HARTLEY OSCILLATOR

Aim: To design, study the operation of Hartley’s Oscillator using BJT.

Design: Design a Hartley’s oscillator circuit to provide oscillations at a frequency of 1MHz.

Use BJT BC547 for which 𝛽 = 200 . Use 𝑅𝑐 = 5.6𝐾Ω. the biasing conditions are as

follows.𝑉𝐶𝐶 = 15𝑉, 𝑉𝐶𝐸 = 7𝑉, 𝐼𝑐 = 1.2𝑚𝐴, 𝑠𝑡𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑓𝑎𝑐𝑡𝑜𝑟 𝑠 = 5.

Use 𝐼𝑐 = 𝛽𝐼𝐵 => 𝐼𝐵 = 6𝜇𝐴.

Apply KVL to the output loop

−𝑉𝐶𝐶 + 𝐼𝑐𝑅𝑐 + 𝑉𝐶𝐸 + 𝐼𝐶𝑅𝐸 = 0.

=> 𝑅𝐸 = 1𝐾Ω.

Apply Thevenin’s theorem to the base circuit, then

𝑉𝐵 =𝑉𝐶𝐶 𝑅2

𝑅1 + 𝑅2 𝑎𝑛𝑑 𝑅𝐵 =

𝑅1 𝑅2

𝑅1 + 𝑅2

We know that the stability factor for self bias circuit is given by,

𝑆 = 1 + 𝛽

1 + 𝛽𝑅𝐸

𝑅𝐸 + 𝑅𝐵

=> 𝑅𝐵 = 4.11𝐾Ω

Apply KVL to input loop, then

−𝑉𝐵 + 𝐼𝐵𝑅𝐵 + 𝑉𝐵𝐸 = 0

=> 𝑉𝐵 = 1.87𝑉

Divide 𝑅𝐵with𝑉𝐵:

𝑅1 =𝑉𝐶𝐶𝑅𝐵

𝑉𝐵=> 𝑅1 = 33𝐾Ω

Also 𝑅𝐵 =𝑅1 𝑅2

𝑅1+𝑅2 => 𝑅2 = 4.7𝐾Ω

We know that 𝑓𝑜 =1

2𝜋(√𝐿1+𝐿2+2𝑀)𝐶 assume that𝐿1 = 𝐿2 = 70𝜇𝐻 𝑎𝑛𝑑 𝐶 = 100𝑃𝐹.

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Apparatus:

Sl. No

Name of the component

Specifications or

range

Quantity

1

Transistors

BC547 or

BC548

1

2

Resistors

33KΩ,1KΩ,4.7KΩ, ,

5.6KΩ,

1,1,1,1

3

Capacitors, Decade capacitance box

4.7µF,

2,1

4

Decade inductance box

---

2

5

CRO

20MHz

1

7

Regulated power supply

(0-30)V,1A

1

8

Connecting wires and probes

-----

Required no

Theory:

An Oscillator is an electronic circuit that provides an A.C output (Oscillations)

without using any external input. All sinusoidal oscillators use the concept of positive

feedback to produce Oscillations. An Oscillator circuit must satisfy Barkhausen’s criterion to

produce oscillations.

Hartley’s oscillators are used at Radio frequencies.

Refer textbook to write theory

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Circuit Diagram:

Model graphs:

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Experimental procedure:

1. Connections are made as per biasing circuit diagram and check the conditions.

2. Connect the circuit diagram as shown in the oscillator, adjust the capacitance to

100PF.

3. Observe the peak to peak output voltage and time period of the output waveform.

Calculate the frequency of oscillation using f= 1/T) Hz.

4. Now vary the Capacitance in appropriate steps and record the frequency in each case.

5. Compare practical frequencies with theoretical values.

Observations:

Sl.No Capacitance(PF) Theoritical frequency

𝑓𝑜 =1

2𝜋(√𝐿1 + 𝐿2 + 2𝑀)𝐶

𝐿1 = 𝐿2 = 70𝜇𝐻

Practical frequency

Precautions:

1. All connections must be done carefully.

2. Switch off power supply before making connections.

Result: Thus Hartley’s oscillator was designed and the frequency of oscillation as obtained

practically. ece4

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