Expt. No. 8

OSCILLATORS

Objectives: a) To study the operation of Phase shift, Wien bridge, Hartley and Colpitts Oscillators.b) To simulate oscillator circuits using PSPICE

Components and equipments required:

1. Oscilloscope (Scope/CRO).2. Function Generators (FG).3. DC power supply.4. Project Breadboard. 5. Resistors.6. Capacitors. 7. Inductors.8. Inductance decade box9. Capacitance decade box10. SL100 BJTs / BEL100N/BC10711. Connection Wires. 12. Oscilloscope Probes.

Circuit diagram:

1. RC phase shift oscillator:

Fig.1

2. Wien Bridge Oscillator:

Fig. 2

3. Hartley Oscillator:

Fig. 3

4. Colpitts Oscillator:

Fig. 4

Theory:

(Prepare yourself)

Notes contents are about:

Phase shift oscillators

LC oscillators

(Should not be more than four pages)

Design:

a) RC Phase Shift Oscillator:

Given that

For amplifier

We have

(Given >> required )

Design for amplifier:

(Typical value of β =100)

Design for feedback network:

Let

(Designed Av > hfe)

(Use 1kΩ potentiometer to get accurate value)

b) Wien Bridge Oscillator :

Given that

For amplifier

We have

Where is the gain of cascade amplifier with feedback. Then feedback factor k ≤ 0.5

Design of amplifier:

(We are using negative feedback, so ignore the loading

effect and use same design for each stage)

(Typical value of β =100)

Design of RC network for Wien Bridge:

So choose R = 10kΩ

For the given fo

Design for feedback network:

(use 10k potentiometer)

c)Hartley Oscillator:

Given that

We have where

And loop gain

According to Barkhausen criteria

Then

Design for amplifier:

Choose

(Designing is similar to amplifier used for RC phase shift oscillator. Use low value for IC, to avoid loading effect at the input and output of the amplifier with feedback LC tuned circuit. Low IC value provide medium matched impedances with the available L1 and L2)

Design of LC circuit:

Choose L1 and L2 according to the availability and the conditions

.

Let L1= 10µF (Use Inductance Decade box) and L2 = 470µF (Available)

Check the conditions at resonant frequency fo. ( Where

)

(If standard capacitor is not available, use Capacitance Decade box)

d) Colpitts Oscillator:

Given that

We have where

And loop gain

According to Barkhausen criteria

Then

Design for amplifier:

Choose

(Designing is similar to amplifier used for RC phase shift oscillator. Use low value for IC, to avoid loading effect at the input and output of the amplifier with feedback LC tuned circuit. Low IC value provide medium matched impedances with the available C1 and C2)

Design of LC circuit:

Choose C1 and C2 according to the availability and the conditions

.

Let C1= 0.1µF and C2 = 0.01µF (Both are available)

Check the conditions at resonant frequency fo. (Where

)

(Use Inductance Decade box)

Pre-lab Assignments:

1. Design RC network of RC phase shift oscillator for frequency 20kHz.

2. Design RC network of Wien bridge oscillator for frequency 15kHz.

3. Design tuned circuit of Hartley oscillator for frequency 500kHz.

4. Design tuned circuit of Colpitts oscillator for frequency 750kHz.

5. Compare phase shift oscillators and tuned oscillators.

6. Define frequency and amplitude stabilization used in oscillators.

Model Waveforms Phase Shift Oscillators

Tuned Oscillators

Simulated Results:

a) RC Phase Shift Oscillator

b) Wien Bridge Oscillator

(Only real time simulation using virtual oscilloscope is possible. SPICE simulation is not possible due to loading effect? Or…….)

c) Hartley Oscillator

d) Colpitts oscillator

(To simulate inductors using SPICE, connect a low resistance of value 10mΩ in series with it.)