Power amplifiers and the use of pulse modulation

Transcription

1 Power ampliiers and the use o pulse modulation Ampliier types Voltage ampliication urrent ampliication (buer, driver) Power ampliication

2 Ampliier characteristics Single sided power supply Double sided power supply The active element (transistor) must be biased to work in its active region Ampliier classes lass A Analog Digital lass B lassab lass lass D lass G lass This class uses pulse modulation

3 lass A uiescent point V I DD max emove D rom load (P ilter) Base bias gives quiescent point lass A cont. 3

4 lass B In rest U= I= No need to remove D or a double sided power supply rossover distorsion 4

5 lass AB Bias diods gives quiescent points This circuit is highly simpliied Tuned circuit π L lass easonably linear in narrow band arrier wave o radio signal 5

6 lass G Adapt the power supply ppy voltage by making it switchable in level In some cases also use separate power transistors or the separate voltage levels lass Dynamically adapt the power supply voltage by modulating it in accordance with the input voltage Used in power amps manuactured by Labgruppen in Kungsbacka 6

7 Switching ampliier, lass D V DD U IN Fully on (saturated) means low voltage high current LOAD Fully o means high voltage no current -V DD In both cases power dissipation U I is small Pulse width modulation (PWM) Pulse train with ixed requency Modulation o the duty cycle Duty cycle pulse time period time % igh input signal high duty cycle Low input signal low duty cycle 7

8 Pulse width modulation (PWM) Duty cycle % Pulse width modulation (PWM) PWM generation 8

9 Demonstration Let aligned channels Periods starts with level changes at the same time 9

10 enter aligned channels PWMPOL= A hannel x t A T hannel y t T Observe the doubled period time Periods doesn t starts with level changes at the same time unless the duty cycles are the same Pulse density modulation (PDM) Simular to PWM Fixed period time The whole period is either high or low

11 Pulse density modulation (PDM) igh signal level igh pulse density Low signal level low pulse density Pulse train Deinitions Pulse time Amplitude Pulse Pulse gap Period time

12 Frequency analysis o PWM signal Frequency components D component Fundamental requency τ T τ T t A 4A cosk t k k τ sin k τ π π T Amplitude o requency component Frequency k Lowpass L ilter L U IN LOAD U OUT

13 3 Lowpass L ilter cont. L L L L L π Lowpass L ilter cont. Second order ilter equations Lowpass ilter ighpass ilter pb LP pb P Lowpass ilter ighpass ilter B d ilt B d t ilt pb BP pb B Bandpass ilter Bandreect ilter

14 4 Lowpass L ilter cont. Second order ilter equations Lowpass ilter ighpass ilter pb LP pb P Lowpass ilter ighpass ilter B d ilt B d t ilt pb BP pb B Bandpass ilter Bandreect ilter Lowpass L ilter cont. L L L π uto requency value Identiy constants L π L uto requency

15 Lowpass L ilter cont. ipple L ilter db-scale values Amplitude (db) uto requency =.7 = Slope -4 db/decade =.5 equal to - db/octave Frequency (z) Demonstration 5

16 Low pass L ilter cont. L ilter db-scale Amplitude (db) =.7 = Low PWM requency Low damping =.5 igh resolution igh PWM requency igh damping Low resolution Frequency (z) Active second order ilter Uin U Uout U U U U U U U A out U 6

17 Active second order ilter cont. U A U in U U out U U out in A A Active second order ilter cont. U A U in U U out I A= U U out in 7

18 8 Active second order ilter cont. π π Active second order ilter cont. U U in U out U U A I A= and = = in out U U

19 Active second order ilter cont. U A U in U out U π π Active second order ilter cont. This won t work in our case! Why? The ilter should be placed ater the power ampliier and the operational ampliier can t handle this power We have to use a passive ilter! 9

20 PWM-requency vs. damping L ilter db-scale Amplitude (db) =.7 = Low PWM requency Low damping =.5 igh PWM requency igh damping Frequency (z) Demonstration

21 PWM-requency vs. resolution n bits resolution n N possible values ( to N-) N possible duty cycles o the PWM-signal Example 3 bits resolution 3 8 possible values (- 7) system clock requency PWM requency 7 PWM-requency vs. resolution cont. We get the roule system clock requency system clock requency PWM requency n N - - The higher the resolution the lower the PWM-requency

22 PWM-requency vs. damping and resolution L ilter db-scale Amplitude (db) =.7 = Low PWM requency Low damping =.5 igh resolution igh PWM requency igh damping Low resolution Frequency (z) PWM-requency vs. damping and resolution We could increase the resolution by increasing N In the same time the PWM period (N-) T gets longer That is the PWM-requency alls And the PWM-requency moves closer to the signal requency It gets harder to ilter out the pulse

23 PWM-requency vs. damping and resolution We have to balance the PWM-requency and the accuracy to it our application Filter problems We need to attenuate the pulse out o our signal igh attenuation ti calls or higher h order ilters It is hard to make passive ilters o higher order Tolerances makes the accuracy low The ilters tend to get bulky mostly because o the inductors 3

24 Sigma-delta modulator Sigma-delta modulation The pulse could be deskribed as a noise loor on our signal emember digital-toanalog converters 4

25 Sigma-delta modulation cont. Using sigma-delta modulation we can shape the noise and move it to higher requencies where it gets more attenuated t even using lower order ilters Typical PWM application Speed control o electrical D motors The motor is a sluggish device that in itsel acts as a lowpass ilter We don t need no separate lowpass ilter 5

26 omplete class D ampliier Two types Ampliier without eedback Ampliier with eedback lass D ampliier without eedback s = signal max Too low or PWM Samplingrequency 44, kz and 6 bits would give a system clock or PWM o 6 44, kz =,89 Gz Too high We lower the number o bits to 3-4 No control o the result without eedback 6

27 lass D ampliier with eedback Noise damping and requency correction in the passband The signal we eed back is analog thereore AD The output signal has ripple thereore lowpass ilter lass D ampliier pros and cons + igh eiciency + Easy integration to digital electronics - The switching gives noise and distorsion - Noise on the power lines directly inluence the output signal 7