FEEDBACK CIRCUITS CLASSIFICATION OF AMPLIFIER CONCEPT OF FEEDBACK TYPES OF FEEDBACK FEEDBACK TOPOLOGIES Reference Books: 1.Electronic Devices and circuit theory, int. edition (Chapter 14) 2.Microelectronics circuits, 5th ed. Sedra and smith (Chapter 8) 3. Microelectronic circuits: analysis and design (Chapter 10)

Classification of Amplifiers Before proceeding with the concept of feedback it is useful to classify amplifiers into 4 basic categories based on their input & output signal relationships. Voltage amplifier Current amplifier Transconductance amplifier Transresistance amplifier

Classification of amplifiers 1. Voltage amplifier if then and if then, hence with represent the open circuit voltage gain.

Classification of amplifiers 2. Current amplifier if then and if then, hence with represent the short circuit current gain.

3.Transconductance amplifier Classification of amplifiers 3.Transconductance amplifier if then and if then, hence with represent the short circuit mutual or transfer conductance

4. Transresistance amplifier Classification of amplifiers 4. Transresistance amplifier if then and if then , hence with represent the open circuit mutual or transfer resistance.

Concept of feedback Valve example As the water nears the specified level, the valve is closed. Negative feedback is most commonly used to control systems.

Types of feedback What is feedback? Feedback is a technique where a proportion of the output of a system (amplifier) is fed back and recombined with input. There are two types of feedback amplifier. Positive feedback Negative feedback

Types of feedback 1.Positive Feedback Positive feedback is the process when the output is added to the input, amplified again, and this process continues. Example: In a PA system, you get feedback when you put the microphone in front of a speaker and the sound gets uncontrollably loud (you have probably heard this unpleasant effect).

Types of feedback 2. Negative Feedback Negative feedback is when the output is subtracted from the input. Example: Speed control. If the car starts to speed up above the desired set-point speed, negative feedback causes the throttle to close, thereby reducing speed; similarly, if the car slows, negative feedback acts to open the throttle. The use of negative feedback reduces the gain. Part of the output signal is taken back to the input with a negative sign.

Negative Feedback Gain Types of feedback Negative Feedback Gain The gain with feedback (or closed-loop gain) Af as follows: The quantity A is called the loop gain, and the quantity (1+A) is called the amount of feedback.

Advantages of Negative Feedback Stabilization of gain make the gain less sensitive to changes in circuit components e.g. due to changes in temperature. Reduce non-linear distortion make the output proportional to the input, keeping the gain constant, independent of signal level. Reduce the effect of noise minimize the contribution to the output of unwanted signals generated in circuit components or extraneous interference.

Advantages of Negative Feedback (cont.) 4. Extend the bandwidth of the amplifier Reduce the gain and increase the bandwidth 5. Modification the input and output impedances raise or lower the input and output impedances by selection of the appropriate feedback topology. Disadvantages of Negative Feedback 1. Circuit gain - Reduce 2. Stability – Tend to be oscillate

Basic structure of feedback amplifier A : open-loop gain B : feedback factor  xs xi xf xo + - A Error signal The gain of the feedback amplifier If AB is very large, the overall gain becomes a function of the feedback network only.

Basic structure of feedback amplifier Basic structure of a single - loop feedback amplifier

Feedback Network This block is usually a passive two-port network. contain resistors, capacitors, and inductors. Usually it is simply a resistive network.

Sampling Network The output voltage is sampled by connecting the feedback network in shunt across the output. Type of connection is referred to as voltage or shunt or node sampling.

Sampling Network (cont.) The output current is sampled by connecting the feedback network in series with the output Type of connection is referred to as current or series or loop sampling.

Comparator or Mixer Network voltage - applied feedback . identified as voltage or series or loop mixing.

Comparator or Mixer Network (Cont.) current - applied feedback identified as current or shunt or node mixing.

Feedback Topologies Four basic feedback topologies based on the parameter to be amplified (voltage or current) and the output parameter (voltage or current). The four feedback circuit can be described by the types of connections at the input and output of circuit.

1. Series-shunt topology Feedback Topologies 1. Series-shunt topology Voltage-mixing voltage-sampling

2.Shunt-series topology Feedback Topologies 2.Shunt-series topology Current-mixing current-sampling

3.Series-series topology Feedback Topologies 3.Series-series topology Voltage-mixing current-sampling

Current-mixing voltage-sampling Feedback Topologies 4.Shunt-shunt topology Current-mixing voltage-sampling

Ideal series-shunt feedback Feedback Topologies Ideal series-shunt feedback Rof Rif Ri Ii Vi Vs Vf Vo AVi Ro Vo

Ideal series-shunt feedback (cont.) Feedback Topologies Ideal series-shunt feedback (cont.) 1. Input resistance with feedback 2.Output resistance with feedback

Practical series-shunt feedback Feedback Topologies Practical series-shunt feedback Rs Vs Rif Rin Rout Rof Vo

Ideal shunt-series feedback Feedback Topologies Ideal shunt-series feedback If Is Io Rif Rof Vi Ii Ri Ro AIi Io

Ideal shunt-series feedback (cont.) Feedback Topologies Ideal shunt-series feedback (cont.) 1. Gain of feedback amplifier 2. Input resistance with feedback 3.Output resistance with feedback

Practical shunt-series feedback Feedback Topologies Practical shunt-series feedback RL Io Rout Rin Is

Ideal series-series feedback Feedback Topologies Ideal series-series feedback Rof Rif - Vf + Vs Io Io Vi Ri AVi Ro Ii

Ideal series-series feedback (cont.) Feedback Topologies Ideal series-series feedback (cont.) 1. Gain of feedback amplifier 2. Input resistance with feedback 3.Output resistance with feedback

Practical series-series feedback Feedback Topologies Practical series-series feedback RL Vs Rs Rout Rin Io

Ideal shunt-shunt feedback Feedback Topologies Ideal shunt-shunt feedback Ii Is If Vi Ri Ro Rof Rif Vo AIi Vo

Ideal shunt-shunt feedback Feedback Topologies Ideal shunt-shunt feedback

Practical shunt-shunt feedback Feedback Topologies Practical shunt-shunt feedback Rs Is Rout Rin RL Vo

Feedback relationship Feedback Topologies Feedback relationship Gain Input resistance Output resistance Without feedback A Ri Ro Series-shunt Series-series Shunt-shunt Shunt-series

Feedback Amplifier Topologies

FEEDBACK CIRCUITS Exercise Consider the non-inverting op-amp circuit with parameters R1 = 10 k, R2 = 30 k, and A = 104. Assume Ri = . Determine the closed-loop voltage gain. If the open-loop gain increases by a factor of 10, what is the percent change in the closed-loop gain? (Ans : 3.9984, 0.036%)