UNIT - III SDC feed back & oscillators


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UNIT - III FEEDBACK OSCILLATORS AND POWER AMPLIFIERS: - Feedback in amplifiers: Basic feedback topologies. Oscillators: Barkhausen’s criterion, sinusoidal oscillators, Phase shift oscillators, Resonant circuit oscillator, a general form of oscillator, the Wein Bridge oscillator, Crystal oscillator. Introduction to power amplifiers and its various types with applications.


Outline Feedback Feedback topologies Feedback Barkhausen’s criteria Types of oscillators Phase shift Resonant circuit oscillator Wein bridge oscillator Crystal oscillator Power Amplifiers and their applications

Classification of Amplifiers:

Classification of Amplifiers Voltage Amplifiers Current Amplifiers Transconductance Amplifier Tranresistance Amplifier

Voltage Amplifiers:

Voltage Amplifiers In Voltage amplifier output voltage (V 0 ) is proportional to input voltage (V i ) Gain is represented by A v For ideal voltage amplifier R i is infinite R o is zero Thevenin’s equivalent of voltage amplifier

Current Amplifiers:

Current Amplifiers In Current amplifier output current I L is proportional to signal current I s Gain is represented by A i For ideal current amplifier R i is zero and R o is infinite Norton’s equivalent of current amplifier

Transconductance Amplifier:

Transconductance Amplifier In Transconductance amplifier output current I L is proportional to signal voltage V s Gain is represented by G m For ideal current amplifier R i is infinite and R o is infinite Thevenin’s equivalent circuit at input Norton’s equivalent circuit at output

Transresistance Amplifier:

Transresistance Amplifier In Transresistance amplifier output voltage V 0 is proportional to signal current I s Gain is represented by R m For ideal current amplifier R i is zero and R o is infinite Norton’s equivalent circuit at input Thevenin’s equivalent circuit at output

Feed Back Topologies:

Feed Back Topologies Voltage Series Feed back Voltage Amplifier

Feed Back Topologies:

Feed Back Topologies Current Shunt Feed back Current Amplifier

Feed Back Topologies:

Feed Back Topologies Voltage shunt Feed back Transresistance Amplifier

Feed Back Topologies:

Feed Back Topologies Current series Feed back Transconductance Amplifier

Transfer gain with feedback:

Transfer gain with feedback

Slide 13:

Gain of Amplifier If a feedback signal X f is connected in series with input then Since Then Overall gain with feedback

Feed Back Types:

Feed Back Types Positive Feed Back When fraction of output ( fed back signal ) is added to input signal ( regenerative / output increases ) V i = V s + V f Negative Feed Back When fraction of output ( fed back signal ) is subtracted from input signal ( degenerative / output decreases ) V i = V s - V f

Negative Feed back:

Negative Feed back Advantages Desensitivity of transfer ratio Bandwidth increase Non-linear distortion decrease Reduction of noise Disadvantages Gain decreases Stability decreases at high frequencies

Positive Feed Back:

Positive Feed Back Advantage Gain of amplifier increases Can be used for making oscillators Disadvantage Non linear distortion Decrease in Band width Increase in Noise

Effect on Input Impedance:

Effect on Input Impedance Voltage series feed back

Slide 18:

Voltage shunt feedback

Effect on output Impedance:

Effect on output Impedance Voltage series feed back Let V is voltage applied at output and short V s Then, V

Slide 20:

Current Series feedback The output impedance with current-series feedback can be determined by applying a signal V to the output with Vs shorted out, resulting in a current I , the ratio of V to I being the output impedance With Vs 0,


Oscillators In our daily life Digital watches, Invertors, Radios , T.V, Computers, Fans, Metal Detectors, Electronic Bells and lots more Pendulum of a clock . If you push on a pendulum to start it swinging, it will oscillate at some frequency -- it will swing back and forth a certain number of times per second. The length of the pendulum controls the frequency. In pendulum potential energy is converted in kinetic energy

Slide 22:

“Oscillators are the circuits which coverts DC Voltage from battery to AC Voltage” Without excitation input signal A simple example If you charge up the capacitor with a battery and then insert the inductor into the circuit, here's what will happen The capacitor will start to discharge through the inductor. As it does, the inductor will create a magnetic field Once the capacitor discharges, the inductor will try to keep the current in the circuit moving, so it will charge up the other plate of the capacitor. Once the inductor's field collapses, the capacitor has been recharged (but with the opposite polarity), so it discharges again through the inductor Frequency will depend upon L and C

