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# E5_3 - 1 Introduction 1.1 Background An instrument capable...

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1. Introduction: 1.1 Background An instrument capable of amplifying every component in a signal by the same amount is an amplifier. Two classes of transistor amplifier are used in the engineering field, i.e. bipolar junction transistors (BJT) and field-effect transistors (FET). In our experiment, only non npn BJT transistor is dealt with. A npn BJT transistor is a transistor in which a thin layer of p-type semiconductor material separates two n-type material. There are three contacts made to a BJT transistor. These are termed as the base (B), collector (C) and emitter (E), which is shown symbolically in Fig. 1. For a linear amplifier, the output signal is an amplified version of the input signal, which is given by V out = ± A V in where A is the gain of the amplifier. In an ideal amplifier, the gain A should be constant over a range of conditions. However, in practice, the gain change as the amplitude and frequency of the input signal varies. To avoid distortion in the output signal, the input signal must be restricted to a certain range of amplitude and frequency values. The gain, A, is a function frequency. The variation of A with frequency of a typical amplifier, shown by Fig 2, is known as the frequency response curve. The gain of the amplifier is constant over a band of frequency and drops at high and low frequency, as shown by the Bode plot in Figure 2 . The Bode plot is a graph of voltage gain against the logarithmic of the signal frequency. The bandwidth is given by B = f H – f L where f H is the upper 3 dB point and f L is the lower 3 dB point.

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Fig. 1 Diagram of a Typical Transistor Schematic Symbol of a Typical transistor Fig .2 Diagram of a Bode plot / Frequency Response of a Typical Amplifier 1.2 Objective (i) To familiarize with a bipolar junction transistor (BJT) (ii) To study the behaviour of a single transistor amplifier (iii) To understand the frequency response of an amplifier 1.3 Apparatus 1. One oscilloscope 2. One signal generator 3. One digital multimeter (DMM) 4. One breadboard with the amplifier soldered on 5. Power supply
2. Theory Fig 3. Common Emitter Configuration. Fig. 3 above shows the circuit that a transistor amplifier is used for signal amplification. The transistor considered is of common emitter variety, so called because the input signal is applied between the base and the emitter, and the output is taken from between the collector and the emitter. There will be a specific dc collector current I C across the external loads. The combination of dc current and voltage constitute the operating quiescent point of the transistors, i.e. Q-point in Fig. 4. As can be seen from the transfer characteristic diagram, the voltage drop across R C varies between 0 and V CC gives the negative reciprocal value of the collector resistance. The negative sign implies that the

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E5_3 - 1 Introduction 1.1 Background An instrument capable...

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