EEE321 — Analogue Electronic Circuits: Lecture Note

About this lecture note

This is the complete lecture note for EEE321 Analogue Electronic Circuits. You can read it online below, open it in the full viewer, or download the PDF for offline study. It pairs with the course's video lectures, so you can read and watch together.

Preview: opening of the note

Course code: EEE 321 Course title: Analogue Electronic Circuits Course credit load: 2 Units Series 1: Single-Stage BJT and FET Amplifiers and Equivalent Circuits • Part 1.1: BJT Amplifier Configurations and Biasing o Sub Part 1.1.1: The bipolar junction transistor as an amplifier o Sub Part 1.1.2: BJT amplifier configurations: CE, CB, CC o Sub Part 1.1.3: DC biasing of a BJT amplifier • Part 1.2: BJT Small-Signal Equivalent Circuit and Parameters o Sub Part 1.2.1: The h-parameter (hybrid) equivalent circuit of a BJT o Sub Part 1.2.2: Current gain and voltage gain of a CE amplifier o Sub Part 1.2.3: Input impedance and output impedance of a CE amplifier • Part 1.3: FET Amplifier Configurations and Biasing o Sub Part 1.3.1: The field effect transistor as an amplifier o Sub Part 1.3.2: FET amplifier configurations: CS, CG, CD o Sub Part 1.3.3: DC biasing of a FET amplifier • Part 1.4: FET Small-Signal Equivalent Circuit and Parameters o Sub Part 1.4.1: The small-signal equivalent circuit of a FET o Sub Part 1.4.2: Current gain and voltage gain of a CS amplifier o Sub Part 1.4.3: Input impedance, output impedance, and power gain of a CS amplifier

Introduction This course examines the design and analysis of analogue electronic circuits. This first Series lays the essential foundation, examining single-stage transistor amplifiers built from a single BJT or FET, their principal configurations, DC biasing, small-signal equivalent circuits, and the calculation of their fundamental gain and impedance parameters. The aim of this Series is to equip you with a sound understanding of BJT and FET amplifier configurations and their principles of operation, DC biasing methods, their small-signal equivalent circuits, and the calculation of current gain, voltage gain, power gain, input impedance, and output impedance. We shall commence this Series by examining BJT amplifier configurations and biasing. Next, we will consider the BJT small-signal equivalent circuit and the calculation of CE amplifier gain and impedance parameters. Following that, our attention turns to FET amplifier configurations and biasing. Finally, we will conclude by examining the FET small-signal equivalent circuit and the calculation of CS amplifier gain and impedance parameters. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1: describe the principle of operation of a BJT amplifier and its three configurations, • SLO 1.2: describe DC biasing methods for a BJT amplifier, • SLO 1.3: draw the h-parameter equivalent circuit of a BJT, • SLO 1.4: calculate the current gain, voltage gain, input impedance, and output impedance of a CE amplifier, • SLO 1.5: describe the principle of operation of a FET amplifier and its three configurations, • SLO 1.6: describe DC biasing methods for a FET amplifier, • SLO 1.7: draw the small-signal equivalent circuit of a FET, • SLO 1.8: calculate the current gain, voltage gain, power gain, input impedance, and output impedance of a CS amplifier.

1.1 BJT Amplifier Configurations and Biasing 1.1.1 The bipolar junction transistor as an amplifier A bipolar junction transistor (BJT) can be used as a small-signal amplifier by superimposing a small alternating input signal onto a fixed DC bias voltage, so that the resulting small variation in base current produces a proportionally much larger variation in collector current, governed by the transistor's current gain, with this larger collector current variation then converted into an amplified output voltage as it flows through a suitably chosen collector (or load) resistor. For the transistor to amplify a signal without distortion, it must be biased so that it operates continuously within its active region, the region of operation in which the base-emitter junction is forward biased and the base-collector junction is reverse biased, throughout the entire cycle of the applied AC input signal. Worked example: A BJT biased with a quiescent (DC) collector current of 2 mA, and driven by a small AC base current that varies the total base current only slightly around its DC value, produces a correspondingly small AC variation in collector current superimposed on the same 2 mA quiescent level, with this AC variation representing the amplified signal. Application: This principle, a small input current or voltage controlling a much larger output current, is the fundamental mechanism underlying every BJT amplifier examined throughout this Series, and correctly establishing and maintaining the transistor's active-region operating point is the essential first step in designing any such amplifier. Fill in the blank: For a BJT to amplify without distortion, it must remain in its ________ region throughout the signal cycle. A small variation in base current produces a proportionally much larger variation in ________ current. 1.1.2 BJT amplifier configurations: CE, CB, CC A common-emitter (CE) amplifier applies the input signal to the base and takes the output from the collector, with the emitter terminal common to both, and is the most widely used BJT amplifier configuration, offering both substantial current gain and substantial voltage gain, though with the output signal inverted (180° phase shift) relative to the input. A common-base (CB) amplifier applies the input signal to the emitter and takes the output from the collector, offering high voltage gain but essentially no current gain and a very low input impedance, while a common-collector (CC) amplifier, or emitter follower, applies the input to the base and takes the output from the emitter, offering substantial current gain but a voltage gain of essentially unity, combined with a very high input impedance and a very low output impedance. Worked example: A common-emitter amplifier is selected to provide the substantial voltage amplification required for the first stage of an audio amplifier, whereas a common-collector (emitter follower) stage might instead be selected as a final output stage specifically for its low

