VIEWS: 14 PAGES: 20 CATEGORY: College POSTED ON: 10/27/2012 Public Domain
ECE, ME – I SEM BASIC ELECTRONICS Feedback amplifiers A) Introduction B) Types of Feedback C) Effect of feedback (positive & negative) on Gain & Bandwidth WHAT DO YOU MEAN BY FEEDBACK? When a part or fraction of output is combined to input, feedback is said to be exist. Types of feedback a) Negative Feedback: When the Feedback voltage or current) is so applied as to weaken the input signal it is in phase opposition to the input signal), It is called negative, inverse or degenerative feedback. b) Positive Feedback: When the Feedback voltage or current) is so applied as to increase the input signal it is in phase same to the input signal), It is called positive, generative feedback. Concept of amplifier feedback(NEGATIVE) xo A Af , if A f A, then negative feedback x s 1 A A f : the closed loop gain of the amplifier A : the open loop gain of the amplifier : feedback coefficient A : loop gain If A 1, then A f 1 / . Thus, the closed-loop gain would be much more stable and is nearly independent of changes of open-loop gain A If A 1, x f x s x s , so xi x s x f 0. Thus in a negative 1 A feedback amplifier, the output takes the value to drive the amplifier input to almost 0 (this is summing point constraints). Positive feedback xo A Af , if A f A, then Positive feedback xs 1 A A f : the closed loop gain of the amplifier A : the open loop gain of the amplifier : feedback coefficient A : loop gain If A 1, then A f inf inite Feedback Types If the feedback network samples the output voltage, it is voltage feedback. If it samples the output current, it is current feedback. The feedback signal can be connected in series or in parallel with the signal source and the amplifier input terminals, so called series feedback and parallel feedback. So, there are four types of negative feedback in amplifier circuits: Series voltage feedback Series current feedback Parallel voltage feedback Parallel current feedback Voltage Series Feedback Voltage-sampling series-mixing (series-shunt) topology Voltage Shunt Feedback Voltage-sampling shunt-mixing (shunt-shunt) topology. Current Series current-sampling series-mixing (series-series) topology Current Shunt current-sampling shunt-mixing (shunt-series) topology Effect of Negative feedback Gain Desensitized Bandwidth increases Gain Desensitivity Feedback can be used to desensitize the closed-loop gain to variations in the basic amplifier. Let’s see how. Assume beta is constant. Taking differentials of the closed-loop gain equation gives… A dA Divide by Af Af dAf 1 A 1 A 2 dAf dA 1 A 1 dA Af 1 A 2 A 1 A A This result shows the effects of variations in A on Af is mitigated by the feedback amount. 1+Abeta is also called the desensitivity amount We will see through examples that feedback also affects the input and resistance of the amplifier (increases Ri and decreases Ro by 1+Abeta factor) Bandwidth Extension in Negative feedback AvL f Avm 1 jf 1 / f Avmf f Avm 1 Avm Avlf Avl / 1 Avl AvLf f Avm /(1 jf 1 / f ) Avm 1 ( Avm /(1 jf 1 / f )) 1 jf 1 / f Avm Notice that the midband gain reduces by (1+AMbeta) while the 3-dB roll-off frequency increases by (1+AMbeta) Divide the numerator and denominator by 1+Avmβ Thus lower cut off frequency reduces. NOTICE THAT THE MIDBAND GAIN REDUCES BY (1+AMBETA) WHILE THE 3-DB ROLL-OFF FREQUENCY INCREASES BY (1+AVMBETA) AvLf f Avmf 1 jf 1" / f Avmf f Avm 1 Avm f 1" f 1 / 1 Avm f2= upper cut off frequency It is increased by (1+Avmβ ) f2 “ = f2(1+Avmβ ) Hence bandwidth increases Advantage of Negative Feedback –Desensitized gain – gain less sensitive to circuit component variations .Gain is stabilized. –Reduce nonlinear distortion – output proportional to input (constant gain independent of signal level) –Reduce effect of noise – input impedance increases and output impedances decreases Extend bandwidth of amplifier. DISADVANTAGES: A) less gain B) possible oscillation if not properly designed. EFFECT OF POSITIVE FEEDBACK ON BANDWIDTH Bandwidth decreases.(derive by yourself as like in Negative Feedback) DISADVANTAGES: A) Very prone to oscillation. B) Poor freq. response, more distortion, more drift. ADVANTAGES: A) High gain