7/9/2016 HANSABA COLLEGE OF ENGINEERING AND TECHNOLOGY, SIDDHPUR.

CHAPTER:13 FEEDBACK AMPLIFIER PREPARED BY: GUIDE BY: Dwivedi Himani K. Jatin P. Patel ( ) Sindhi Dhanvanti P. ( )

The purpose of this exercise is to become familiar with the concept of feedback through practical investigations. This technique is frequently used in most areas of electrical engineering. The text of the exercise is made relatively comprehensive in order to give general information about feedback and aim therefore at the practical part of this special laboratory exercise. Feedback in amplifiers gives better performance in several important ways, including: a) Increased stability in the amplification. The gain is less dependent on the parameters of the amplifier elements. INTRODUCTION

b) Feedback reduces distortion in the amplifier. c) The bandwidth of the amplifier is increased. d) It is easier to achieve desired input and output impedances. These advantages are achieved at the expence of gain which is less than the amplifier gain without feedback. Stability, nonlinear distortion, bandwidth requirements and impedance matching are very important concepts that are part of a long list of problems, f.ex. in telecommunications.

A negative feedback amplifier (or feedback amplifier) is an electronic amplifier that subtracts a fraction of its output from its input, so that negative feedback opposes the original signal.electronicamplifiernegative feedback The applied negative feedback improves performance (gain stability, linearity, frequency response, step response) and reduces sensitivity to parameter variations due to manufacturing or environment.step response Because of these advantages, many amplifiers and control systems use negative feedback. FEEDBACK AMPLIFIER

BLOCK DIAGRAM OF FEEDBACK AMPLIFIER

Can increase or decrease input impedance (depending on type of feedback)impedance Can increase or decrease output impedance (depending on type of feedback) Reduces distortion (increases linearity) Increases the bandwidth Desensitizes gain to component variations Can control step response of amplifierstep response PROPERTIES OF FEEDBACK AMPLIFIER

May lead to instability if not designed carefully Amplifier gain decreases Input and output impedances of a negative feedback amplifier (closed-loop amplifier) become sensitive to the gain of an amplifier without feedback (open-loop amplifier)—that exposes these impedances to variations in the open loop gain, for example, due to parameter variations or nonlinearity of the open-loop gain DISADVANTAGE

SIGNAL FLOW ANALYSIS

A principal idealization behind thie formulation of the Introduction is the network's division into two autonomous blocks (that is, with their own individually determined transfer functions), a simple example of what often is called 'circuit partitioning‘,which refers in this instance to the division into a forward amplification block and a feedback block. In practical amplifiers, the information flow is not unidirectional as shown here. Frequently these blocks are taken to be two-port networks to allow inclusion of bilateral information transfer.two-port networks SIGNAL FLOW ANALYSIS

Casting an amplifier into this form is a non-trivial task, however, especially when the feedback involved is not global (that is directly from the output to the input) but local (that is, feedback within the network, involving nodes that do not coincide with input and/or output terminals)

Using NF: The properties of the feedback amplifier are modified in respect with the properties of the basic amplifier: the gain is reduced other properties are improved: the gain sensitivity is reduced the nonlinear distortion is reduced the bandwidth is extended the effect of the noise is reduced the input and output impedances are enhanced EFFECT OF NEGATIVE FEEDBACK AMPLIFIER

Series-Shunt Feedback, also known as series voltage feedback, operates as a voltage-voltage controlled feedback system. The error voltage fed back from the feedback network is in series with the input. The voltage which is fed back from the output being proportional to the output voltage, Vo as it is parallel, or shunt connected. SERIES-SHUNT FEEDBACK SYSTEM

For the series-shunt connection, the configuration is defined as the output voltage to the input voltage. Most inverting and non-inverting operational amplifier circuits operate with series-shunt feedback producing what is known as a “voltage amplifier”. As a voltage amplifier the ideal input resistance, Rin is very large, and the ideal output resistance, Rout is very small. Then the “series-shunt feedback configuration” works as a true voltage amplifier as the input signal is a voltage and the output signal is a voltage, so the transfer gain is given as: Av = Vout ÷ Vin.

Shunt-Series Feedback Systems Shunt-Series Feedback, also known as shunt current feedback, operates as a current-current controlled feedback system. The feedback signal is proportional to the output current, Io flowing in the load. The feedback signal is fed back in parallel or shunt with the input as shown.

SHUNT-SERIES FEEDBACK SYSTEM

For the shunt-series connection, the configuration is defined as the output current to the input current. In the shunt-series feedback configuration the signal fed back is in parallel with the input signal and as such its the currents, not the voltages that add. This parallel shunt feedback connection will not normally affect the voltage gain of the system, since for a voltage output a voltage input is required. Also, the series connection at the output increases output resistance, Rout while the shunt connection at the input decreases the input resistance, Rin. Then the “shunt-series feedback configuration” works as a true current amplifier as the input signal is a current and the output signal is a current, so the transfer gain is given as: Ai = Iout ÷ Iin.

Feedback Circuit The function of the feedback circuit is to return a fraction of the output voltage to the input of the amplifier. shows the feedback circuit of negative voltage feedback amplifier. It is essentially a potential divider consisting of resistances R1 and R2. The output voltage of the amplifier is fed to this potential divider which gives the feedback voltage to the input.

Thank you