Feedback of Amplifier Circuits IConcept of amplifier feedbackAmplifier negative feedback: reduce nonlinear distortionAmplifier negative feedback: noise reductionAmplifier negative feedback typesSlide 6Effect of negative feedback on gainNegative feedback on input impedanceNegative feedback on output impedanceSome practical feedback network in amplifiersDesign of negative feedback amplifiersAn example of feedback voltage amplifier1 Feedback of Amplifier Circuits I•Feedback is to return part of the output to the input for a circuit/system (amplifiers in our context)•Feedback is very useful in Control Theory and Systems and is well researched•Amplifier circuit can have negative feedback and positive feedback. Negative feedback returns part of the output to oppose the input, whereas in positive feedback the feedback signal aids the input signal.•Both negative feedback and positive feedback are used in amplifier circuits•Negative feedback can reduce the gain of the amplifier, but it has many advantages, such as stabilization of gain, reduction of nonlinear distortion and noise, control of input and output impedances, and extension of bandwidthGraphs are from Prentice Hall2Concept of amplifier feedbackgain loop:tcoefficienfeedback :amplifier theofgain loopopen the:amplifier theofgain loopclosed the:feedback negative then , if ,1AAAAAAAxxAffsof.0 so,1,1 If fsissfxxxxAAxxA• Thus, the closed-loop gain would be much more stable and is nearly independent of changes of open-loop gain• Thus, in a negative feedback amplifier, the output takes the value to drive the amplifier input to almost 0 (this is summing point constraints)../1then ,1 IffAA3Amplifier negative feedback: reduce nonlinear distortion010-for 98.9100for 99.9ofofxAxA•If a pre-amplifier with gain 1000 is placed before the nonlinear one so that the whole amplifier is used with negative feedback, and the gain for whole amplifier becomes: which greatly reduce the nonlinear distortion.•This is achieved through compensatory distortion of the input signal1A4Amplifier negative feedback: noise reduction22222112121112212)()()(1)(1)()()()()()()()()()(AxxSNRAAAtxAAAAtxtxtxAtxtxtxAtxtxtxtxnoisesnoisesoonoiseos2211)()()()()(noisesnoisesoxxSNRAtxAtxtx•If an amplifier (assumed to be noise free or very low noise) is placed before the noisy amplifier, then the Signal-to-Noise (SNR) ratio is greatly enhanced (by a factor equal to the preceding amplifier gain) •As a summary, negative feedback is very useful in amplifier circuits. It can help stabilize the gain, reduce nonlinear distortion and reduce noise.•Also, as will be shown later, negative feedback in amplifiers can also control input and output impedance.5Amplifier negative feedback types6Amplifier negative 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 (corresponding to (a) in previous slide)Series current feedback (corresponding to (b) in previous slide)Parallel voltage feedback (corresponding to (c) in previous slide)Parallel current feedback (corresponding to (d) in previous slide)In voltage feedback, the input terminals of the feedback network are in parallel with the load, and the output voltage appears at the input terminals of the feedback block.Whereas in current feedback, the input terminals of the feedback network are in series with the load, and the load current flows through the input of the feedback block.As a result, a simple test on the feedback type is to open-circuit or short-circuit the load. If the feedback signal vanishes for an open-circuit load, then it is current feedback. If the feedback signal vanishes for a short-circuit load, it is voltage feedback.7Effect of negative feedback on gainvvvffAAAAAA1 so,1,generalIn mmmffGGGAAA1 so amplifier, uctance transcondaby modeled is this,1 generalIn mmmffRRRAAA1 so amplifier, stance transresiaby modeled is this,1 generalIn •In series voltage feedback, input signal is voltage and output voltage is sampled, so it is natural to model the amplifier as a voltage amplifier.•Amplifier employing series current feedback is modeled as a transconductance amplifier.•Amplifier employing parallel voltage feedback is modeled as a transresistance amplifier.•Amplifier employing parallel current feedback is modeled as a current amplifier.iiiffAAAAAA1 so amplifier,current aby modeled is this,1 generalIn8Negative feedback on input impedance)1(ARRiifiR)1/(ARRiif•For series feedback, the following model can be used for analysis of input impedance (the output x could be either voltage or current) If the input impedance of the open-loop amplifier is Ri, then the closed-loop impedance isso, series feedback (either current or voltage) increase the input impedance•Similarly, the effect of parallel feedback on input impedance can be analyzed using a similar model, the closed-loop input impedance would then beso, parallel feedback decrease the input impedance feedback negativefor 1 notice ),1(  AβARRiif9Negative feedback on output impedance)1/(ARRoof•For voltage feedback, (it could be either series or parallel feedback), the closed-loop impedance isso, voltage feedback decrease the output impedance•Similarly, for current feedback (either series or parallel feedback), the closed-loop impedance is so, current feedback increase the output impedance•As a summary, negative feedback tends to stabilize and linearize gain, which are desired effects.•For a certain type of amplifier, negative feedback tends to produce an ideal amplifier of that type. •For example, series