transistors

[Andy]

Where are a few good tutorial sites to read at that explain how audio is affected and just the general how-to's of BJT's and FET's?

[R.G.]

I don't know about the sites, but the how-to's are pretty easy.

A BJT is a reverse biased diode set up so you can "poison" the holdoff it normally does by injecting charge into the region of the diode that would otherwise hold off conduction.

The collector-base is always reverse biased (in circuits which we're interested in) and as such holds off the power supply voltage. No current flows except the leakages. The way diodes prevent conduction in the reverse direction is by the internal fields pulling all the charge carriers out of the junction region.   In a BJT, the base region is set up to squirt conductive charge carriers into this depleted region; when it does, proportionately more charge flows through than you squirted in because now the region conducts the charges that were held out from the collector. The emitter sucks them all down.

Notice that the base-emitter substantially doesn't conduct with a bias VOLTAGE of less than about 0.4V, and never gets above about 0.7V. The CURRENT through the base-emitter varies over orders of magnitude while the voltage stays the same. This is, by the way, the entire trick to biasing the BJT. The base-emitter is always 0.6V, and everything else has to work outwards from that.

For general how-tos, assume that the BJT is a simple current amplifier; you put current in the base, and hfe times that much current comes through the collector and 1+hfe times that much goes out the emitter.  Any modern transistor has gain so high (over 100) and capacitances so low that you can ignore the other stuff. Shape frequency response with things outside the transistor.  With certain exceptions  - like germanium - assume the modern BJT is a perfect current amplifier until you get up to designing variable bias multiplier cells.

JFETs are the best analogy to the garden hose that we've found yet. Depletion mode JFETs are a conductive sliver of silicon surrounded by a reverse-biased-diode gate region. If you short the gate to the source, a current flows that's limited only by the geometry and doping level of the drain-source sliver of silicon. That current is the max the JFET can normally pass , and is called Idss.

If you place a forward bias on the JFET gate, the gate current goes into the drain-source channel, but has a pretty small effect on current flow otherwise. However, if you reverse bias the gate-channel, the electrical field inside the JFET from that voltage between the gate region and conductive channel starts depleting charge carriers from a region near the gate, just as it did in ordinary diodes and the collector base of a BJT.

In this case, the depleted region makes the conducting sliver of silicon have a smaller effective cross section, just as stepping on a garden hose makes part if its cross section smaller. The effective channel resistance goes UP because its area has gone down.  Garden hose effect.

When you get enough reverse voltage on the gate-channel, the depletion region goes all the way through the channel, and the device is off. Real off. Reverse biased diode off.  

The problem with JFETs is that unlike BJTs, the reverse cutoff voltage varies substantially from type to type and device to device within a type. There is none of this "base-emitter is always 0.6V" stuff. Some JFETs have Vgsoff of only about 0.2V, notably the J201, everyone's mu-amp darling. Some JFETs won't shut off until you get    -10 to -15V on the gate. JFETs are hell to bias reliably, and almost always require selection or tinkering of resistances in the circuit to get the bias right.

Think of JFETs as a voltage variable current source. Current varies from a max of Idss down to 0, with Id=Idss at Vgs=0, and Id = 0 at Vgs = Vgsoff, which varies all over the map.

JFETs are even better at high frequency response than BJTs. Ignore the JFET contribution to frequency response and shape tone with other parts. This approximation will work fine until you're designing distortion compensated voltage variable resistors and JFET diffamps.

Did I confuse the issue?

[petemoore]

Could some one explain to me in greater detail the definitions of foreward bias, reverse bias, and reverse voltage?  
 Also reverse cutoff voltage?

[bwanasonic]

Could some one explain to me in greater detail the definitions of foreward bias, reverse bias, and reverse voltage?  
 Also reverse cutoff voltage?

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html#c1

Kerry M

[Rob Strand]

I think the OP had the right idea of asking *where* to find this information- sorry, I dont' keep track of site with this type of info, but I have seen *heaps* of sites.

If you what to know this stuff you really have to study it, either using the web, or from a book.  I'm afraid there aren't any short cuts and you have to find something which discusses thing at your depth/speed.

forward biased:        A semiconductor junction when the (conventional)
                               current flow is from the P to N region.  Usually
                               this is the high current region of operation of a
                               diode (/junction).

forward voltage:        Voltage drop across junction when forward biased,
                               sometimes called the forward bias voltage.

reverse biased:        A semiconductor junction when the (conventional)
                               current flow is from the N to P region.  Usually
                               this is the low current/ blocked region of operation
                               of a diode (/junction).

reverse voltage       Voltage across junction when reverse biased.

reverse cutoff voltage:    Perhaps best called the pinch-off voltage.
                               Applies to JFETs and is the gate to source voltage
                               which causes the JFET to have no current flow
                               between the drain and source.  At the pinch-off
                               voltage the JFET channel gets physically pinched off.
                               There is a point on the channel which is depleted
                               of mobile charge carriers (necessary for current
                               flow) which blocks any current flow.

The forward voltages are ususally considered positive and the reverse ones  negative - but at the end of the day it depends on which end of the junction you are calling the + end.

You can take all these snippets and try to piece things together but it just won't work, you need something which is a go to woe discussion.

[Rob Strand]

Try here and see how you go,
http://www.101science.com/transistor.htm