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Author Topic: Is this an appropriate circuit to drive my 8 ohm speaker using PWM?  (Read 6506 times)
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That's a rather Lo-Fi approch, but with square wave input who cares, it will make noise.  smiley-wink

Transistor is good noise filter.
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Not really.  In fact it is more often the opposite.

In order to switch a bipolar transistor, some amount of current required, isn't it?
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Yes. And every active component (and many passive components) are all sources of noise and/or distortion.

Sounds a bit paranoidal.  smiley
"This is my HiFiduino, baby." (c)
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Okay I built the circuit, and the wonderful smell of magic smoke started to come out both bjts after decreasing resistance in the pot to 0 (I had to replace both). By the way, I kept the 4k and 1k voltage divider in the circuit. How do I calculate the power dissaption of the bjts in this type of circuit?
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Okay, here is a more appropriate circuit.

The MOSFET I used is IRF3707Z, any higher current logic level MOSFTETwill work, such as the sparkfun sells.
The speaker is from mpa.com
http://www.mpja.com/prodinfo.asp?number=14618+SP
90dB. I use it in a fencing club to make a warble when a touch is scored, can be heard very well.

The current thru your BJT is (Vsource - Vce)/(sum of resistance in the rest of the path)

So (5V-0.5v)/(8ohm+pot)


* MOSFET_amp.jpg (34.99 KB, 960x720 - viewed 46 times.)
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I think I figured it out, since with 0 ohms at the pot, the voltage output of the opamp would be 1V, which means 12 - 1 = 11V must be dropped across the bjt. 1V/8ohms = 0.125 A ----> power disipation of bjt = 11V*0.125A = 1.375W > 0.6 W max of the transistor. Is this correct?
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Which circuit do you have wired up? There have been several posted.
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This one:

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Yes, for full on (12V - Vce)/8 = ~1.44 amp.
P=I^2R = 16W goin thru the speaker
P= IV = ~0.5*1.44 = 720mW dissipating in the transistor.

2N4401 absolute max 600mA - 1.44A would smoke 'em!
« Last Edit: March 03, 2011, 03:23:39 pm by CrossRoads » Logged

Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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