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1 Output Stages and Power Amplifiers. Microelectronic Circuits - Fifth Edition Sedra/Smith2 Copyright 2004 by Oxford University Press, Inc. Figure 14.1.

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Presentation on theme: "1 Output Stages and Power Amplifiers. Microelectronic Circuits - Fifth Edition Sedra/Smith2 Copyright 2004 by Oxford University Press, Inc. Figure 14.1."— Presentation transcript:

1 1 Output Stages and Power Amplifiers

2 Microelectronic Circuits - Fifth Edition Sedra/Smith2 Copyright 2004 by Oxford University Press, Inc. Figure 14.1 Collector current waveforms for transistors operating in (a) class A, (b) class B,

3 Microelectronic Circuits - Fifth Edition Sedra/Smith3 Copyright 2004 by Oxford University Press, Inc. Figure 14.1 (Continued) (c) class AB, and (d) class C amplifier stages.

4 Microelectronic Circuits - Fifth Edition Sedra/Smith4 Copyright 2004 by Oxford University Press, Inc. Figure The popular TO3 package for power transistors. The case is metal with a diameter of about 2.2 cm; the outside dimension of the seating plane is about 4 cm. The seating plane has two holes for screws to bolt it to a heat sink. The collector is electrically connected to the case. Therefore an electrically insulating but thermally conducting spacer is used between the transistor case and the heat sink.

5 Microelectronic Circuits - Fifth Edition Sedra/Smith5 Copyright 2004 by Oxford University Press, Inc. Figure 14.2 An emitter follower (Q 1 ) biased with a constant current I supplied by transistor Q 2.

6 Microelectronic Circuits - Fifth Edition Sedra/Smith6 Copyright 2004 by Oxford University Press, Inc. Figure 14.3 Transfer characteristic of the emitter follower in Fig This linear characteristic is obtained by neglecting the change in v BE1 with i L. The maximum positive output is determined by the saturation of Q 1. In the negative direction, the limit of the linear region is determined either by Q 1 turning off or by Q 2 saturating, depending on the values of I and R L.

7 Microelectronic Circuits - Fifth Edition Sedra/Smith7 Copyright 2004 by Oxford University Press, Inc. Figure 14.4 Maximum signal waveforms in the class A output stage of Fig under the condition I = V CC /R L or, equivalently, R L = V CC /I.

8 Microelectronic Circuits - Fifth Edition Sedra/Smith8 Copyright 2004 by Oxford University Press, Inc. Figure 14.5 A class B output stage.

9 Microelectronic Circuits - Fifth Edition Sedra/Smith9 Copyright 2004 by Oxford University Press, Inc. Figure 14.6 Transfer characteristic for the class B output stage in Fig

10 Microelectronic Circuits - Fifth Edition Sedra/Smith10 Copyright 2004 by Oxford University Press, Inc. Figure 14.7 Illustrating how the dead band in the class B transfer characteristic results in crossover distortion.

11 Microelectronic Circuits - Fifth Edition Sedra/Smith11 Copyright 2004 by Oxford University Press, Inc. Figure 14.8 Power dissipation of the class B output stage versus amplitude of the output sinusoid.

12 Microelectronic Circuits - Fifth Edition Sedra/Smith12 Copyright 2004 by Oxford University Press, Inc. Figure 14.9 Class B circuit with an op amp connected in a negative-feedback loop to reduce crossover distortion.

13 Microelectronic Circuits - Fifth Edition Sedra/Smith13 Copyright 2004 by Oxford University Press, Inc. Figure Class B output stage operated with a single power supply.

14 Microelectronic Circuits - Fifth Edition Sedra/Smith14 Copyright 2004 by Oxford University Press, Inc. Figure Class AB output stage. A bias voltage V BB is applied between the bases of Q N and Q P, giving rise to a bias current I Q given by Eq. (14.23). Thus, for small v I, both transistors conduct and crossover distortion is almost completely eliminated.

15 Microelectronic Circuits - Fifth Edition Sedra/Smith15 Copyright 2004 by Oxford University Press, Inc. Figure Transfer characteristic of the class AB stage in Fig

16 Microelectronic Circuits - Fifth Edition Sedra/Smith16 Copyright 2004 by Oxford University Press, Inc. Figure Determining the small-signal output resistance of the class AB circuit of Fig

17 Microelectronic Circuits - Fifth Edition Sedra/Smith17 Copyright 2004 by Oxford University Press, Inc. Figure A class AB output stage utilizing diodes for biasing. If the junction area of the output devices, Q N and Q P, is n times that of the biasing devices D 1 and D 2, and a quiescent current I Q = nI BIAS flows in the output devices.

