Analog Lect 13 14022012 Generators

8/12/2019 Analog Lect 13 14022012 Generators

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EEE C364/INSTR C 364

Analog Electronics

Lecture 13

14-02-2012


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A triangular wave which goes from -12 V to +12 V is

applied to the inverting input of the opamp. Assume that

the output of the opamp swings from +15 V to -15 V. The

voltages at the input of the noninverting input switches

between .. and ..

-

+ +3

2

6

7

4

OP2 !OPAMP

10

k

10k

10k

V1 15

V2 15

Vi


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T

Time (s)

0.00 1.00m 2.00m 3.00m

Output

-20.00

-10.00

0.00

10.00

20.00

-

++3

2

6

7

4

OP2 !OPAMP

10k

10k

10k

V1 15

V2 15

Vi

30VVi


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Oscillators

Oscillation: an effect that repeatedly andregularly fluctuates about the mean value

Oscillator: circuit that produces oscillation

Characteristics: wave-shape, frequency,

amplitude, distortion, stability


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Application of Oscillators

Oscillators are used to generate signals:

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.


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Linear Oscillators

1. Wien Bridge Oscillators

2. RC Phase-Shift Oscillators

3. LC Oscillators

Non-linear Oscillators

Multivibrators or Relaxation oscillators1. Astable 2. Bistable 3. Monostable


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Positive feedback


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How to generate a signal.

Regenerative

feedback

Amplifier

Oscillator

1v vA


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Effects of Avon oscillator operation.

Barkhausen criterion: Av=1

Av1

Av=1


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Properties of Linear Oscillators

For sinusoidal input is connected

Linear because the output is approximately sinusoidal

A linear oscillator contains:

- a frequency selection feedback network

- an amplifier to maintain the loop gain at unity

+

+ Amplifier (A)

Frequency-Selective

Feedback Network ()

Vf

Vs VoV

Positive

Feedback


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Basic Linear Oscillator

+

+

SelectiveNetwork

(f)Vf

Vs VoV A(f)

)( fso VVAAVV and of VV

A

A

V

V

s

o

1If V

s

= 0, the only way that Vo

can be nonzero

is that loop gain A=1 which implies that

0

1||

A

A (Barkhausen Criter ion)


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No input s ignal is needed. Noise at the desired oscillation frequencywill likely be present at the input and when picked up by the

oscillator when the DC power is turned on, it will start the oscillator

and the output will quickly buildup to an acceptable level.

Wien Bridge Oscillator


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Wien Bridge Oscillator

Frequency Selection Network

Let

11

1

CX

C and

111 CjXRZ 2

2

1

CX

C

22

22

1

22

2

11

C

C

C jXR

XjR

jXR

Z

Therefore, the feedback factor,

)/()(

)/(

222211

2222

21

2

CCC

CC

i

o

jXRXjRjXR

jXRXjR

ZZ

Z

V

V

222211

22

))(( CCC

C

XjRjXRjXR

XjR

Vi Vo

R1 C1

R2C2

Z1

Z2


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can be rewritten as:

)( 2121221221

22

CCCCC

C

XXRRjXRXRXR

XR

For Barkhausen Criter ion, imaginary part = 0, i.e.,

02121 CC XXRR

Supposing,

R1=R2=R and XC1= XC2=XC,

2121

21

21

/1

11or

CCRRCC

RR

f

f

R

XRjRX

RX

CC

C

0

0

C

C

22

f

fj3

1

X

X

Rj3

1

)(3

0.2

0.22

0.24

0.26

0.28

0.3

0.32

0.34

Feedb

ack

factor

-1

-0.5

0

0.5

1

Phase

Frequency

=1/3

Phase=0

f(R=Xc)


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Example

Rf

+

R

R

C

CZ1

Z2

R1

Vo

By setting , we get

Imaginary part = 0 and

RC

1

3

1

Due to Barkhausen Criter ion,

Loop gain Av=1

where

Av : Gain of the amplifier

1

131

R

RAA

f

vv

21

R

RfTherefore, Wien Bridge Oscillator

)(3 22 CC

C

XRjRX

RX

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Analog Lect 13 14022012 Generators