E5_2 - CONTENTS PAGE 1 INTRODUCTION.2 1.1 Background.2 1.2...

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CONTENTS PAGE 1. INTRODUCTION ................................................................................................ 2 1.1 Background ................................................................................................... 2 1.2 Objectives ..................................................................................................... 2 1.3 Apparatus ...................................................................................................... 2 2. THEORY .............................................................................................................. 3 3. PROCEDURE ....................................................................................................... 4 3.1 Quiescent point ............................................................................................. 4 3.2 Gain at signal frequency of 1 kHz ............................................................. 4 3.3 Frequency response of the transistor amplifier .......................................... 4 4. CALCULATIONS and RESULTS ...................................................................... 5 5. DISCUSSION ....................................................................................................... 7 5.1 Precautions and errors ................................................................................. 8 5.1.1 Precautions ............................................................................................ 8 5.1.2 Errors .................................................................................................... 8 5.2 Comments on the accuracy of different measuring devices ...................... 8 5.3 Comments on the observation of non linearity of transfer characteristics ............................................................................................... 9 5.4 Comments on the transfer curve ................................................................ 9 5.5 Comments on experiment 4.3 ..................................................................... 9 6. CONCLUSION ................................................................................................... 10 7. REFERENCE ...................................................................................................... 10 8. APPENDICES
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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 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. In practice, however, not all signal frequencies are subjected to same degree of amplification (A V ). When the transistors' bandwidth is extended, it approaches the band limits and the gain will begin to drop. Amplifier instability of such is usually minimized by one same process known as negative feedback, accompanied by diminution of the amplifier's gain. In the middle frequency range, the amplification is constant and is well illustrated in experiment by the Bode plot. Bode plot is graph of voltage gain in decibels form against the logarithmic of the signal frequency ( in Fig. 2). 1.2 Objective (i) To verify type theoretical finding of the values of I C , V CE and A 0 . (ii) Compare the results obtained when different instrument of measurement is used. (iii) Determine the transfer characteristic of the transistor. (iv) Determine the bandwidth and the frequency response of the transistor. 1.3 Apparatus 1. One oscilloscope 2. One signal generator 3. One digital multimeter 4. One breadboard with the amplifier soldered on 5. Power supply 2
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Fig.1 Symbolic representation of Fig 2 Idealized Bode plot of frequency a transistor amplifier response in the higher frequency range. 2. Theory Fig. 3 Common emitter configuration Above, Fig. 3 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 emitter, and the output is taken from between the collector and 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 as the input signal varies. The load line joining the Q- point and the
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