Basic transistor question

Emp_Fab

I'm plodding my way through some electronics books to refresh my memory (it's been a loooooong time since I did my ancient O-level !) and I'm having trouble with a simple concept.

Take this diagram...

 

and this text...  "The bias current has to flow through R2, so its value determines how much is available to the transistor’s base; the higher R2’s resistance, the less bias current there will be. The transistor passes current from collector to emitter in proportion to how much current passes from base to emitter, as shown by the arrows."

I get it that R1 & R2 form a voltage divider and that a current will flow through them from the supply to ground.  What I don't get is the statement that increasing the value of R2 will reduce the bias current.  Surely, if you increase R2, then the voltage at the base will be higher, and if the base voltage is higher then the current from the base to emitter will be higher, not lower ?  I can see that increasing R2 will obviously reduce the current through R1 and R2, but how does that affect the base-emitter current (especially if the voltage at the base has increased as a result of upping R2) ?

Danny1969

In your circuit raising the value of R2 will increase the base current and the base voltage, I think the wording in the article is misleading  \ wrong 

ICBM

Yes, that's correct. They've either misplaced R1 and R2 on the diagram or they've got higher and lower the wrong way round.

Emp_Fab

Bastards.  That's the VERY FIRST schematic illustration in the bloody book.  Drives me mad when things aren't properly proof-read - especially technical stuff.  It leaves you pulling your hair out as you end up questioning your understanding of the fundamentals.

When I was doing an O.U. degree, the textbooks I would get (produced by the O.U.) were diabolical for errata.  You'd spend hour upon hour scratching your head as to why the result you got didn't match the answer in the book, time and time again.  Then two weeks later a booklet (not a sheet - a fucking booklet of at least six A4 pages) would land on the doormat with all the errata.  Then you'd realise that you were right all along.

Just to confirm what Danny & ICBM said, I recreated the circuit on http://www.falstad.com/circuit/ and sure enough....  increasing R2's value increases the base current.  Thanks chaps !

robinbowes

Which book do we need to avoid?

Emp_Fab