Unit 1 NEGATIVE FEEDBACK IN AMPLIFIERS Outline 1

Unit 1 NEGATIVE FEEDBACK IN AMPLIFIERS

Outline 1. 1 Concept of feedback 1. 2 Gain of negative feedback amplifiers 1. 3 Advantages and disadvantages of negative feedback 1. 4 Advantages of negative feedback 1. 5 Effect of negative feedback 1. 6 Types of negative feedback

1. 1 Concept of feedback q q Invented in 1928 by Harold Black – engineer in Western Electric Company, methods to stabilize the gain of amplifier for use in telephone repeaters. In feedback system, a signal that is proportional to the output is fed back to the input and combined with the input signal to produce a desired system response.

Feedback Amplifier - Concept Basic structure of a single - loop feedback amplifier

Basic Feedback Concept Basic configuration of a feedback amplifier

Continue………. Amplifier: The circuit which amplifies the weak input signal to the strong output signal is known as amplifier. Feedback: the process of injecting a fraction of output energy of some devise back to the input is known as feedback. Types of feedback: positive feedback negative feedback

1. 1. 1 positive feedback: Positive feedback: if the feedback energy is in phase with the input signal, then it is called a positive feedback. It is also known as regenerative feedback or direct feedback. A phase shift of 180 is produced by amplifier and phase shift 180 is introduced by feedback network so total 360 phase shift.

Continue…. . q. Positive feedback is the process when the output is added to the input, amplified again, and this process continues. q. Positive feedback is used in the design of oscillator and other application.

Advantages Increase the gain of the amplifier q Its capability of increasing the power of the original signal. q Disadvantages q It causes increased distortion and instability Application q Used in oscillator.

1. 1. 2 Negative feedback: Positive feedback: if the feedback energy is out of phase with the input signal, then it is called a negative feedback. q It is also known as degenerative feedback or inverse feedback. q A phase shift of 180 is produced by amplifier and phase shift 0 is introduced by feedback network so total 180 phase shift. q

Continue…. . q. Negative feedback is when the output is subtracted from the input. q. The use of negative feedback reduces the gain. q. Part of the output signal is taken back to the input with a negative sign.

Advantages Stability in the gain of the amplifier. q Reduction in distortion and noise. q Disadvantages q reduced the voltage gain. Application q Used in amplifier.

1. 1. 3 comparison of positive and negative feedback. No. parameter Positive feedback Negative feedback 1 Overall phase shift O or 360 180 2 Feedback signal and input signal In phase Out of phase 3 Input voltage increase decreases 4 Output voltage increase decreases 5 Voltage gain increase decreases 6 Stability poor better 7 Application oscillator amplifier 8 noise increase decreases

1. 2 Gain of negative feedback amplifiers A feedback amplifiers consists of two parts: an amplifier and feedback circuit. When negative feedback is applied: 1. The gain of the amplifier is reduced. 2. The voltage applied to the amplifier is extremely small. 3. The feedback fraction B is always between 0 or 1.

1. 2. 1 open loop voltage gain q A= Vo/Vi Vin Amplifier Where A = open loop voltage gain Vo= output voltage Vi= input voltage Vout

1. 2. 2 Closed loop voltage gain

Continue……. Vi= Vs +Vf and Vf= β Vo For positive feedback Vi= (Vs+ β Vo) Vo=A Vi, Vo=A(Vs+ β Vo) Vo=AVs+ A β Vo, Vo+A β Vo= AVs Vo(1+A β)= AVs Vo/Vs=A/(1+A β) but Vo/Vs= Af Af= A/(1+A β) A β= feedback factor β= feedback ratio (1+A β) = loop gain

1. 3 Advantages and disadvantages of negative feedback 1. 2. 3. 4. 5. Advantages: Gain Sensitivity – variations in gain is reduced. Bandwidth Extension – larger than that of basic amplified. Noise Sensitivity – may increase S-N ratio. Reduction of Nonlinear Distortion Control of Impedance Levels – input and output impedances can be increased or decreased

Disadvantages of negative feedback 1) 2) 3) Circuit Gain – overall amplifier gain is reduced compared to that of basic amplifier. Stability – possibility that feedback circuit will become unstable and oscillate at high frequency. more no. of amplifiers are required.

Applications of negative feedback 1. 2. 3. 4. In all electronics amplifiers. In regulated power supply. In wideband amplifiers. In special circuits like bootstrapping.

1. 4 Advantages of negative feedback 1. 4. 1 gain stability: Af=A/(1+A β) If A β >>1 Then Af= 1/ β Gain depends only on feedback factor β, and feedback circuit is resistive network so gain is stable.

1. 4. 2 Reduction of non linear distortion: Df = D/(1+A β) Here (1+A β) >>1 So Df will be reduced.

1. 4. 3 Reduction of noise Nf = N/(1+A β) Here (1+A β) >>1 So Nf will be reduced.

1. 4. 4 Improvement in the frequency response

Continue…. . When the negative feedback is given in amplifier, the upper cutoff frequency is increased by factor (1+Aβ) and lower cutoff frequency decreases by factor (1 -A β). q

1. 4. 5 Increased in bandwidth The gain bandwidth product is Ax β W = Afx β Wf So β Wf= (A/Af)x β W Here Af<A So A/Af>>1 So bandwidth is increased. q

1. 5 Effects of negative feedback 1. 5. 1 Input impedance of amplifier.

Continue…… Voltage series feedback circuit, Ii=Vi/Zi But Vi=Vs-Vf Ii= Vs-Vf/Zi And Vf= β vo Ii=Vs- β vi/Zi Also Vo=Avi Ii=Vs- β AVi/Zi so Vs/Ii= Zi(1+A β) Zif=Zi(1+A β) So input impedance is increased by factor (1+A β)

1. 5. 2 output impedance of amplifier.

Continue…… Voltage series feedback circuit, Vo=Io. Zo+AVi But Vi=Vf so Vo=Io. Zo-AVf And Vf= β vo V 0= Io. Zo-A β Vo Vo/Io= Zo/(1+A β) Z 0 f = Zo/(1+A β) So output impedance is decreased by factor (1+A β)

1. 6 Types of negative feedback. Classification of amplifiers 1)Use 1. 1 voltage amplifier 1. 2 power amplifier 2) Frequency capabilities 2. 1 audio frequency 2. 2 RF amplifiers 3) Coupling methods 3. 1 RC coupled 3. 2 transformer coupled 4) Method of 4. 1 Class A operations 4. 2 class B 4. 3 class C

Types of amplifiers Classify amplifiers into 4 basic categories based on their input (parameter to be amplified; voltage or current) & output signal relationships: q q Voltage amplifier (series-shunt) Current amplifier (shunt-series) Transconductance amplifier (series-series) Transresistance amplifier (shunt-shunt)

Feedback Configuration Series: connecting the feedback signal in series with the input signal voltage. Shunt: connecting the feedback signal in shunt (parallel) with an input current source

Types of feedback 1) 2) 3) 4) Voltage series feedback Voltage shunt feedback Current series feedback Current shunt feedback

1. 6. 1 voltage series feedback (shunt –derived or series –fed)

Continue…. . Input Resistance, Rif Output Resistance, Rof Or Assume Vi=0 and Vx applied to output terminal. Input current Rif with feedback Or Input current Rof with feedback

Circuit diagram of voltage series feedback

1. 6. 2 voltage shunt feedback (shunt –derived or shunt –fed)

Continue…… The feedback samples a portion of the output voltage and converts it to a current – current-tovoltage amplifier. The circuit consist of a basic amplifier that converts the error current to an output voltage with a gain factor, Az and that has an input resistance, Ri. The feedback circuit samples the output voltage and produces a feedback current, Ifb, which is in shunt with the input current, Ii.

Continue… Input Resistance, Rif Output Resistance, Rof Or Assume Vi=0 and Vx applied to output terminal. Input current Rif with feedback Or Input current Rof with feedback

Circuit diagram of voltage shunt feedback

1. 6. 3 current series feedback (series –derived or series–fed)

Continue…. . q q q The feedback samples a portion of the output current and converts it to a voltage – voltage-tocurrent amplifier. The circuit consist of a basic amplifier that converts the error voltage to an output current with a gain factor, Ag and that has an input resistance, Ri. The feedback circuit samples the output current and produces a feedback voltage, Vfb, which is in series with the input voltage, Vi.

Continue…. Input Resistance, Rif Output Resistance, Rof Or Assume Ii=0 and Ix applied to output terminal. Input current Rif with feedback Rof with feedback

Circuit diagram of current series feedback

1. 6. 4 current shunt feedback (series –derived or shunt–fed)

Continue… Shunt input connection decrease input resistance – avoid loading effects on the input signal current source. Series output connection increase the output resistance - avoid loading effects on the output signal due to load connected to the amplifier output.

Continue… Input Resistance, Rif Output Resistance, Rof Or Assume Ii=0 and Ix applied to output terminal. Input current Rif with feedback Rof with feedback

Circuit diagram of current shunt feedback

Feedback Amplifier