Operational amplifier

1. Done by,
Dr. D. Veera Vanitha, AP/ECE,
School of Engineering,
Avinashilingam Institute for Home Science and
Higher Education for Women, Coimbatore.

2.  Oscillator is an electronic circuit that generates a periodic
waveform on its output without an external signal source. It
is used to convert dc to ac.
 They are used in Communications systems, digital systems
(including computers) and test equipments.
 Applications
 Used as a local oscillator to transform the RF signals to IF
signals in a receiver
 Used to generate RF carrier in a transmitter
 Used to generate clocks in digital systems
 Used as sweep circuits in TV sets and CRO

3.  An oscillator is a circuit that produces a repetitive
signal from a dc voltage.
 The feedback oscillator relies on a positive feedback of
the output to maintain the oscillations.
 The effect of positive or regenerative feedback is to
increase the systems gain, i.e, the overall gain with
positive feedback applied will be greater than the gain
without feedback
 The relaxation oscillator makes use of an RC timing
circuit to generate a non sinusoidal signal such as square
wave
Sine wave
Square wave
Sawtooth wave

4. Based on the feedback components used, the types of
oscillators are:
1. RC oscillators
◦ Wien Bridge
◦ Phase-Shift
◦ Twin T Oscillator
2. LC oscillators
◦ Hartley
◦ Colpitts
◦ Clapp
◦ Crystal
3. Unijunction / relaxation oscillators

5. Basic Linear Oscillator

+
+
SelectiveNetwork
(f)
Vf
Vs Vo
V
A(f)
)( fso VVAAVV   and of VV 
A
A
V
V
s
o


1
If Vs = 0, the only way that Vo can be nonzero is that loop gain A=1 which
implies that
0
1||




A
A (Barkhausen Criterion)
Noise signals and the transients associated with the circuit turning on provide the initial source
signal that initiate the oscillation

6. Basic principles for oscillation
In phase
Noninverting
amplifier
V f V o
A v
Feedback
circuit

7.  RC feedback oscillators are generally limited to
frequencies of 1 MHz or less. The types of RC oscillators
that we will discuss are the Wien-bridge and the phase-
shift.
 It is a low frequency oscillator which ranges from a few
kHz to 1 MHz.

8. Wien-bridge Oscillator
• The loop gain for the oscillator is;
• where,
• and
      














sp
p
ZZ
Z
R
R
sβsAsT
1
2
1
sRC
R
Zp


1
sC
sRC
Zs


1

9. Wien-bridge Oscillator
• Hence
• Substituting for s
• For oscillation frequency f0
 
 












RC/jRCjR
R
jT
001
2
0
13
1
1


 
 












/sRCsRCR
R
sT
13
1
1
1
2
 
 












RC/jRCjR
R
jT


13
1
1
1
2

10. Wien-bridge Oscillator
• Since at the frequency of oscillation, T(j)
must be real i.e., for zero phase condition, the
imaginary component must be zero
0
1
0
0 
RCj
RCj


RC
1
0  0
1
0
0 
RCj
RCj


RCj
RCj
0
0
1

 
  1
2
0  RC 10  RC
RC
1
0 
12)
0
(  RCj   1
2
0
2
 RCj 
  12
0
.1  RC

12. RC Phase-Shift Oscillator
• The phase shift oscillator utilizes three RC
circuits to provide 180º phase shift that when
coupled with the 180º of the op-amp itself
provides the necessary feedback to sustain
oscillations.
• The gain must be at least 29 to maintain the
oscillations.
• The frequency of resonance for the this type is
similar to any RC circuit oscillator:
62
1
RC
fr



13. Phase-Shift Oscillator
• The gain must be at least 29 to maintain the oscillations
62
1
RC
fo

 292

R
R
Gain