1 EE362L, Spring 2009 H-Bridge Inverter as an Audio Amplifier

2 There is only one music station, so please use the signup sheet (with one-hour time slots) and be considerate of others who are waiting Before conducting the music tests with CD player and speakers, you use any lab bench to perform the first steps in the lab document

3 Efficiency is much higher than the 50% max of a linear amplifier But the distortion is higher than a linear amplifier The H-Bridge works best for bass amplification and loud music In our case, f tri = 20kHz (which should be more than 30 times f cont for best performance) So, for best performance, f cont < 20,000 / 30 = 667Hz f tri of 150kHz would extend the best performance range to 5kHz The H-Bridge Inverter is a Class D Amplifier !

4 Begin by Using the Benchtop Waveform Generator to Produce the 100Hz and 1kHz V cont Signals H-Bridge Inverter Three Headlight Bank V cont Benchtop Waveform Generator Transformer/ DBR

5 Using 100Hz Sinewave Input Input Output Waveform generator output Inverter output Dead spots at zero crossings are characteristic of PWM because of blanking

6 100Hz Output, THD  2% FFT of inverter output with 100Hz input signal Save screen snapshot #1 1kHz span, 500Hz center

7 With 1kHz Input, the Output THD Remains  2% FFT ofinverter output with 1kHz input signal Save screen snapshot #2 10kHz span, 5kHz center !

8 For the Music Test, Replace the Three Headlight Bank with Four Speakers Use one-half of the transformer winding to power a DBR so that you don’t accidentally raise Vdc above 20V 495µF of series capacitance blocks DC current flow to the speakers Four 8Ω speakers in series become a 32Ω load The sound quality is best when loud, but do not raise Vdc above 20V or it will be too loud

9 With the LC Filter and 32Ω Speaker Load, the Resonant Spike is Pronounced Gives you a treble boost – try with and without the 10µF cap

10 Transfer Function Without 10µF Output Capacitor (but leaving the 100µH inductor in) Roll-off due to 459µF series capacitance Roll-off due to 100µH inductor Speakers do not respond much anyway

11 Use the Mono Output of a CD Player as Your Vcont Signal Switch channels A and B outputs to 1. the speakers, or 2. the summing transformers Two small 1:1 audio transformers at the back of the CD player add channels A and B to produce a mono V cont signal for the H-Bridge inverter !

12 The CD player is also equipped with channel A and B auxiliary audio inputs. Use the AUX mode. An iPod interface cable is provided so that you can play your iPOD into the CD player, and use the CD player to boost the resulting mono signal to the 3-4 volts needed for Vcont. By itself, the iPod only produces about one-tenth of volt, which is not large enough for Vcont. When using an iPod, turn the iPod volume to the max, and the CD player volume to the max. iPod update

13 Mono Signal Used as V cont for the H-Bridge Inverter (Avoid Clipping) Signal not exceeding the 4V triangle wave peak, so m a < 1

14 CD player output Inverter output Top curve: Audio output of CD player to inverter, Bottom curve: Outputof inverter to speakers (scope set to average over one cycle) Save screen snapshot#3 Operation with the CD Player Start with Vdc = 0V Raise the CD player volume to the maximum Raise the m a control potentiometer to the maximum You will hear weak music with Vdc = 0 because the MOFSET gates are firing Sound quality is best when loud. Gradually raise Vdc to 15V as long as the distortion is not excessive Input Output

15 Use My CD or Yours Track 1. The Hollies, “Long Cool Woman in a Black Dress” Track 2. Eagles, “Peaceful Easy Feeling” Track 3. America, “Sister Golden Hair” Track 4. Electric Light Orchestra, “Don’t Bring Me Down” Track 5. The Beach Boys, “Help Me, Rhonda” Track 6. The Moody Blues, “Your Wildest Dreams” Track 7. Charlie Dore, “Pilot of the Airways” Track 7. Ronnie Milsap, “Any Day Now”

16 The average current from 15Vdc will be about 0.1 to 0.15A The 2-3 Watts from the DC power supply will produce a surprisingly loud output If you are bold, you can use 20V (about Watts). And your inverter can produce 200W! Use the Clamp-On Ammeter for Power Supply Current (be sure to zero the meter as described in the document)

17 Questions?