18 Microelectronic Circuits - Fifth Edition Sedra/Smith18 Copyright 2004 by Oxford University Press, Inc. Figure A class AB output stage utilizing a V BE multiplier for biasing.

19 Microelectronic Circuits - Fifth Edition Sedra/Smith19 Copyright 2004 by Oxford University Press, Inc. Figure A discrete-circuit class AB output stage with a potentiometer used in the V BE multiplier. The potentiometer is adjusted to yield the desired value of quiescent current in Q N and Q P.

20 Microelectronic Circuits - Fifth Edition Sedra/Smith20 Copyright 2004 by Oxford University Press, Inc. Figure A class AB output stage with an input buffer. In addition to providing a high input resistance, the buffer transistors Q 1 and Q 2 bias the output transistors Q 3 and Q 4.

21 Microelectronic Circuits - Fifth Edition Sedra/Smith21 Copyright 2004 by Oxford University Press, Inc. Figure The Darlington configuration.

22 Microelectronic Circuits - Fifth Edition Sedra/Smith22 Copyright 2004 by Oxford University Press, Inc. Figure The compound-pnp configuration.

23 Microelectronic Circuits - Fifth Edition Sedra/Smith23 Copyright 2004 by Oxford University Press, Inc. Figure A class AB output stage utilizing a Darlington npn and a compound pnp. Biasing is obtained using a V BE multiplier.

24 Microelectronic Circuits - Fifth Edition Sedra/Smith24 Copyright 2004 by Oxford University Press, Inc. Figure A class AB output stage with short-circuit protection. The protection circuit shown operates in the event of an output short circuit while v O is positive.

25 Microelectronic Circuits - Fifth Edition Sedra/Smith25 Copyright 2004 by Oxford University Press, Inc. Figure Thermal-shutdown circuit.

26 Microelectronic Circuits - Fifth Edition Sedra/Smith26 Copyright 2004 by Oxford University Press, Inc. Figure The simplified internal circuit of the LM380 IC power amplifier. (Courtesy National Semiconductor Corporation.)

27 Microelectronic Circuits - Fifth Edition Sedra/Smith27 Copyright 2004 by Oxford University Press, Inc. Figure Small-signal analysis of the circuit in Fig The circled numbers indicate the order of the analysis steps.

28 Microelectronic Circuits - Fifth Edition Sedra/Smith28 Copyright 2004 by Oxford University Press, Inc. Figure Power dissipation (P D ) versus output power (P L ) for the LM380 with R L = 8. (Courtesy National Semiconductor Corporation.)

29 Microelectronic Circuits - Fifth Edition Sedra/Smith29 Copyright 2004 by Oxford University Press, Inc. Figure Structure of a power op amp. The circuit consists of an op amp followed by a class AB buffer similar to that discussed in Section The output current capability of the buffer, consisting of Q 1, Q 2, Q 3, and Q 4, is further boosted by Q 5 and Q 6.

30 Microelectronic Circuits - Fifth Edition Sedra/Smith30 Copyright 2004 by Oxford University Press, Inc. Figure A class AB amplifier with MOS output transistors and BJT drivers. Resistor R 3 is adjusted to provide temperature compensation while R 1 is adjusted to yield the desired value of quiescent current in the output transistors. Resistors R G are used to suppress parasitic oscillations at high frequencies. Typically, R G = 100.

31 Microelectronic Circuits - Fifth Edition Sedra/Smith31 Copyright 2004 by Oxford University Press, Inc. Figure Capture schematic of the class B output stage in Example 14.6.

32 Microelectronic Circuits - Fifth Edition Sedra/Smith32 Copyright 2004 by Oxford University Press, Inc. Figure Several waveforms associated with the class B output stage (shown in Fig ) when excited by a 17.9-V, 1-kHz sinusoidal signal. The upper graph displays the voltage across the load resistance, the middle graph displays the load current, and the lower graph displays the instantaneous and average power dissipated by the load.

33 Microelectronic Circuits - Fifth Edition Sedra/Smith33 Copyright 2004 by Oxford University Press, Inc. Figure The voltage (upper graph), current (middle graph), and instantaneous and average power (bottom graph) supplied by the positive voltage supply (+V CC ) in the circuit of Fig

34 Microelectronic Circuits - Fifth Edition Sedra/Smith34 Copyright 2004 by Oxford University Press, Inc. Figure Waveforms of the voltage across, the current through, and the power dissipated in the pnp transistor Q P of the output stage shown in Fig

35 Microelectronic Circuits - Fifth Edition Sedra/Smith35 Copyright 2004 by Oxford University Press, Inc. Figure Transfer characteristic of the class B output stage of Fig


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