Linear Vbe-IC transistor favorites?
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 9th June 2004, 07:20 AM #1 hjelm   diyAudio Member     Join Date: Nov 2003 Location: Västerås Linear Vbe-IC transistor favorites? Hi all. I vould like to know if anyone has a particular favorite transistor for an application i am considering. I am doing current mirrors and have the intuitive idea that Vbe vs IC linearity is the most critical part, if it is linear in a region i can adjust the emitter resistors to acheive more gain and thus adjust for base currents, which also look more like a resistive load when the Vbe Ic caracteristic is linear. The basic requirements are: Not impossible to get a hold of. PNP devices primarely. IC linear region +-1.2mA Max current small signal transistor <200mA High beta (if possible) Spice models available. Are the 2SA1455k (ROHM i think) any good and are there any spice models of them. __________________ Hjelm
 9th June 2004, 07:49 AM #2 hjelm   diyAudio Member     Join Date: Nov 2003 Location: Västerås Ah and ft should be as high as possible, or at least well above audio band since the application is an iv stage. __________________ Hjelm
 9th June 2004, 08:37 AM #3 mirlo   diyAudio Member   Join Date: Jul 2002 Location: San Diego I think you mean logarithmic Vbe vs Ic, right? The "ideal" transistor's collector current is given by Ic = Is*e^(Vbe/VT) where VT = kT/q, the thermal voltage, about 26 mV at room temp, and Is a scaling constant that depends on the emitter area. Anyway, even in an ideal transistor, this is pretty far from linear. On the other hand, in the ideal transistor, Gm = d(Ic)/d(Vbe) = Ic/VT Which is a linear relationship. Or maybe you mean Ib vs Ic linearity, or constancy of beta vs Ic? In that case, I'd probably just look for a high beta transsitor to minimize the beta nonlinearity, or use a beta-immune current mirror topology. Or maybe the question is, how does one make the most linear current mirror. Which is probably open to all sorts of debate. -- mirlo
 9th June 2004, 09:49 AM #4 hjelm   diyAudio Member     Join Date: Nov 2003 Location: Västerås Thanks, I was trying to avoid the physics of the transistor but it comes up and bites you whenever you try. The thing i am searching for is as you say the gm. The gmneeds to be constant or as slow moving as possible over at least the +-1.2 mA delta ic. I suppose linear current mirrors is the thing i am searching for but my gut feeling tells me that the requirements on the transistors differ depending on type of current mirror you use. __________________ Hjelm

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