Electronics II Laboratory
EXPERIMENT NO.(6)
LC OSCILLATORS
Object
1. To examine the operation of both a Colpitts & Hartley oscillator
2. To verify oscillation conditions
Equipment
1. Power- supply
2. Oscilloscope
3. Operational amplifier (A741)
4. Decade capacitor
5. Decade inductor
6. 100 ohm resistor
7. Potentiometer of 100kohm
Theory
High frequency oscillators are generally LC oscillator, for example
Colpitts oscillator and Hartley oscillator. The frequency of the oscillation
is proportional to 1 / LC . The circuit diagram for Colpitts oscillator is
shown in Fig.(1) . The Colpitts oscillator must have a loop gain of unity
and will have zero degree phase shift at frequency (fo) of the tuned
circuit. The output developed across the tank circuit. The positive
feedback required for oscillation is derived by capacitive tapping of the
tank.
The oscillation frequency can be derived to be:

1
LC1C2 / C1  C2
Jassim K. Hmood
1
3-2011
Electronics II Laboratory
Therefore, 2 
1
LC
if C1=C2=C
In practice, normally reactance of C1 is kept small so it is not
shunted by the input impedance of the amplifier. So, always Cl is taken
larger than C2.
Hence,  
1
LC
and the capacitor C1 controls feedback portion while
C2 affects the frequency of oscillation.
The same analysis can be done for the Hartley oscillator whose
circuit is shown in Fig.(2).
The frequency of oscillation is:
1

L1  L2C
Procedure
(A) Colpitts oscillator
fo 
1
2 LC
1. Connect the circuit shown in Fig.(1), with L=3mH, Cl=C2=2F. Draw
the output & measure (fo).
2. For C1=C2=2f, vary (L) from (3 to 9)mH. Then measure output
frequency in each case.
3. For L=3mH, C2=2F, vary Cl from (2F to 8F). Then measure
output frequency in each case.
4. For L=3mH, C1=2F, vary C2 from (2uF to 8F). Then measure
output frequency in each case.
B) Hartley oscillator
fo 
Jassim K. Hmood
1
2 L1  L2C
2
3-2011
Electronics II Laboratory
1. Connect the circuit
shown in Fig.(2), Ll=L2=2mH, draw the output
& measure (fo).
2. For L1=L2=2mH, vary (C) from (2F to 8F), measure fo for each
case.
3. For L2=2mH, C=5F, vary L1. from (lmH to 8mH) measure output in
each case.
4. For Ll=2mH, .C=5F, vary L2 from (lmH to BmH), measure output in
each case.
Graphs
Plot the following graphs:
1. C1, C2 against amplitude of O/P
2. Ll, L2 against amplitude of O/P
3. L, C against frequency
Discussion
1. Compare between the experimental & theoretical values of (fo).
2. What is the general condition of oscillation?
3. Comment on your results.
100k 50%
100k 50%
Rf
Rf
+15V
+15V
100
+
741
100
+
eo
-15V
-15V
C1
0.2uF
C2
0.1uF
L1
2mH
L2
1mH
1uF
L
3mH
Fig.(2): Hartley oscillator
Fig.(1): Colpitts oscillator
Jassim K. Hmood
741
3
3-2011
eo