audioguru Posted May 8, 2004 Report Share Posted May 8, 2004 MP and ANTE,Maybe they use more transistors than is necessary in order to reduce the gamble of a high-gain transistor "hogging" the current. Also, extra transistors are probably cheaper than having many emitter resistors.You didn't comment about my explanation for emitter resistors that I posted previously:Ante,Zapco either matched the paralleled transistors, or had good luck.Think of emitter resistors as a form of negative feedback. When a high-gain transistor attempts to conduct a large collector current, then that current creates a voltage-drop across the emitter resistor, which reduces the base-emitter voltage and therefore reduces base-drive.A transistor with less gain will attempt to conduct less current and therefore will have less base-drive reduction. So the gains of the transistors are equalized.Without emitter resistors, when a high-gain transistor is paralleled with a low-gain one, then the high-gain transistor will conduct more current than the low-gain one, which results in unbalanced current sharing. Without balanced sharing, the high-gain transistor may exceed its maximum current and/or thermal rating and blow-up. Quote Link to comment Share on other sites More sharing options...
ante Posted May 8, 2004 Report Share Posted May 8, 2004 audioguru,I don Quote Link to comment Share on other sites More sharing options...
MP Posted May 8, 2004 Report Share Posted May 8, 2004 closer than what you might think ante. What you have stated is approximately what I was saying but in a different way. I have not had much use for square wave inverters in the past.Linear is quite different as I have found on my bench, though. And of course, the sinewave inverters are a little different because of this.A comment about changing the transistors: Why would you do anything different? If the circuit has caused a break down in one transistor, I would be willing to believe the others have had a hard life as well. I always replace components as sets, too.MP Quote Link to comment Share on other sites More sharing options...
audioguru Posted May 8, 2004 Report Share Posted May 8, 2004 Ante,Yes, I agree that transistors from the same batch will have the same gain and therefore can be paralleled without emitter resistors. I also agree that changing them all together maintains their matching. Quote Link to comment Share on other sites More sharing options...
audioguru Posted May 11, 2004 Report Share Posted May 11, 2004 MP,Back to the "darlington transistors" argument. Power darlingtons also use base-emitter resistors too:http://www.fairchildsemi.com/ds/MJ/MJE803.pdf Quote Link to comment Share on other sites More sharing options...
MP Posted May 12, 2004 Report Share Posted May 12, 2004 Sure, lots of devices use this array. As it stated in the pdf that I posted, it is for speed of transition. I do not understand where you think there is an argument....unless you mean you are arguing with the contents of the article that I posted.MP Quote Link to comment Share on other sites More sharing options...
hotwaterwizard Posted May 15, 2004 Report Share Posted May 15, 2004 Here is another Amp Schematic.Pyramid PB300 and there are no mosfet here either.Click on image for higher resolution Quote Link to comment Share on other sites More sharing options...
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