|10th March 2012, 02:12 PM||#11|
Join Date: Nov 2004
Actually, having higher fT transistors in the buffer is a good thing. You want to be something like a factor of 3 faster than the opamp (GBW / gain) to avoid instability or having to reduce loop gain. (Alternatively, "a lot slower" is another - if not too well-performing - option.)
If, for example, you want to operate an opamp with a 20 MHz GBW at 3x gain, you'll want at least 20 MHz of buffer fT. A BD139/140 pair can do it but they need to be run a little warm.
Note that practical fT depends on current plus transistor tf and Cbc/Cbe. (Formula given here, for example. fT vs. Ic also may be given in the datasheet.) This is why transistors with higher power handling and thus bigger dies tend to be slower. Cheapo oldschool power transistors like TIP31, TIP41 or BD24x tend to reach about 3 MHz only.
Excessive fT may be a problem since emitter followers like to oscillate when fed by an inductive source impedance, and an opamp output is about as inductive as they come (its output impedance steadily rises as its open-loop gain falls, up to the point where it runs out of OLG altogether). Using a feedback cap on the opamp or artifically increasing Ccb (making the transistor slower) can help in such cases.
The biggest problem with any TO-92 case transistor is cooling. That kind of case has a thermal resistance of like 200 K/W in free air and not a lot under 100 on a heatsink. You'd probably be well advised not to run one at more than about 10 mA in a typical headphone buffer circuit, and I wouldn't expect massive amounts of output power.
2N2222s or BC337s aren't a bad compromise for TO-92. Still quite fast and reasonably robust. BC639s already need a good bit more current but are more rugged in return, though in that case they can't really live up to their full potential. Philips used to package the same die in TO-126 as BD139, those weren't bad parts (fairly little beta droop, too). The BD139s you can buy today can handle more current, but their capacitances are about twice as high, and beta droop starts to set in above about 200 mA already. Still quite useful for a headphone buffer though.
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