8/13/2019 Oscillators 1p

1/29

Oscillator Circuits

8/13/2019 Oscillators 1p

2/29

II. Oscillator Operation

For self-sustaining oscillations:

the feedback signal must positive

the overall gain must be equal to one (unity gain)

8/13/2019 Oscillators 1p

3/29

If the feedback signal is not positive or the gain is less than one, then the oscillations will

dampen out.

If the overall gain is greater than one, then the oscillator will eventually saturate.

8/13/2019 Oscillators 1p

4/29

Types of Oscillator Circuits

A. Phase-Shift Oscillator

B. Wien Bridge Oscillator

C. Tuned Oscillator Circuits

D. Crystal Oscillators

E. Unijunction Oscillator

8/13/2019 Oscillators 1p

5/29

A. Phase-Shift Oscillator

Frequency of the oscillator: (the frequency where the phase shift is 180)

Feedback gain = 1/[152 j (63) ] where = 1 / (2fRC)Feedback gain at the frequency of the oscillator = 1 / 29

The amplifier must supply enough gain to compensate for losses. The overall gain must be

unity. Thus the gain of the amplifier stage must be greater than 1/, i.e. A > 29

The RC networks provide the necessary phase shift for a positive feedback. They also

determine the frequency of oscillation.

62

10

RCf=

8/13/2019 Oscillators 1p

6/29

Example of a Phase-Shift Oscillator

FET Phase-Shift Oscillator

8/13/2019 Oscillators 1p

7/29

Example 1

8/13/2019 Oscillators 1p

8/29

BJT Phase-Shift Oscillator

C

Cfe

R

R

R

Rh 42923

iehRR

8/13/2019 Oscillators 1p

9/29

Phase-shift oscillator using op-amp

8/13/2019 Oscillators 1p

10/29

B. Wien Bridge Oscillator

So frequency of oscillation is 22110 21

CRCRf=

1

1

1

1

4

3

2

143

4

21

2

===

R

R

Z

ZRR

R

ZZ

Z

V

VV

V

V

a

bd

o

i

2and,2

1

thenandWhen

4

30

2121

=

R

R

RC f

CCCRRRfrequencynoscillatiotheatphasezerohaveshouldi.e.,,2

1

21

2

Z

Z

ZZ

Z

1

2

2

1

4

30C

C

R

R

R

R

8/13/2019 Oscillators 1p

11/29

Calculate the resonant frequency of the Wien bridge oscillator shown above

Hz7.3120)101)(1051(2

1

2

1930

== RCf

Example 2

8/13/2019 Oscillators 1p

12/29

C. Tuned Oscillator Circuits

Tuned Oscillators use a parallel LC resonant circuit (LC tank) to provide the oscillations.

There are two common types:

Colpitts The resonant circuit is an inductor and two capacitors.

Hartley The resonant circuit is a tapped inductor or two inductors and one

capacitor.

8/13/2019 Oscillators 1p

13/29

Colpitts Tuned Oscillator Circuit

FET based Colpitts Oscillator

21

21where,2

10

CC

CCC

LCf eq

eq =Frequency of oscillations:

8/13/2019 Oscillators 1p

14/29

Transistor Colpitts oscillator.

8/13/2019 Oscillators 1p

15/29

Op-amp Colpitts oscillator.

8/13/2019 Oscillators 1p

16/29

Hartley Tuned Oscillator Circuit

FET Hartley Oscillator

1221 2where,2

10 LLLL

CLf eq

eq

Frequency of oscillations:

8/13/2019 Oscillators 1p

17/29

BJT based Hartley Oscillator

8/13/2019 Oscillators 1p

18/29

D. Crystal Oscillators

The crystal appears to the rest of the circuit as a resonant circuit.

8/13/2019 Oscillators 1p

19/29

Crystal Resonant Frequencies

The crystal has two resonant frequencies:

Series resonant: RLC determine the resonant frequency. The crystal has a low impedance.Parallel resonant: RL and CM determine the resonant frequency. The crystal has a high

impedance.

The series and parallel resonant frequencies are very close, within 1% of each other.

8/13/2019 Oscillators 1p

20/29

Series Resonant Crystal Oscillator

FET Crystal Oscillator

8/13/2019 Oscillators 1p

21/29

Parallel Resonant Crystal Oscillator

BJT Crystal Oscillator

8/13/2019 Oscillators 1p

22/29

Crystal Oscillator using Op-amp

8/13/2019 Oscillators 1p

23/29

E. Unijunction Oscillator

Frequency of oscillations:

where = 0.4 to 0.6

is a rating of the unijunction transistor.

)(CRf TT = 11ln1

0

8/13/2019 Oscillators 1p

24/29

Unijunction Oscillator Waveforms

The unijunction oscillator (or relaxation oscillator) produces a sawtooth waveform.

DBBBP

BBBB

VVVV

VVV

112

8/13/2019 Oscillators 1p

25/29

8/13/2019 Oscillators 1p

26/29

Unijunction transistor (UJT): basic construction.

8/13/2019 Oscillators 1p

27/29

UJT Unijunction Transistor

The UJT is also basically a switching device.

Schematic Symbol:

8/13/2019 Oscillators 1p

28/29

UJT Basic Operation

Even though the UJT is a switching device it works very differently from the SCR variety

of devices.

The equivalent circuit indicates that the UJT is like a diode and a resistive voltage divider

circuit. The resistance exhibited by RB1 is variable; it is dependent on the value of current

IE.

8/13/2019 Oscillators 1p

29/29