Colpitt’s Oscillator

Aim:

To study the operation of Colpitt’s Oscillator

Apparatus:

1.     Analog board AB67

2.   DC Power Supplies +12V from external source or ST2612 analog lab

3.   Oscilloscope 20 MHz, Caddo 802 or equivalent

4.   2 mm patch cords

Theory:

Oscillators are circuits that produce specific, periodic waveforms such as square, triangular, sawtooth, and sinusoidal. Two requirements for oscillation are:

1. The magnitude of the loop gain AvB must be at least 1, and

2.            The total phase shift of the loop gain AvB must be equal to 0^o or 360^o.  If  the amplifier causes a phase shift of 180^o, the feedback circuit must provide an additional phase shift of 180^o so that the total phase shift around the loop is 360^o.

The Colpitt oscillator is  one of  the  simplest and best known oscillators and  is used extensively in circuits, which work at radio frequencies. The transistor is in voltage divider bias which sets up Q-point of the circuit.In the circuit note that Vout is actually the ac voltage across C2. This voltage is fed back to the base and sustains oscillations developed across the tank circuit, provided there is enough voltage gain at the oscillation frequency.

The resonant frequency of the Colpitt oscillator can be calculated from the tank circuit used. We can calculate the approximately resonant frequency as

1

ResonantFrequency(f_r) =

2π√LC

Here, the capacitance used is the equivalent capacitance the circulating cur- rent passes through. In Colpitt oscillator the circulating current passes through the series combination of C1 and C2. Therefore equivalent capacitance is,

C equivalent) =   C1C2

(                                         C₁ + C₂

Procedure:

1.     Connect +12V DC power supplies at their indicated position from external source or ST-2612 Analog Lab.

2.   Connect a patch cord between points a and b and another patch chord between points d and g1.

3.   Switch ON the power supply.

4.   Connect oscilloscope between points f and g2 on AB–67 board.

5.   Record the value of output frequency on oscilloscope.

6.   Calculate the resonant frequency using equation mentioned.

7.    Compare measured frequency with the theoretically calculated value.

8.   Switch off the supply.

9.   Remove the patch chord connected between points a and b and connect it between points a and c.

10.   Remove the patch chord connected between points d and g1 and connect it between points e and g2.

11.     Follow the procedure from point 4 to 8.

12.   Connect +5V supply instead of +12V supply ly and follow the procedure from point 2 to point 11.

Result:

•    When patch cords are connected across C₁ and C₂

Practically calculated Output frequency (on CRO): 1.073 MHz Theoretically calculated values

C_equ: 14.98 nF

Resonant frequency (f_r): 1.18  MHz Output voltage amplitude : 1.90 V (Vp-p)

•    When patch cords are connected across C₃ and C₄

Practically calculated Output frequency (on CRO): 1.76 MHz Theoretically calculated values

C_equ: 4.71 nF

Resonant frequency (f_r): 2.12  MHz Output voltage amplitude : 3.58 V (Vp-p)