App_II_CH1_Feedback.pdf - -CHAPTER 1 FEEDBACK AMPLIFIERS-1.1 INTRODUCTION In electronics I we have seen small-signal voltage gain and other

App_II_CH1_Feedback.pdf - -CHAPTER 1 FEEDBACK...

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EENG2202 1 --------------------------------------------------------------------------------------------------------------------- CHAPTER 1 FEEDBACK AMPLIFIERS --------------------------------------------------------------------------------------------------------------------- 1.1 INTRODUCTION In electronics I, we have seen small-signal voltage gain and other characteristics of transistors are function of transistor parameters. In general, these parameters vary with temperature and they have range of values for a given type of transistor. This means that the Q-point, voltage gain, and other circuit properties can vary from one circuit to another, and can be function of temperature. Such transistor circuit characteristics can be made essentially independent of the individual transistor parameters by using feedback. Feedback is the process where by a portion of the output is returned to the input to form part of the system excitation. There are two basic types of feedback: negative feedback & positive feedback. Negative feedback : In this case the feedback signal is out of phase with the input signal. The amplifier introduces a 180 0 phase shift into the circuit, while the feedback network does not. Positive feedback: In this case the feedback signal is in phase with the amplifier input signal. Both the amplifier and the feedback signal introduce a 180 0 phase shift. The result is a 360 0 phase shift around the loop, causing the feedback signal to be in phase with the original signal. The positive feedback is used in oscillator circuits. In this chapter, we will concentrate on negative feedback. Negative feedback circuits bring a number of improvements with some reduction of the gain, among them being: 1 Better stabilized voltage gain : variations in the gain as a result of change in transistor parameter are reduced by feedback. This reduction in sensitivity of gain is one of the most attractive features of negative feedback. 2. Increase in bandwidth: the bandwidth of the circuit that incorporates negative feedback is larger than the basic amplifier. 3. Reduction of non-linear distortion: negative feedback reduces the large signal distortion that could arise due to the non-linear response of the transistor. 4. Minimize noise sensitivity: negative feedback may increase signal to noise ratio if noise is generated with the feedback loop.
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EENG2202 2 5. Control of impedance level : the input and output impedances can be increased or decreased using the appropriate type of negative feedback circuit. On the other hand, negative feedback circuits have disadvantages like reduction in the overall gain (the price required to improve circuit performance), and instability (oscillation) at high frequencies. 1.2 Basic representation of feedback amplifiers In feedback representation we may sample the output voltage or current by means of a suitable sampling network and apply this signal to the input through the feedback two port networks as shown below. At the input the feedback signal is combined with the signal through the mixer network and is feed into the amplifier. The signal source of the feedback amplifier is either a signal voltage or a signal current.
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