Electronic Circuits - Part 2

The reader may wonder why the amplifier stages studied in previous chapters are not suited to high-power applications. Suppose we wish to deliver 1 W to an 8- speaker. Approximating the signal with a sinusoid of peak amplitude VP, we express the power absorbed by the speaker as:
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Electronic Circuits - Part 2Teacher: Dr. LUU THE VINH& The chapter outline is shown below. The chapter outline is shown below. Basic Stages Large-Signal Heat Efficiency and Efficiency and Basic Stages Large-Signal Heat PA Classes Considerations Dissipation Dissipation PA Classes ConsiderationsEmitter Follower Emitter Follower Omission of  Power Ratings Efficiency of PAs Omission of Power Ratings Efficiency of PAsPush−Pull Stage Push−Pull Stage PNP Transistor Thermal Runaway  Classes of PAs PNP Transistor Thermal Runaway Classes of PAs High-Fidelity High-Fidelity and Improved Design Design and Improved Variants Variants PA - Power Amplifier 13.1 General Considerations•• The reader may wonder why the amplifier stages studied The reader may wonder why the amplifier stages studied in previous chapters are not suited to high-power in previous chapters are not suited to high-power applications. Suppose we wish to deliver 1 W to an 8Ω- applications. Suppose we wish to deliver 1 W to an 8Ω- speaker. Approximating the signal with a sinusoid of peak speaker. Approximating the signal with a sinusoid of peak amplitude VPP, we express the power absorbed by the amplitude V , we express the power absorbed by the speaker as: speaker as: (13.1) Where VPP = p2 denotes the root mean square (rms) value Where V = p2 denotes the root mean square (rms) value of the sinusoid and RLL represents the speaker impedance. of the sinusoid and R represents the speaker impedance. For RLL= 8Ω and Pout = 1 W, For R = 8Ω and Pout = 1 W, VP = 4V Also, the peak current flowing through the speaker is given by Also, the peak current flowing through the speaker is given by Important observations1) The resistance that must be driven by the amplifier is1) The resistance that must be driven by the amplifier is much lower than the typical values (hundreds to much lower than the typical values (hundreds to thousands of ohms) seen in previous chapters. thousands of ohms) seen in previous chapters.2) The current levels involved in this example are much2) The current levels involved in this example are much greater than the typical currents (milliamperes) greater than the typical currents (milliamperes) encountered in previous circuits. encountered in previous circuits.3) The voltage swings delivered by the amplifier can hardly3) The voltage swings delivered by the amplifier can hardly be viewed as “small” signals, requiring a good be viewed as “small” signals, requiring a good understanding of the large-signal behavior of the circuit. understanding of the large-signal behavior of the circuit.4) The power drawn from the supply voltage, at least 1W,4) The power drawn from the supply voltage, at least 1W, is much higher than our typical values. is much higher than our typical values.5) A transistor carrying such high currents and sustaining5) A transistor carrying such high currents and sustaining several volts (e.g., between collector and emitter) several volts (e.g., between collector and emitter) dissipates a high power and, as a result, heats up. High- dissipates a high power and, as a result, heats up. High- power transistors must therefore handle high currents power transistors must therefore handle high currents and high temperature. and high temperature.12.1 General Considerations•• Based on the above observations, we can predict the parameters of Based on the above observations, we can predict the parameters of interest in the design of power stages: interest in the design of power stages:(1) “Distortion,” i.e., the nonlinearity resulting from large-signal operation. A (1) “Distortion,” i.e., the nonlinearity resulting from large-signal operation. A high-quality audio amplifier must achieve a very low distortion so as to high-quality audio amplifier must achieve a very low distortion so as to reproduce music with high fidelity. In previous chapters, we rarely dealt reproduce music with high fidelity. In previous chapters, we rarely dealt with distortion. with distortion.(2) “Power efficiency” or simply “efficiency,” denoted by η and defined as: (2) “Power efficiency” or simply “efficiency,” denoted by η and defined as:For example, a cellphone power amplifier that consumes 3 W from the For example, a cellphone power amplifier that consumes 3 W from thebattery to deliver 1 W to the antenna provides η≈ 33,3%. In previous battery to deliver 1 W to the antenna provides η≈ 33,3%. In previouschapters, the efficiency of circuits was of little concern because the chapters, the efficiency of circuits was of little concern because theabsolute value of the power consumption was quite small (a few absolute value of the power consumption was quite small (a fewmilliwatts). milliwatts).(3) “Voltage rating.” As suggested by Eq. (13.1), (3) “Voltage rating.” As suggested by Eq. (13.1), higher power levels or load resistance values higher power levels or load resistance values translate to large voltage swings and (possibly) high translate to large voltage swings ...