Power Amplifiers. Class B Class AB

Transcription

1 ower Amplifiers Class B Class AB

2 Class B he circuit each transistor conducts for a half of every signal period complementary pair push-pull arrangement

3 peration vi ( 0.6; 0.6) (off), (off) v 0 vi v 0.6 v I v (on), (off) BE vi 0.6 vi v 0.6 v I v BE (off), (on) v 0.6 I i 0 i i i C C 0 i 0 i i C i C 0 v o max CEsat v omin CEsat

4 oltage transfer characteristic crossover distortions

5 Waveforms Assume the signal magnitude sufficiently high to neglect the crossover distortions. v ˆ o ( t) ˆ o sin t o

6 owers. Efficiency S S S ˆ o sin tdt S rms I rms S 0 ˆ ˆ o S 0 For ˆ i S S For ( t) dt ˆ o ˆ o 0 max ˆ o S max sint dt Average efficiency: S ˆ ˆ ˆ 4 Maximum average efficiency: for ˆ. % 4 78 max 5

7 he amplifying transistor S Maximum efficiency: What is the maximum average power dissipated by a transistor depending on the output voltage magnitude? d dˆ 0 Maximum average power: Maximum instantaneous power: i C max ˆ S ˆ o ˆ o ˆ o o o 4 ˆ ˆ p max max v CEmax 4 4 ˆ ˆ

8 Class AB - Crossover distortion, specific to class B amplifier can be virtually eliminated

9 Class AB Amplifier. Basic Circuit biasing the complementary output transistors at a small output current Bias 0.6 I I S e Bias v I - positive v t v t v t I Bias BEn v I - negative v t vi t Bias BEp vi t I

10 oltage ransfer Characteristic (C) Solutions to generate Bias???

11 Biasing using Diodes should allow the currents in the diodes and in the bases of the transistors, even for maximum output current ˆ 8 = +5 β=50 =00Ω, I D =.5mA =?, I =? (each ) is the driver transistor from previous amplifier stage

12 Biasing using BE Multiplier BB BE

13 Short-Circuit rotection Elements for short-circuit protection:, En for v o > 0, Ep for v o < 0 When v o > 0 If rotection against the effect of short-circuiting the output i En 0,6 v ( b) ; i If i increases, when i En 0,6 BE, on ( c); i max En 0,6 En

14 Short-Circuit rotection Effective in ensuring device safety Disadvantage: under normal operation up to 0.6 appears across each protection resistor. he voltage swing will be reduced by that much, in each direction Negative feedback, protection of the output transistor against thermal runaway

15 Illustration I Bias mA - adjusts the biasing voltage between BE and 3.5 BE C - in the bases of n and p it is the same variable input voltage i max =0.6A

16 Use of Compound ransistors with Higher Current Gain Necessary for high output currents he Darlington npn configuration he compound pnp configuration preferred in IC

17 roblem ± = ± 5 500Ω 50Ω a) What is the expression v (v i ) for v i [-0;0]? lot the v (v i ) C. What is the operating class of this amplifier? What is the maximum theoretical average efficiency? b) lot v (t) for: () v i (t)= 0.sint []; () v i (t)= 0sint []. c) For v i (t)= 0sint [], v (t) can be well approximated by a sinewave (neglecting crossover distortions). Compute: the average output power, the average power consumption, and the average efficiency of the stage. lot the collector-emitter voltage and collector current for and. d) ropose a solution to eliminate crossover distortion.