Savitribai Phule Pune University SE ETCElex Revised Syllabus

Savitribai Phule Pune University SE (E&TC/Elex) Revised Syllabus 2015 Course Unit V Feedback Amplifiers and Oscillators By Pramod Chavan, KJCOEMR, Pune January 9, 2022 1

Syllabus- Feedback Amplifier • Four Types of amplifiers • Feedback topologies • Effect of feedback on terminal characteristics of amplifiers • Examples of voltage series and Current Series FET feedback amplifiers and their analysis January 9, 2022 2

Oscillators • • • Barkhausen Criterion Stability with feedback FET RC Phase-Shift Oscillator General form of LC Oscillator Hartley Oscillator Colpitts Oscillator January 9, 2022 3

Feedback Concept • Feedback is one of the fundamental processes in electronics. It is defined as the process whereby a portion of the output signal is fed to the input signal in order to form a part of the system-output control. • Feedback is used to make the operating point of a transistor insensitive to both manufacturing variations in as well as temperature. • There is another type of feedback called positive or regenerative feedback in which the overall gain of the amplifier is increased. Positive feedback is useful in oscillators and while establishing the two stable states of flip-flop. January 9, 2022 4

Classification of Amplifiers Thevenin’s equivalent circuits of a voltage amplifier January 9, 2022 5

Classification of Amplifiers contd. Norton’s equivalent circuits of a current amplifier January 9, 2022 6

Classification of Amplifiers contd. Transconductance Amplifier January 9, 2022 7

Classification of Amplifiers contd. Transresistance Amplifier January 9, 2022 8

Block Diagram of a Feedback Amplifier January 9, 2022 9

Basic elements of feedback amplifier: � Input Signal: The signal source is modeled either by a voltage source Vs in series with a resistance Rs , or by a current source is in parallel with a resistance Rs. � Output Signal: The output can either be the voltage across the load resistance or the current through it. It is the output signal that is desired to be independent of the load and insensitive to parameter variations in the basic amplifier. � Sampling Network: The function of the sampling network is to sample the output signal and fed to feedback network. This signal is either voltage or current.

Sampling Network January 9, 2022 11

Basic elements of feedback amplifier: • Comparator or mixer Network: The two very common networks used for the mixing of the input and feedback signals are `1. series mixing- used to compare the signal voltage Vs and feedback signal Vf. The amplifier input signal Vi is proportional to the voltage difference Vs -Vf that results from the comparison. 2. Shunt mixing- source current Is and feedback current If are compared. The amplifier input current Ii is proportional to the difference Is & If. • Basic Amplifier: • The basic amplifier is one of the important parts of the feedback amplifier. • The circuit amplifies the difference signal that results from comparison and this process is responsible for de-sensitivity and control of the output in a feedback system.

Comparator or Mixer Network January 9, 2022 13

Schematic Representation of a Single. Loop Feedback Amplifier January 9, 2022 14

Feedback Amplifier Topologies Voltage-series or series-shunt feedback January 9, 2022 15

Feedback Amplifier Topologies contd. Current-series or Series-series feedback January 9, 2022 16

Feedback Amplifier Topologies contd. Current-shunt or shunt-series feedback January 9, 2022 17

Feedback Amplifier Topologies contd. Voltage-shunt or Shunt-shunt feedback January 9, 2022 18

Signals and Ratios in Feedback Amplifiers January 9, 2022 19

Block diagram of ideal feedback amplifier:

PROPERTIES OF NEGATIVE FEEDBACK: ** Reduction in gain � Negative feedback has the following advantages: (i) Negative feedback increases the input impedance of the voltage amplifier. (ii) The output impedance of the voltage amplifier can be further lowered by negative feedback. (iii) The transfer gain Af of the amplifier with a feedback can be stabilized against the variations of h or hybrid parameters of the transistors, or the parameters of the other active devices used in the amplifier. (iv) Negative feedback increases the frequency response and the bandwidth of the amplifier. (v) Negative feedback increases the linear range of operation of the amplifier. (vi) Negative feedback causes reduction in noise.

SENSITIVITY: • The sensitivity of transfer gain of the feedback amplifier AF with respect to the variations in the internal amplifier gain A is defined as the ratio of the fractional change in gain with the feedback to the fractional change in gain without the feedback. • The gain sensitivity S of the feedback amplifier is given by: where, d. Af /Af = fractional change in gain with the feedback; d. A/A = fractional change in gain without the feedback.

