Oscillators: Need of oscillator and difference between oscillator and amplifier

 

Oscillators:  Need, advantages, types of oscillator, Barkhausen criteria and difference between oscillator and amplifier 

 

Oscillator:

          Oscillator is electronic device which is used to generate output signals of specific frequency without using any input signal. Input to oscillator is DC voltage and output is like sinusoidal waveform hence we can say that oscillators are able to convert DC energy into AC energy.

 

Need of oscillator:

          Many electronics circuits needs source of energy which have specific frequency (from few Hz to several MHz) and oscillators are needed to achieve this.  Electronics equipment like RADAR (Radio Detection and Ranging), TV receivers, radio receivers, signal generators uses oscillators.  

 

Advantages of using oscillators:

1.     There are no moving parts in oscillator hence there is no noise, wear and tear.

2.     Oscillators can produce signal of wide frequency range ranging from 20Hz to several MHz

3.     The output frequency (oscillation frequency) can be changed easily

4.     Oscillators generally have good frequency stability

5.     Oscillators have high efficiency

 

Applications of oscillators:

1.     Signal generators

2.     RADAR

3.     TV receivers

4.     Radio receivers

 

Types of oscillators

          Depending upon the output signals, there are two basic types of oscillators.

1.     Sinusoidal/Harmonic oscillators

a.     Tuned oscillator/LC oscillators

i.       Hartley oscillator

ii.     Colpitts oscillator

b.     RC oscillator

i.       Phase shift oscillator

ii.     Wein bridge oscillator

c.      Crystal oscillator

2.     Non-sinusoidal/Relaxation oscillators

a.     UJT relaxation oscillator

b.     Astable multivibrator

 

Difference between oscillator and amplifier:

In last 3 articles, we have seen about feedback amplifiers and now all readers comes know that, positive feedback is used for oscillators where negative feedback is used in amplifiers. Following table provides some differences between oscillator and amplifier on the basis of some parameters.

S. N.	Parameter	Oscillator	Amplifier
1	Feedback	Positive	Negative
2	Total phase shift	3600	1800
3	External signal	Not needed	Needed
4	Importance	To signal frequency and shape	Amplification
5	Function	Generate signal	Amplify signal
6	Applications	RADAR, TV and radio receivers, signal generators	Audio amplifier, signal amplifiers

•             Conditions for oscillations (Barkhausen’s criteria)

To generate the signals of desired frequencies, oscillator must satisfy some conditions. These conditions are also called as “Barkhausen criteria”.

1.  Positive feedback is required

2.  (Total phase shift of circuit must be 0⁰ or 360⁰)

3.  The product of voltage gain and feedback must be greater than or equal to       one.

mV*Av = 1 (also represented as β)

where,

mV = Feedback

Av = Voltage gain without feedback

 Now voltage gain with feedback (Avf) is given by 

 

Avf = Av/(1-mV*Av)

 

or

 

Avf = Av/ (1-βAv)

If product of voltage gain and feedback must be greater than or equal to one, denominator in above equation tends to zero which means feedback gain Avf will increase and tends to infinite. The output frequency is now totally different than input and hence it works as oscillator.