3398356--oscillators.ppt

7/28/2019 3398356--Oscillators.ppt

1/16


2/16

2

Oscillators

It converts dc power supply to the ac power in the load ( just

opposite to rectifier )

It incorporates active and passive components

It delivers an output voltage of given waveform without the

application of an external input signal

Classification of Oscillators

S. Kal, IIT-Kharagpur


3/16

3

Oscillators



4/16

4

Let switch S1 be closed and S2 be opened initially. (f=0)i = s and f= B() 0 = B() . A() . ior f /I = B() . A() = open loop gain

Principle of Sinusoidal Oscillation



5/16

5

Both A and B are functions of frequency. If for a particularfrequency, = 0 , B(0 ) . A(0 ) = 1 then f = i = s

Now if S1 is open and S2 is closed to close the loop, then sincef = i at = 0 , the feed back signal will be in phase with theinput signal and has the same magnitude. Hence the system

will sustain oscillation at the particular frequency 0 ( = 2f ),even ifs is withdrawn.

The condition of oscillation, also called Barkhausen criterion,

is

B (0) . A (0) = 1




6/16

6

Since A and B are complex quantities, it gives twoalternative sets of conditions

1) Re [B(0 ) . A(0 )] = 1 B(0 ) . A(0 ) = 12) Im [B(

0

) . A(0

)] = 0 B(0

) . A(0

) = 0

The first condition means that the signal fed back to the

input should be of the same magnitude as the input signal,

while the second condition dictates that the feedback should

be positive with zero phase shift.

The second condition determines the frequency of

oscillation.




7/16

7

If the first condition is satisfied, but not the second condition,

oscillation will die out ( or decay ) because the input signal willgradually decay due to phase cancellation of signals fed backto the input after successive trips round the loop.

The practical oscillators do not require an input signal, s totrigger oscillation.Then how does oscillation grow ? And, from what ?




8/16

8

The random movement of electrons in conductors andresistors, random emission of carriers in a transistor anddiode, random electron-hole recombination phenomena etc.produce random fluctuation of voltage of very smallmagnitude ( nV - V range ) called electrical noise.

Noise has a broad spectrum consisting of all frequencies.

The noise voltage at = 0 is the starting or triggering signalfrom which oscillation grows. Other frequency components cannot grow because they do

not satisfy the phase reinforcement condition, viz., net phaseshift = 0.

For the starting of oscillation, in fact, AB should beslightly greater than unity. But in the steady state, AB = 1and AB = 0.




9/16

9

Thus the condition of oscillations are : (i) the magnitude of

the loop gain must be equal to unity and (ii) the feed back

must be of regenerative type ( positive feed back , phase of

AB is either 0 or integer multiples of 3600 )




10/16

10

The basic amplifier ( CE ) provides a phase shift of 1800

, and thefeed back network provides another 1800 of phase shift, so that

the total phase shift is 3600 or 00 ( Note that any integral multiple

of 2 or 3600 is equivalent to 00 phase shift).

Some Oscillator Circuits

1. RC Phase Shift Oscillator ( using BJT )



11/16

11

The maximum phase shift provided by each CR section of the

feedback network is limited to 900 for which RC 0 ( = tan-11/CR for each RC section). R and C are adjusted such thateach section provides a phase shift of 600 at the oscillationfrequency. So, at least three CR sections will be required to

produce a phase shift of 1800.

In this connection, feedback signal is coupled through the feed

back resistor R` in series with the amplifier stage inputresistance ( Ri ) such that (R`+Ri = R).

The frequency of oscillation is given byfo = 1/ [2RC ( 6 + 4Rc / R)]

The condition of oscillation is given by

hfe(min) = 4Rc/ R + 23 + 29.R / RcS. Kal, IIT-Kharagpur


12/16

12

The output of the Op Amp is fed to a three stage RC networkwhich provides the needed 1800 of phase shift (at an attenua-tion factor of 1/29) . If the Op Amp provides gain ( set by

resistors R1 and Rf , A = - R1/ Rf) of greater than 29, a loop gaingreater than unity results and the circuit acts as an oscillator.

The frequency of oscillation is given by,

f0 = 1 / [ 2RC6 ]

2. RC Phase Shift Oscillator using Op Amp



13/16

13

A balanced bridge is used as a feed back network, which isWien bridge

The active element is an op Amp which has a very largepositive voltage gain (non-inverting mode) Av, negligibleoutput resistance, very high input resistance. It is furtherassumed that Av is constant over the range of frequency ofoperation of this circuit.

3. Wien Bridge Oscillator



14/16

14

Analysis of the bridge results,R3/R4 = R1/R2 + C1/C2

Frequency of oscillation is given by :

f0 = 1 / (2 R1R2C1C2 )

In practical circuit,R1 = R2 = R ( say) and C1 = C2 = C (say) f0 = 1 / 2RC and R3 /R4 = 2 or R3 = 2 R4

Thus a ratio of R3/R4 greater than 2 will provide

sufficient loop gain for circuits to oscillate at thefrequency, f0 = 1 / 2RC.

3. Wien Bridge Oscillator



15/16

15

4. High Frequency Tuned Oscillators

Hartley Oscillator Colpitts Oscillator

f0 = 1/ (2 LC) f0 = 1/ [2 (LC1C2/(C1+C2))S. Kal, IIT-Kharagpur


16/16

16

5. Non Sinusoidal Oscillators (Astable Multivibrator)

Circuit diagram WaveformS. Kal, IIT-Kharagpur

3398356--oscillators.ppt

Documents