ECEN3250 Lab 8 Audio Power Amplifier

Transcription

1 Lab 8 Audio Powr Amplifir ECE Dpartmnt Univrsity of Colorado, Bouldr 1

2 Prlab assignmnt Rad txtbook stions 5.1, 5., 5., 5., and 1.

3 Introdution An audio powr amplifir taks audio signal v i from a sour (.g. CD playr) and produs an output voltag =Av i to driv a loudspakr. A spakr Th output powr an b found as audio in v i P out = V o,rms / = I o,rms whr V o,rms is th rms valu of th output voltag and I o,rms is th rms output urrnt. Sin th spakr s impdan is rlativly low ( =8Ω is typial), th amplifir must hav a low output impdan, and must b apabl of driving th spakr with signifiant output urrnt. For xampl, assuming =8Ω, th rms output urrnt of a 100 W audio amplifir is about.5 A. In this lab, th objtiv is to dsign an audio powr amplifir aording to th following spifiations: A = 10, = 100 Ω P out,max = 15 mw V o,rms =.5 V Sinwav V o,pak =5 V, V opp = 10 V

4 Stp 1: Simpl op-amp amplifir R = 15 V LF5 Th starting point for th powr amplifir is th simpl op-amp basd amplifir shown hr. Th gain is idally 10. Construt th op-amp amplifir. Do not forgt th doupling apaitors for th d supply voltags. Using a 1 KHz sinwav from th lab wavform gnrator, vrify th gain. Adjust th input signal amplitud to 1Vpp so that th output pak-to-pak voltag is 10 Vpp (output pak-to-pak amplitud should b 10 V) In th rport, inlud th iruit diagram, and a labld skth of th and wavforms

5 Stp : Op-amp amplifir with load = 15 V R LF5 i o 100 Ω Adjust th input sinwav signal to 1Vpp amplitud at 1 KHz Load th output of th op-amp with =100Ω In th rport, inlud th iruit diagram, and a labld skth of th and wavforms. You should obsrv that th output wavform is svrly distortd. Explain why in th rport. Hint: th op-amp maximum output urrnt is limitd to about 5mA. S th attahd plots from th op-amp data shts. 5

6 Stp : Op-amp amplifir with BJT buffr R = 15 V = 15 V LF5 v b b N90 b N90 i o 100 Ω An npn (N90) and a pnp (N90) an b onntd as shown hr to form a lass-b buffr whr th output urrnt is supplid by on of th BJTs, whil th op-amp sours or sinks a signifiantly smallr urrnt (β tims smallr than th output urrnt). Not that th positiv output urrnt (for > 0) is supplid by th npn; th ngativ urrnt (for < 0) is sunk by th pnp. Construt and tst th buffrd op-amp amplifir using th 1Vpp, 1KHz sinwaput. You should obsrv that th output wavform is signifiantly diffrnt ompard to th wavform obsrvd in Stp. Howvr, th output wavform is still visibly distortd. In th rport, skth and xplain th wavforms, v b and. Hint: rad txtbook Stion 1. (th d. Stion 9.). Exprimnt with hanging th amplitud of th input signal and dsrib your obsrvations in th rport.

7 Stp : Improvd buffrd amplifir R = 15 V = 15 V LF5 v b b b N90 i o N Ω An improvd vrsion of th buffrd amplifir is shown hr. Th fdbak rsistor R is onntd aftr th BJT buffr. Th op-amp output v b tnds to orrt th nonlinarity introdud by th BJT buffr and th output should b muh losr to th idal sinwav than in th amplifir of Stp. Construt th improvd buffrd op-amp amplifir and tst using th 1Vpp, 1KHz sinwav input. In th rport, skth, labl and xplain th wavforms, v b and.

8 Stp : Improvd buffrd amplifir R = 15 V = 15 V LF5 v b b N90 b N90 i o 100 Ω Prpar a PSpi simulation fil orrsponding to th improvd buffrd amplifir in Stp. Us th transistor modls from th 50.lib library. Consult th on-lin PSpi rfrn manual about how to inlud th BJTs in th simulation fil. In th rport show th simulation rsults for th wavforms, v b and. Compar to th xprimntal wavforms and ommnt on th rsults. Comput and rport th output powr P out, th powr P npn dissipatd by th npn (N90), th powr P pnp dissipatd by th pnp (N90), th powr P CC takn from VCC, th powr P EE takn from VEE, and th powr ffiiny of th amplifir, η = P out /(P CC P EE ). Exprimnt with hanging th amplitud and frquny of th input signal and dsrib your obsrvations in th rport. 8

9 Extra rdit xprimnt Construt and tst a buffrd op-amp amplifir using th LF5 op-amp followd by a MOS buffr onsisting of an NMOS (ZVN10) and a PMOS (ZVP10) instad of th BJTs Rpat Stp xprimnt and rport tasks for th MOS buffrd amplifir Compar th prforman of this MOS buffrd amplifir to th prforman of th BJT buffrd amplifir of Stp in trms of th output signal distortion, ffiiny and bandwidth This xtra-rdit assignmnt is worth up to xtra-rdit points 9