" snipped-for-privacy@aol.com" bravely wrote to "All" (16 Feb 06 13:11:27) --- on the heady topic of "Re: Emitter Follower"
ZZ> From: snipped-for-privacy@aol.com ZZ> Xref: core-easynews sci.electronics.repair:358523
ZZ> Dave;
ZZ> I really hope it's not college level, this should be in high school. ZZ> Common collector amps are the simplest to design.
ZZ> Actually I'm a bit squeaky when it comes to common emitter and ZZ> especially common base stages' actual input impedance. In general in ZZ> those stages I use a heavy handed approach, make it damped (shunted) ZZ> and linear.
ZZ> Anyway, now that you got me on the subject, people have looked at me ZZ> funny for saying this; All bipolar transistors operate as common ZZ> emitter. No exceptions.
ZZ> This was in response to the inability of a buddy of mine in trade ZZ> school to understand the concept. I had to draw it about 15 different ZZ> ways but he finally got it. I almost went into sign language !, that ZZ> is I was to the point of holding up three figners and saying sternly ZZ> "OK, my thumb is the emitter ok ?" I didn't have to resort to that but ZZ> it was close.
ZZ> It was a success, and he actually doesn't make more money, but he ZZ> moved from 90 hours a week to 40 hours a week to make about $50,000. ZZ> Not bad, it's just about a whole dollar every day.
ZZ> To the OP;
ZZ> In common collector (emitter follower) the resistance presented by the ZZ> base is approximately the load resistance times the current gain of ZZ> the transistor. This current is applied (V / RL X hfe) in the direction ZZ> of whatever is biasing the emitter. As long as the collector is solidly ZZ> connected to the Vcc, and input is reference to a propely bypassed Vcc ZZ> return this is close enough. When the collector voltage is allowed to ZZ> vary, like by a resisor, this is not accurate.
ZZ> I am not abject to giving help on school projects, but if it is please ZZ> say so. For one, this cheap and dirty formula dfoesn't work with ZZ> anything else. If you don't learn it the right way you will fall flat ZZ> on your face later.
ZZ> Also, if RL=50 ohms, your wimpy little transistor will have to ZZ> dissipate at the very least 720 mW even during quiescence. A 2N2222 is ZZ> going to have a problem with that, you will at least need a heatsink. ZZ> In fact you might need Freon !
ZZ> If you're talking about buffing a soundcard output, this is not the ZZ> way to go. If you simply get an older soundcard that doesn't require ZZ> powered speakers you are fine. They will feed 50 ohms no problem. A ZZ> sig generator, OK, mine for example is 30 volt @ 600 ohms output. But ZZ> then what would stop you from just feeding that to the soundcard ?
JURB,
If his card has only Line-Outs then he would be better off with a pair of amplified speakers. If he insists on DIY then perhaps an 8-pin itty bitty LM386N etc, pair might do. All the preamp gain he needs and it will drive 1 watt into 8 ohms quite happily without too much fuss. Just don't use much gain because the little bastard has a noisy input.
If you insist on just 3-pins, then checkout the application note for the TL431 precision voltage reference which makes a radical use of it as a 400 milli-Watt phono-amplifier. However it uses a small audio transformer 330:8 ohms. The beauty of using a transformer is that at idle it dissipates the maximum power it ever will. So if it is too hot then bolt it to a big heatsink and it will be safe.
A*s*i*m*o*v
... A fail-safe circuit will destroy others.