Thank you for your kind reply, Vasile. Yes, of course I agree with what you say, if the problem is one of a switching application. These are simple academic facts. I could not tell from what I had seen, exactly what problem it was that need to be solved, and why the junction voltage would be an issue. I could not tell, perhaps, due to my own stupidity, if the question was one of measurement or one of switching. If it was simply a choice between silicon or germanium it would seem pointless to bring it up. Furthermore, in a switching application it is wise to avoid driving the device into saturation because of the junction capacitance and longer switching times that result; if speed is a factor in the design requirement. Thank you again for your kind and somewhat informative reply. It seems from your response that the question was one of switching. Can you also satisfy my curiosity as to what the actual application is? What is to be switched? Is there some issue about the junction voltage? Speed? I can admit to sometimes being ignorant or even stupid, but I must admit to always to being curious. Respectfully, Rich ----- Original Message ----- From: "Vasile Surducan" To: "Microcontroller discussion list - Public." Sent: Sunday, May 14, 2006 3:12 AM Subject: Re: [EE]: NPN Transistor with low switch on voltage? > On 5/14/06, Rich Graziano wrote: >> I find this one rather confusing. The junction voltage is fixed by some >> relationships given in quantum physics. As far as I know (But what do I >> know?) there are not a variety of devices that successfully avoid the >> parameters that determine the junction potential restrictions. >> >> It would seem to me that any switching application that requires >> switching >> at a very low threshold, say in milivolts or microvolts, > > At this low levels the only way for secure "switching" is the > operational amplifier used as comparator with positive hysteresis > (trigger schmidt). > > where "switching" = pure non linear application (fast transient > between blocked and saturated state - and viceversa- of a transistor). > >> could be achieved >> by selecting the appropriate gain (Beta) for the device; Often a >> Darlington >> arrangement for small currents or a fet differential to a reference, or >> several other design approaches. > > Sure if you're thinking to drive a load proportionaly (a variation of > Ib or Vbe is causing a variation on load voltage or current). A > darlinghton is not suited for "switching", (because is incapable of > fast response, it has important leackage current) and that's the ideea > why you can't say almost nothing about beta in switching applications. > > On the other hand a standard darlington (2x PNP ot 2x NPN) require 2xVbe > so > if he want to sense 100mV there is no way just with one transistor...:) > Sure you can design structures with NPN-PNP pairs where the Vbe is > still 0.6V-0.7V and h21e goes up to 600 or more. > > >> >> The only caveat I see in the extremely low level operation is the noise, > > and temperature dependence which cause instability, > >> linearity and hysteresis over the range. > > ... you can't talk about linearity when switch On-Off, just when drive > proportionally the load > > > > These also can be addressed in the >> design error budget and essential performance spec. Why would someone be >> looking for a different junction voltage? > > because it doesn't know the way how to solve the problem, so it looks > for a germanium transistor instead of a comparator or something else > >> If indeed that is what is being >> sought. >> >> I am at a loss to understand why this is a junction problem. > > Of course is not.This is hapenning when the question is asked wrong or > the originator does not now exactly what he intend to do and let the > answerers to guess. > > Vasile > >> >> >> ----- Original Message ----- >> From: "Vasile Surducan" >> To: "Microcontroller discussion list - Public." >> Sent: Saturday, May 13, 2006 3:21 PM >> Subject: Re: [EE]: NPN Transistor with low switch on voltage? >> >> >> > On 5/13/06, Spehro Pefhany wrote: >> >> At 12:38 PM 5/13/2006 +1200, you wrote: >> >> >> >> >> >> >Tell us how you are trying to do an unknown thing and we may not be >> >> >able to help. >> >> >Tell us what you want to achieve and we'll tell you how to achieve >> >> >it. >> >> > RM >> >> >> >> I think what he wants to do is to take an audio signal (line level?) >> >> and modulate a fan controller based on the expected power dissipation >> >> in an audio amplifier, so that the fan tends to be quiet when the >> >> music is quiet, and loud when the music is blasting. >> > >> > >> > This simple trick and needs one: transistor, termistor (Kohm range), >> > resistor, capacitor, ventilator. >> > >> > Thermistor is sensing the radiator's temperature on which final >> > transistors are mounted. Transistor's load is a low current ventilator >> > connected from colector to Vcc, an RC from the transistor base to >> > ground and the thermistor from base to Vcc. Heating the termistor will >> > decrease his resistance so the transitor will be turned ON and the >> > ventilator will blow faster. It's a common scheme used in many >> > computer power supplies. >> > >> > Vasile >> > >> > >> > >> >> >> >> Something like a precision rectifier (couple of op-amps) into a PIC >> >> ADC >> >> and PWM output would fill the bill. The front end might not have to be >> >> than complex, of course. >> >> >> >> >Best regards, >> >> >> >> Spehro Pefhany --"it's the network..." "The Journey is the >> >> reward" >> >> speff@interlog.com Info for manufacturers: >> >> http://www.trexon.com >> >> Embedded software/hardware/analog Info for designers: >> >> http://www.speff.com >> >> ->>Test equipment, parts OLED displys >> >> http://search.ebay.com/_W0QQsassZspeff >> >> >> >> >> >> -- >> >> http://www.piclist.com PIC/SX FAQ & list archive >> >> View/change your membership options at >> >> http://mailman.mit.edu/mailman/listinfo/piclist >> >> >> > >> > -- >> > http://www.piclist.com PIC/SX FAQ & list archive >> > View/change your membership options at >> > http://mailman.mit.edu/mailman/listinfo/piclist >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist