Simple Info About Bipolar Junction Transistors and How to Use Them! -Steve Walz rstevew@armory.com Somebody wrote: >I think, either my transistors are dead, or I'm hooking them up wrongly. >I hooked the emitter to ground. The base is biased with a pot to about >0.75V. > >So, if the reading between ground and base is 7.5V shouldn't my reading >between collector and ground be amplified? It's not. It's only about >one tenth of the ground to base voltage. The current readings (ground >to base, ground to collector) give similar one to one tenth readings. > >Lastly, even if I vary the pot down to zero so the ground to base >voltage goes way up (slowly varying up to 9V) I still get only about one >tenth of the base voltage at the emitter. > >Have I hooked these transistors up incorrectly or are they all broken. -------------------------------------------- Hahahahah! Sorry. But you'll find this funny later on!! So I am laughing WITH you, it's just that there will be a "time-delay" before you "get the joke"!! You HAVE successfully saturated the transistor as soon as you brought the base to ground voltage up to 0.75V, presuming it's NPN of course. When you further took it all the way to 9 Volts, you further saturated it!! The problem seems to be that YOU think that all a transistor needs to perform amplification is base voltage!! Wrongo!! It also needs a resistor-limited collector supply from the positive of the voltage supply!! In other words, connect a resistor, say 1K ohm, to the collector and the other end of that resistor to the supply positive, at least close to 9V's! THEN you will be able to explore all the three regions of the behavior of an NPN bipolar junction transistor (BJT)!! They are: -- Cut-off: where the base-emitter voltage has not yet reached about .62 to .65 volts, so that the inherent diode comprising the base-emitter junction is not yet turned-on. The collector will be pulled to 9V by the collector resistor and the supply, seeing as the transistor is not yet acting. -- Linear Region: where the slight variation of the base-emitter voltage and thus its current causes the collector to begin to sink current more, the more the base voltage with respect to the grounded emitter goes up! The collector resistor and the transistor's collector-emitter path to ground act as two resistors in series which divide the total supply voltage into two voltages at the collector, the output of this primitive amplifier!! As you dial it up you will see that the base voltage rising causes the collector voltage to fall, since the current in the collector-emitter path is rising and likewise in the collector's resistor, increasing it's share of the voltage drop by Ohm's Law. This decreases the transistor's share of the voltage, and you see the voltage drop from 9V to nearly nothing as you increase the base voltage and thus it's current through the base-emitter junction. In this region, you will note that the base CURRENT and collector CURRENT are a fixed ratio, Ic/Ib, called the GAIN. You should also realize that this simple DC amplifier is an inverting amplifier, as it both amplifies and inverts the base voltage in its effect on the collector voltage!! However, this is because of the way we set it up, since the collector current paces base current in the Linear Region pretty precisely! -- Saturation: This region is where one can no longer supply more base current and get a proportional rise in collector current, as before in the Linear Region. In fact the current simply won't increase in the circuit because the collector to emitter path is conducting as well as it EVER can at that collector voltage!! Thus, the collector is as close to ground, the emitter, as it will ever go, or one gets little further drop in it until one overvoltages the base and explodes the poor little transistor!! The collector resistor is often selected just for this, to set the maximum current that the supply voltage will be allowed to push through the collector to emitter path, so that the Icmax is not exceded. In doing that, the resistor hogs almost all the voltage drop, and there is very little across the collector to emitter path, now matter HOW much you crank up the base voltage!! If the base doesn't get cooked, you will be lucky!! ;-) -- Uses: The switching function of a transistor is its simplest role, and it is accomplished by swiftly as possible passing right through the Linear Region from Cut-off to Saturation, from all the way OFF to all the way ON, which is why we call this a switching function!! We slam it ON and slam it OFF, and we don't WANT any in-between, as that uses more POWER (HEAT LOSS) in between, just because Volts time Amps = Watts, and thus: ON is all amps and almost no volts, and OFF is all volts and almost no Amps!! So low power consumption!! And the less time spent lingering in-between, the faster the "logic" can be. The other use is as a linear amplifier of voltage levels or current values, and it is achieved by setting the quiescent, or "resting" base voltage and current so that the collector assumes a value of exactly half the supply voltage, and then, using a capacitor to isolate anything but AC signals from the base, we pipe in a slight additional signal on top of the same quiescent signal, and it causes a greater change in the collector current by the GAIN ratio, and consequently a GAIN in the signal when seen, inverted, as in upside-down on an oscilloscope, compared to its input signal, at the collector as the changing or AC portion, which can be extracted there, amplified, with another capacitor!! And that's about it in this configuration. To figure out other configurations, simply remember, that whatever the current through a transistor's collector-emitter path, it has to take on whatever is left of the voltage that gets dropped across it after the resistors get their legal share under Ohm's Law!! The bipolar transistor, therefore, doesn't HAVE any such thing as a known "resistance" at a given base current, as it will change with the collector voltage!!! And make SURE that you resist ANY and ALL temptations to try to THINK that way, because it WON'T WORK!! It will only screw up your thinking!! In fact what it has is called "transconductance", and it is not a fixed number, either, it's an equation that is quite complicated!! It's not really important to know unless you wind up designing transistors, either!! The GAIN ratio will work just fine for most all of any engineer's calculations!! AND the voltage and current MAXimums for base and collector!! ;-) -Steve -- Copyright 1997 Richard Steven Walz All Commercial Rights Reserved All Non-Commercial Rights Free -- -Steve Walz rstevew@armory.com ftp://ftp.armory.com:/pub/user/rstevew -Electronics Site!! 1000 Files/50 Dirs!! http://www.armory.com/~rstevew Europe:(Italy) http://ftp.cised.unina.it/pub/electronics/ftp.armory.com