>> OK. So you'd wire them both up in essentially the same way?
> 
> Essentially, yes, but you need to bear in mind the differences between 
> the two when designing your circuit.  The main difference is that a FET 
> is controlled by input voltage, and the BJT by input current.  So in 
> most designs with BJTs you'll see some resistors dotted around to 
> control the current.

See, in theory the way a transistor works is supposed to be simple. 
However, every transistor circuit I've ever looked at seems so damned 
complicated...

> Good point about FET is that they have almost infinite input impedance, 
> which is often useful for low-power operation or when measuring very low 
> powered signals.  Good point about BJT is that the output resistance is 
> higher, so good for driving higher powered devices rather than pure 
> digital signals.

...and this is why I like to stick to logic circuits. You don't have to 
worry about obscure concepts like impedance or capacitance. (I mean, 
unless you're designing "high performance" circuits anyway.)

>> So why is it that (say) a simple NAND gate involves 8 transistors, 3 
>> diodes and 25 resistors? That doesn't make any sense to me... 
>> Logically it looks like any possible 2-input gate should require 1 or 
>> 2 switches and nothing else.
> 
> A NAND gate needs 4 FETS and no resistors:
> 
> http://en.wikipedia.org/wiki/Image:CMOS_NAND.svg
> 
> Inside an IC, resistors are HUGE compared to FETs, there is no way you 
> could make a modern CPU if you needed to include resistors for each gate.

Even so, you can make a NAND gate out of just 2 electromechanical 
relays. Why does it require twice that number when you use transistors? 
(Additionally, I can't make any sense out of that diagram. The wiring 
seems to have no reason or rhyme to it.)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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