Slide 23:

Oscillations produced in L-C (Tank Circuit) are not sustained – die out due to resistance loss In order to get sustained oscillation we require energy source This accomplished by Amplifier So, we can conclude that an oscillator circuit must contain ‘Active Device' (Transistor/Op-amp) ‘Feedback circuit’ ( in order to get output without input excitation)

Slide 24:

Block diagram of oscillator

Slide 25:

To start the oscillation with the constant amplitude, positive feedback is not the only sufficient condition. Oscillator circuit must satisfy the following two conditions known as Barkhausen conditions: The product of gain of amplifier 'A' and the gain of feedback network 'β' has to be unity . The phase shift through the amplifier and feedback network has to be 360° or 0°. These conditions are called Barkhausen’s criteria and these conditions are true only at one frequency

Slide 26:

Barkhausen's criterion is a necessary condition for oscillation, not sufficient . There are some circuits which satisfy the criterion but do not oscillate How Oscillations starts ? Ans. Thermal Noise Voltage. Every resistor has certain free electrons. At room temperature these free electrons move randomly and generate a noise voltage across the resistor due to collisions. Hence, the resistor acts as small ‘ac’ voltage source

Classification of oscillators :

Classification of oscillators Sinusoidal Oscillators – Oscillators which are capable of generating sin wave. ranging from low audio frequencies(Hz) to ultrahigh radio (MHz) and microwave frequencies (GHz) Nonsinusoidal/Relaxation oscillators – Other type of waveforms like square, triangular etc.

Types of sinusoidal oscillators:

Types of sinusoidal oscillators R-C oscillators Phase shift oscillator L-C oscillators (resonant oscillators) Hartley oscillator Collpit’s oscillator Crystal Oscillator ( a pizeo-electric material)

R-C oscillators:

R-C oscillators Phase shift oscillator The phase shift oscillator is suited to the range of frequencies (few Hz to 200 kHz) Transistor is utilized in common emitter configuration with voltage divider bias which give phase shift of 180 0 Feedback network gives 180 0 phase shift

Phase shift oscillator:

Phase shift oscillator If the value of all R and C is equal then the frequency of oscillation can be determined by the formula Gain at this frequency should be 29 Frequency can be changed by changing C or R

Slide 31:

Problem - find frequency of oscillations

Resonant circuit Oscillator :

Resonant circuit Oscillator L-C Oscillators (R-F oscillators) Hartley oscillator Colpitt’s oscillator Uses an L-C circuit in the feedback loop : To provide necessary phase shift To act as a resonant filter that passes only the desired frequency

Hartley Oscillator:

Hartley Oscillator Feed back circuit consists of two inductors and one capacitor Frequency of oscillations is given by Gain of amplifier should be decided by condition AB = 1 , here B = L 1 /L 2

Colpitt’s Oscillator:

Colpitt’s Oscillator Uses two C and one L Frequency of oscillations is given by

Wein Bridge oscillator:

Wein Bridge oscillator The circuit uses two RC networks connected to the positive terminal to form a frequency selective feedback network R 7 ,C 3 ( Z 1 )act as high pass filter and R 8 ,C 4 ( Z 2 ) act as low pass filter Z1 Z2

Slide 36:

Resonant frequency is given by R = R 7 = R 8 and C = C 3 = C 4 Feedback factor B = 1/3 So gain of amplifier should be 3

Crystal Oscillator:

Crystal Oscillator The Piezoelectric Effect Quartz exhibits piezoelectric effect. When a changing mechanical stress is applied across the crystal to cause it to vibrate, a voltage develops at the frequency of mechanical vibration. Conversely, when an ac voltage is applied across the crystal, it vibrates at the frequency of the applied voltage. The greatest vibration occurs at the crystal's natural resonant frequency . Which is determined by the physical dimensions and by the way the crystal is cut.

Slide 39:

From equivalent circuit it is clear that it consists of series as well as parallel resonant circuit At series resonance inductive reactance is equal to capacitive reactance C s At parallel resonance inductive reactance is equal to capacitive reactance C m So,crystal can be used in hartley or colpitts oscillator in place of the tank circuit

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