HYM report .docx - Feedback Lab report Department...

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Feedback Lab report Department: Electronic and Electrical Engineering Candidate Number: 17006013 Name: Yumeng Hu (Fiona Hu)
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Abstract This report consists 4 pats. The first part gives an introduction and theoretical background of amplifier feedback operation. Following by the experiment and results sections. In the experiment part, all the tasks for the experiment are described and answered. The final part is the conclusion and discussion about the results regarding the theory. This helps to gain a better understanding of the concepts in a practical content. The experiment is about investigating the open-loop, closed-loop configuration in terms of feedback and stability. Introduction The aim of this experiment is to investigate amplifiers’ behaviour when feedback applies to them. Both open loop system (no feedback) and closed loop system (with feedback) will be analysed and compared by using an unknown characteristics amplifier. The first part of the experiment look at open loop characterization and non-linear behaviour. The second part investigates the feedback and the Gain-Bandwidth-Product for close loop configuration. The last part is focusing on the amplifier stability. The reason of doing this experiment is simply that it can help us to achieve a better understanding of feedback principle in a more practical context. Procedures and measurements will be presented along with the results. Theoretical background Firstly, general concepts of feedback will be explained here before moving to the experiment. Feedback is feeding some of the amplifier’s output back to the input. A feedback circuit contains two parts as shown below in figure 1. One part is the amplifier with a gain of A, which A is the ratio of output voltage V out to the input voltage V dif . The other part is the feedback circuit with a gain of ? , which is called the feedback ratio. Such that V out = A*V dif =V in ? *V out. Through derivation we can get the equation V out Vin = A 1 + A β . If the open loop gain A is very high, then the closed loop gain will be dominated by the feedback fraction ? . There are two types of feedbacks depending on whether the signal is enhancing or opposing the input signal. Positive feedback happens when the feedback signal aids the input signal, which increases the gain of the amplifier due to both the input signal and feedback signal introduces a phase shift of 180 o . Thus, there will be a 360 o resultant phase shift. However, there is a trade-of as increasing the gain causes increase in distortion and instability in the circuit. Such that the positive feedback is not recommended for amplifiers. For negative feedback, the feedback signal opposes the input signal. The amplifier leads a 180 o phase shift into the circuit but zero phase shift happens in feedback network. Thus, the Fig 1. Generalised system employing negative
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resultant feedback voltage is 180 o out of phase with the input signal V in . Although the gain of negative feedback is reduced, the benefits cannot be ignored: it improves the stability of the gain, reduces distortion and reduces noise. Also, it increases the input impedance and
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