PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Chapter 8 - Feedback1 - Desensitize The Gain2 - Reduce Nonlinear Distortions3 - Reduce The Effect of Noise4 – Control The Input And Output Impedances5 – Extend The Bandwidth Of The AmplifierBasic structure of a feedback amplifier. To make it general, the figure shows signal flow as opposed to voltages or currents (i.e., signals can be either current or voltage). The open-loop amplifier has gain A  xo = A*xi Output is fed back through a feedback network which produces a sample (xf) of the output (xo)  xf = xoWhere  is called the feedback factor The input to the amplifier is xi = xs – xf (the subtraction makes feedback negative) Implicit to the above analysis is that neither the feedback block nor the load affect the amplifier’s gain (A). This not generally true and so we will later see how to deal with it. The overall gain (closed-loop gain) can be solved to be: A is called the loop gain 1+A is called the “amount of feedback”Source ALoadxsxfxixoAAxxAsof1The General Feedback StructureThe General Feedback Structure (Intro to Simulink)xsxixfxoAxoA xixixsxfxf xoAfxoxsA1 A  A  1 A feedabck factor loop gain amount of feedabckxfA 1 A xsFinding Loop GainGenerally, we can find the loop gain with the following steps: –Break the feedback loop anywhere (at the output in the ex. below) –Zero out the input signal xs –Apply a test signal to the input of the feedback circuit –Solve for the resulting signal xo at the output If xo is a voltage signal, xtst is a voltage and measure the open-circuit voltage If xo is a current signal, xtst is a current and measure the short-circuit current –The negative sign comes from the fact that we are apply negative feedbackAxs=0xfxixoxtstAxxAxAxAxxxxxxtstotstfiofitstf gain loop0Negative Feedback Properties Negative feedback takes a sample of the output signal and applies it to the input to get several desirable properties. In amplifiers, negative feedback can be applied to get the following properties –Desensitized gain – gain less sensitive to circuit component variations –Reduce nonlinear distortion – output proportional to input (constant gain independent of signal level) –Reduce effect of noise –Control input and output impedances – by applying appropriate feedback topologies –Extend bandwidth of amplifier These properties can be achieved by trading off gainGain DesensitivityFeedback 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… Divide by Af 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) 21AdAdAf AdAAAAAdAAdAff11112AAAf1Bandwidth ExtensionWe’ve mentioned several times in the past that we can trade gain for bandwidth. Finally, we see how to do so with feedback… Consider an amplifier with a high-frequency response characterized by a single pole and the expression: Apply negative feedback beta and the resulting closed-loop gain is: •Notice that the midband gain reduces by (1+AMbeta) while the 3-dB roll-off frequency increases by (1+AMbeta)  HMsAsA1     MHMMfAsAAsAsAsA1111Basic Feedback TopologiesDepending on the input signal (voltage or current) to be amplified and form of the output (voltage or current), amplifiers can be classified into four categories. Depending on the amplifier category, one of four types of feedback structures should be used (series-shunt, series-series, shunt-shunt, or shunt-series) Voltage amplifier – voltage-controlled voltage source Requires high input impedance, low output impedance Use series-shunt feedback (voltage-voltage feedback) Current amplifier – current-controlled current source Use shunt-series feedback (current-current feedback) Transconductance amplifier – voltage-controlled current source Use series-series feedback (current-voltage feedback) Transimpedance amplifier – current-controlled voltage source Use shunt-shunt feedback (voltage-current feedback)series-shuntshunt-seriesseries-seriesshunt-shuntExamples of the Four Types of Amplifiers•Shown above are simple examples of the four types of amplifiers. Often, these amplifiers alone do not have good performance (high output impedance, low gain, etc.) and are augmented by additional amplifier stages (see below) or different configurations (e.g., cascoding). vINRDvOUTVbRDvOUTiINVbiOUTiINvINiOUTVoltageAmpTransconductanceAmpTransimpedanceAmpCurrentAmpvINRDvOUTVbRDvOUTiINVbiINvINiOUTRDiOUTRDlower Zoutlower Zouthigher gain higher gainSeries-Shunt Feedback Amplifier(Voltage-Voltage Feedback)Samples the output voltage and returns a feedback voltage signal Ideal feedback network has infinite input impedance and zero output resistance Find the closed-loop gain and input resistance The output resistance can be found by applying a test voltage to the output So, increases input resistance and reduces output resistance  makes amplifier closer to ideal VCVS  ARVAVVRVVRRVVIVRAAVVAVVAVVVVVViiiiiisiiisisifsofosofsiof11ARRoof1The Series-Shunt Feedback AmplifierThe Ideal SituationThe series-shunt feedback amplifier: (a) ideal structure; (b) equivalent circuit.AfVoVsA1 A RifVsIiVsViRiRiVsVi RiVi A ViViRif Ri1 A  Zifs( ) Zis( ) 1 A s( )  s( ) Zofs( )Zos( )1 A s( )  s( )Series-Series Feedback Amplifier(Current-Voltage Feedback)For a transconductance amplifier (voltage input, current output), we must apply the appropriate feedback circuit Sense the output current and feedback a voltage signal. So, the feedback current