
Basic Principles Of Sinusoidal Oscillator.
The basic structure of a sinusoidal oscillator consists of an amplifier and a frequency selective network connected in a positive
feedback loop as shown in fig.
A feedback amplifier is one that produce a feed back voltage Vf which is in phase with the input signal. A phase shift of
180o is produced by the amplifier and a further phase shift of 180o is introduced by the feedback network. Thus the signal is
shifted by 360o and fed to the input. That is feedback voltage is in phase with the input signal. But, oscillator is a circuit which
produces oscillations without any external signal source .A signal Vin is first supplied to the network and removed .Then a
feedback signal Vf is still applied to the input signal. The oscillator will respond to this signal and Vf will be amplified and send to
the o/p. the feed back n/w will send a portion of the o/p again back to the i/p. Hence the amplifier receives another i/p cycle
and another o/p cycle is produced. This process continues and amplifier will produce oscillations without any ext input.
A GENERAL FORM OF LC OSCILLATORS
Many resonant-circuit oscillators can be expressed by the general structure as shown below (fig
1).The active element may be an operational amplifier, a BJT amplifier or an FET. In the analysis that follows we
assume an active device with infinite input resistance.Fig2 shows a linear equivalent circuit of fig1, using an amplifier
with negative gain –Av and output resistance Ro. Clearly the topology is that voltage series feedback.
COLPITTS OSCILLATOR
The Colpitts oscillator is shown below. In this two capacitors are placed across a common inductor as shown below so
that C1, C2 and L forms the tank (tuned) circuit.
Working:
When the power supply is turned on, C1 and C2 get charged .These capacitors then discharge through
the coil L, setting up oscillations whose frequency depends on the values of L, C1,and C2.The oscillations across C1
are applied to the Base-Emitter junction and appears in the amplified form in the collector circuit. The amount of
feedback depends on the values of C1 and C2. Smaller the C1 the greater will be the feedback (from Eqn (5)). The
capacitors C1 and C2 act as a simple voltage divider. Therefore the points 1 and 2 are 180o out of phase. A further
180o phase shift is introduced by the amplifier. Hence a proper positive fed back is obtained (for sustained un-damped
oscillation).
HARTLEY OSCILLATOR
It is also an LC tuned oscillator and has the following advantages
Adaptability to wide range of frequencies.
Easy tuning.
Working:
A Hartley oscillator using BJT is shown below. When the
power supply is turned on due to some transient disturbances in the circuit the
collector current starts increasing and charges the capacitor C. C then discharges
through L1 and L2 setting up oscillations. These oscillations across L1 are applied
to the base-emitter junction and appear in the amplified form at the collector.
The coil L2 couples collector circuit energy back by means of mutual induction
b/w L1 and L2. In this way, energy is continuously supplied back to the tank (L1-
L2-C1) ckt to overcome the losses in it. Consequently continues un-damped
oscillations will obtain. The loop gain and the frequency of oscillations can be
found by preceding the steps as explained under the section 2.