output impedance, allowing it to drive a low-impedance load, such as a loudspeaker, without a significant drop in signal level. Application: Choosing the correct configuration, CE, CB, or CC, for a particular position within a larger amplifier circuit depends entirely on which combination of current gain, voltage gain, input impedance, and output impedance is required at that specific point, a decision examined in more quantitative detail in the remainder of this Series. Fill in the blank: A common-emitter amplifier inverts the output signal by ________ degrees relative to the input. A common-collector amplifier, or emitter follower, has a voltage gain of approximately ________. 1.1.3 DC biasing of a BJT amplifier DC biasing establishes the fixed, quiescent operating point (or 'Q-point') of a BJT amplifier, and must position this Q-point centrally within the transistor's active region so that the maximum possible symmetrical AC signal swing can be accommodated without the transistor being driven into cut-off or saturation. Voltage divider bias is the most widely used practical biasing method, using two resistors connected across the supply voltage to fix the DC base voltage, combined with an emitter resistor that provides negative feedback stabilisation, automatically compensating for variations in transistor current gain and temperature, giving considerably better Q-point stability than the simpler fixed bias method. Worked example: A BJT biased using the voltage divider method with a fixed base voltage of 2 V and an emitter resistor that develops approximately 1.3 V across it at the desired quiescent current maintains a stable emitter current of Ie = 1.3/Re, largely independent of the individual transistor's current gain, since the base voltage itself is fixed by the resistor divider rather than depending on the base current drawn. Application: Voltage divider bias, precisely because of its superior stability against transistor-to-transistor variation and temperature drift, is the standard biasing method used throughout the great majority of practical, mass-produced BJT amplifier circuits, and a sound understanding of how it achieves this stability is essential before the small-signal amplifier analysis examined in the next Part of this Series can be properly undertaken. Fill in the blank: DC biasing establishes the quiescent operating point, also called the ________ point, of a BJT amplifier. Voltage divider bias uses an emitter resistor to provide negative ________ stabilisation.

Figure 1.1: The common-emitter, common-base, and common-collector BJT amplifier configurations. Pilot questions 1.1 1. Explain why a BJT must remain in its active region to amplify without distortion. (Linked to SLO 1.1) 2. Describe the common-emitter amplifier configuration. (Linked to SLO 1.1) 3. Describe the common-base amplifier configuration. (Linked to SLO 1.1) 4. Describe the common-collector amplifier configuration. (Linked to SLO 1.1) 5. Distinguish between fixed bias and voltage divider bias. (Linked to SLO 1.2) 1.2 BJT Small-Signal Equivalent Circuit and Parameters 1.2.1 The h-parameter (hybrid) equivalent circuit of a BJT The h-parameter (or hybrid) equivalent circuit represents a BJT, for small-signal AC analysis, as a simple two-port network described by four hybrid parameters: hie, the input impedance with the output short-circuited; hre, the reverse voltage transfer ratio; hfe, the forward current gain; and hoe, the output admittance with the input open-circuited.

For most practical hand-calculation purposes, the simplified h-parameter model is used, in which the comparatively small effects of hre and hoe are neglected entirely, reducing the equivalent circuit to just an input resistance hie and a current source hfe×Ib representing

This is the opening of the note. Read the full note or download the PDF to continue.