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AMPLIFIERS AND OSCILLATORS 6/9/2015www.noteshit.com2

Differential amplifier An amplifier which amplifies the difference between the two input signals is called differential amplifier Fig /9/2015www.noteshit.com3

CLASSIFICATION Differential amplifier is classified as 1 Voltage amplifier 2. Current amplifier 3. Tran conductance amplifier 4. Tran resistance amplifier 6/9/2015www.noteshit.com4

CHARACTERISTICS The characteristics of differential amplifier 1.Infinite input resistance 2.Zero output resistance 3.Infinite gain 4.Infinite Bandwidth 6/9/2015www.noteshit.com5

BASIC DIFFERENTIAL AMPLIFIER Basic differential amplifier has two input voltage signals. 6/9/2015www.noteshit.com6

Cont V O =A d (V 1 -V 2 ) Where A d is called differential gain, (V 1 -V 2 ) Is called differential voltage V d = (V 1 -V 2 ). A d =V O / V d if V 1 =V 2 then V d, and V O would become zero. In practical differential Amplifier depends on both difference and average common level of two inputs. 6/9/2015www.noteshit.com7

Cont…. Common mode signal is V C = (V 1 +V 2 ) /2 V O =A C (V 1 +V 2 ) /2 or V O =A C V C. Where V C is called as common mode gain. The Total output voltage is V O = A d V d +A C V C. 6/9/2015www.noteshit.com8

COMMON MODE REJECTION RATIO The ability of a differential amplifier to reject a common mode signal is expressed by CMMR =ρ=20 log A d/ A C Differential amplifier has two modes Common mode and differential mode. 6/9/2015www.noteshit.com9

DIFFERENTIAL MODE 6/9/2015www.noteshit.com10

Cont…  The input signals are same in magnitude but 180 degree phase shift  Q 1 is positive going and Q 2 is negative going.  R E has same current but 180 degree phase shift,so AC current in R E is zero.  The output of collector one and two are equal but opposite in phase. 6/9/2015www.noteshit.com11

Cont…..  Therefore the total output voltage of differential mode is V d = (V 1 -V 2 ) V 1 = Voltage across collector one V 2 = Voltage across collector two  Therefore total output voltage is twice as large as signal voltage from either collector to ground. 6/9/2015www.noteshit.com12

COMMON MODE 6/9/2015www.noteshit.com13

Cont..  The input signals of Q 1 and Q 2 are equal in magnitude and phase.  RE carries current (2I E ) and provides negative feedback.  The output voltage at collector one and two are same in phase and magnitude  Therefore Vo= 0 in common mode. 6/9/2015www.noteshit.com14

Feedback amplifiers and oscillators Feedback is a process of injecting some energy from the output and then return it to the input.The amplifier which uses this is called feedback amplifier. Classification of feedback amplifiers 1. Positive Feedback amplifiers 2. Negative Feedback amplifiers 6/9/2015www.noteshit.com15

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com16

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com17

Feedback amplifiers and oscillators Types of feedback connections 6/9/2015www.noteshit.com18

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com19

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com20

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com21

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com22

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com23

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com24

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com25

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com26

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com27

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com28

Feedback amplifiers and oscillators 6/9/2015www.noteshit.com29

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Oscillators 6/9/2015www.noteshit.com33

Oscillator Introduction of Oscillator Linear Oscillator –Wien Bridge Oscillator –RC Phase-Shift Oscillator –LC Oscillator Stability 6/9/2015www.noteshit.com34

Oscillators Oscillation: an effect that repeatedly and regularly fluctuates about the mean value Oscillator: circuit that produces oscillation Characteristics: wave-shape, frequency, amplitude, distortion, stability 6/9/2015www.noteshit.com35

Application of Oscillators Oscillators are used to generate signals, e.g. –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. 6/9/2015www.noteshit.com36

Linear Oscillators 1.Wien Bridge Oscillators 2.RC Phase-Shift Oscillators 3.LC Oscillators 4.Stability 6/9/2015www.noteshit.com37

Integrant 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 6/9/2015www.noteshit.com38

Basic Linear Oscillator and If V s = 0, the only way that V o can be nonzero is that loop gain A  =1 which implies that (Barkhausen Criterion) 6/9/2015www.noteshit.com39

Wien Bridge Oscillator Frequency Selection Network Let and Therefore, the feedback factor, 6/9/2015www.noteshit.com40

 can be rewritten as: For Barkhausen Criterion, imaginary part = 0, i.e., Supposing, R 1 =R 2 =R and X C1 = X C2 =X C, 6/9/2015www.noteshit.com41

Example By setting, we get Imaginary part = 0 and Due to Barkhausen Criterion, Loop gain A v  =1 where A v : Gain of the amplifier Therefore, Wien Bridge Oscillator 6/9/2015www.noteshit.com42

RC Phase-Shift Oscillator  Using an inverting amplifier  The additional 180 o phase shift is provided by an RC phase-shift network 6/9/2015www.noteshit.com43

Applying KVL to the phase-shift network, we have Solve for I 3, we get Or 6/9/2015www.noteshit.com44

The output voltage, Hence the transfer function of the phase-shift network is given by, For 180 o phase shift, the imaginary part = 0, i.e., and, Note: The –ve sign mean the phase inversion from the voltage 6/9/2015www.noteshit.com45

LC Oscillators  The frequency selection network (Z 1, Z 2 and Z 3 ) provides a phase shift of 180 o  The amplifier provides an addition shift of 180 o Two well-known Oscillators: Colpitts Oscillator Harley Oscillator 6/9/2015www.noteshit.com46

For the equivalent circuit from the output Therefore, the amplifier gain is obtained, 6/9/2015www.noteshit.com47

The loop gain, If the impedance are all pure reactances, i.e., The loop gain becomes, The imaginary part = 0 only when X 1 + X 2 + X 3 =0  It indicates that at least one reactance must be –ve (capacitor)  X 1 and X 2 must be of same type and X 3 must be of opposite type With imaginary part = 0, For Unit Gain & 180 o Phase-shift, 6/9/2015www.noteshit.com48

Hartley OscillatorColpitts Oscillator 6/9/2015www.noteshit.com49

Colpitts Oscillator Equivalent circuit In the equivalent circuit, it is assumed that:  Linear small signal model of transistor is used  The transistor capacitances are neglected  Input resistance of the transistor is large enough 6/9/2015www.noteshit.com50

At node 1, where, Apply KCL at node 1, we have For Oscillator V  must not be zero, therefore it enforces, 6/9/2015www.noteshit.com51

Imaginary part = 0, we have Real part = 0, yields 6/9/2015www.noteshit.com52

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