SENSITIVITY & DESENSITIVITY: The inverse or reciprocal of sensitivity is called De-Sensitivity (D) indicates the fraction by which the voltage gain has been reduced due to feedback.

Effect of Negative Feedback on Amplifier Characteristics January 9, 2022 24

Voltage-series feedback circuit used to calculate i/p & o/p resistances January 9, 2022 25

Current-shunt feedback circuit used to calculate i/p & o/p resistances January 9, 2022 26

Feedback amplifier analysis January 9, 2022 27

Oscillators are circuits that produce a continuous signal of some type without the need of an input. These signals are used in Communications systems, digital systems (including computers), and test equipment make use of oscillators.

Oscillators

Oscillators • An oscillator is a circuit that produces a repetitive signal from a dc voltage. • The feedback type oscillator which rely on a positive feedback of the output to maintain the oscillations. • The relaxation oscillator makes use of an RC timing circuit to generate a non-sinusoidal signal such as square wave.

Oscillators • The feedback oscillator is widely used for generation of sine wave signals. • Positive feedback is the condition wherein a portion of the output voltage of an amplifier is fed back to the input with no net phase shift, resulting in a reinforcement of the output signal. Positive feedback is also referred to as regenerative feedback.

An amplifier with tr gain A & feedback network not yet connected to form a closed loop January 9, 2022 32

The Barkhausen Criterion II • The Barkhausen Criterion calls for two requirement for the loop gain. First, the magnitude of the loop gain must be unity. Second, the phase angle of the loop gain must be zero the frequency of oscillation. (e. g, if a non-inverting amplifier is used, then the phase angle of must be zero. For a inverting amplifier, the phase angle should be 180) • In real oscillator design, we usually design loop-gain magnitude slightly larger than unity at the desired frequency of oscillation. Because a higher gain magnitude results in oscillations that grow in amplitude with time, eventually, the amplitude is clipped by the amplifier so that a constant-amplitude oscillation results. January 9, 2022 33

The Barkhausen Criterion II • On the other hand, if exact unity loop gain magnitude is designed, a slight reduction in gain would result in oscillations that decays to zero. • One important thing to note is that the initial input Xin is not needed, as in real circuits noise and transient signals associated with circuit turning on can always provide an initial signal that grows in amplitude as it propagates around the loop (assuming loop gain is larger than unity). January 9, 2022 34

Oscillator Operation Start-Up Conditions – For oscillation to begin, the loop gain A ß around the positive feedback loop must be greater than 1 so that the output voltage can build up to a desired level. – Then A ß decrease to 1 and maintains the desired magnitude

• Barkhausen criterion: βAv = 1 • If this criterion is not met, one of the following occurs: (a) If βAv < 1, the oscillations die out after a few cycles. (b) If βAv > 1, the oscillator drives itself into saturation and cutoff clipping.

RC Phase-shift oscillator

Phase-shift oscillator • Utilizes three RC circuits to provide 180º phase shift that when coupled with the 180º of the FET amplifier 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. Frequency of the oscillator

LC - OSCILLATORS • RF oscillators often use LC tank circuits to control the frequency of oscillation. The tank circuits are tapped to control the amount of feedback. • Hartley oscillators use tapped coils while Colpitts oscillators use capacitive taps. January 9, 2022 39

Basic configuration of resonantcircuit oscillator January 9, 2022 40

Colpitts Oscillator

Colpitts Oscillator • The Colpitts Oscillator is a discrete LC amplifier that uses a pair of tapped capacitors and an inductor to produce the regenerative feedback necessary for oscillation. • Uses an LC circuit in the feedback loop : • To provide necessary phase shift • To act as a resonant filter that passes only the desired frequency • Approximate frequency of oscillation :

Example question of the Colpitts Oscillator 1. Determine the frequency of oscillation for the oscillator in the figure.

Solution 1.

**The Clapp Oscillator • The Clapp is similar to the Colpitts with exception to the additional capacitor C 3 in series with the inductor in the resonant feedback circuit • The same formula applies as the Colpitts. • Total capacitance : • Approximate frequency : • C 3 provide more accurate and stable frequency of oscillation

‘ THANK YOU pramodp. chavan@gmail. com 8888890098 January 9, 2022 46