On 5/14/06, Rich Graziano wrote: > 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. Have no ideea. :) 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. This is a good point indeed. But unfortunately in practice you can't avoid saturation only by adding a resonant circuit in parallel with the load (changing from switching to pseudo-linear). This is possible only with inductive loads and not everywhere. In most of the switching schematics, as high is pomped the base current, as fast will be the transient OFF-ON (assuming everything is ok with timings and load). For faster switching from ON to OFF the base is reversed polarised (the simplest tehnique is to use a small volatge zenner or two series diodes in parallel with a capacitor in the base circuit; when driving ON the capacitor is charged, when driving OFF -usually with 0V- the voltage across the capacitor is reversed on BE junction allowing a faster turn off). > > 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. I have no clue, fortunately I didn't ask the question. :) greetings, Vasile > 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 > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist