Analog and Digital Systems Design Laboratory - BECL305
EXPERIMENTS NO. 1
Aim: - Design and set-up BJT/FET i) Colpitts Oscillator ii) Crystal Oscillator.
Components Required: -
Sl. No Components Range Quantity
1. NPN transistor SL100 1
2. Resistors 27K,4.7K,1.2K, 270Ω 4
3. Capacitors 0.47µF & 47µF 2+1
4. Decade Capacitor Box 2
5. Decade Inductor Box 1
Theory: -
Colpitts Oscillator: The oscillators that employ L&C elements are called tuned oscillators. Hartley
and Colpitts’s oscillators belong to LC-tuned oscillators. Its feedback or tank circuit consists of 2
capacitors in series and one inductor in parallel. The amplifier is a single-stage amplifier and has a 180-
degree phase shift. So this tank circuit provides a 180-degree phase shift due to inductor & capacitor
connection to make total phase shift of the circuit as 360 degrees at the frequency of oscillation. When
the feedback is adjusted so that !" = ! , sustained oscillation occurs at the output.
Crystal Oscillator: Crystal oscillators are made from quartz. A crystal can be operated in the series
resonant or parallel resonant mode. In the series mode crystal offers minimum impedance at resonance
and in parallel mode it offers maximum impedance and is inductive.
Design of Biasing Circuit:-
Let VCC =12V, Ic = 4mA.
Since the quiescent point is in the middle of the load line for the amplifier,
VCE = 50% of VCC = 6V. VRE = 10% of VCC = 1.2V.
To Find RE : VRE= VCC/10=1.2V
IE .RE = 1.2V
RE=270 Ω
To Find RC :
Apply KVL to the loop
VCC - ICRC - VCE - VRE = 0
12 - (4*10-3) RC - 6 - 1.2 = 0
RC= 1.07K Ω
`Use RC= 1.2K Ω
Dept. of ETE, BMSIT&M, Bengaluru Page 34
Analog and Digital Systems Design Laboratory - BECL305
To Find R1 & R2 : VBE + VRE = 0.7 + 1.2 = 1.9V
VCC R2
VB = -----------
R1+R2
!" '(
!#$!"
= '))
= 0.158
Let R2 = 4.7KΩ R1 = 25K Ω Use 27K Ω.
Circuit Diagrams:
a) Colpitts Oscillators:
Tank circuit design for Colpitts oscillator
f=100KHZ
# = # " !! !"
Let C1 = 0.01uF; C2 =0.01uF ; C = C1C2/(C1+C2 ) = 0.005 x 10-6 uF
L = 1/ 4*(100K)2*π2(0.005*10-6) = 500µH
Dept. of ETE, BMSIT&M, Bengaluru Page 35
Analog and Digital Systems Design Laboratory - BECL305
b) Crystal Oscillator:
Procedure: -
1. Connect the components on the breadboard as per the diagram.
2. Switch on Vcc and verify the biasing ckt and note down the values.
3. Also observe the waveform across the output using CRO.
4. If wave is not observed, connect a variable 1K pot in series with RE resistor. Adjust this
potentiometer to get the output.
5. Draw the waveform on the graph sheet and note down the frequency.
Expected waveform:
Result: - The frequency of oscillation obtained for the
a) Colpitts Oscillator is equal to _________ KHz
b) Crystal Oscillator is equal to _________ MHz
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