[sdiy] current mirrors

[Benjamin Riggs]

it's all a bit obvious the more i think about it. i knew the current is mirrored in both collectors due to both transistors sharing the same base voltage (and temperature re the Ebers-Moll equation), but at 3 in the morning, it didn't click that each transistor has its own base Vbe v Ic current relationship. if the two transistors don't share the same Vbe v Ic relationship, of course the current is going to be a perfect mirror. duh

cheers all

B
  ----- Original Message ----- 
  From: Scott Bernardi 
  To: Synth-DIY list 
  Sent: Thursday, November 27, 2003 5:06 AM
  Subject: Re: [sdiy] current mirrors


  Ooo! Ooo! I know!  (picture little kid jumping up and down with his hand in the air).
  The Vbe vs. Ic curves should be matched.  I.e., for a given Vbe they should give the same collector current, and visa-versa. The transistors should be at the same temperature also since that greatly affects the Vbe vs. Ic relationship.
  A two transistor current mirror will have an error due to the base currents. One way you can decrease that error (by a factor of Beta) is to add a third transistor with its B-E junction replacing the B-C short on the "input" side of the mirror.The collector can go to the supply rail (positive rail for npn mirror, negative rail for pnp) The transistor doesn't have to match.
  Here's a link I found about current mirrors:
  http://www.eas.asu.edu/~thornton/Teaching/ece334/lectures/lecture_16.pdf

  Paul Schreiber wrote:

Yes, that's the whole *point* of being "matched"  :)

Now, exactly WHAT is "matched" in a 'matched transistor pair'?
.....Anybody.....Buhler?........(yes, I know the answer, but I want you to find out yourself).

Paul S.
dang cold (cough cough)



----- Original Message ----- 
From: "Benjamin Riggs" <benj at iinet.net.au>
To: "Synth-Diy" <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, November 26, 2003 12:41 PM
Subject: [sdiy] current mirrors


  i am hardly an expert when it comes to transistor theory, but i think i
understand the basic operation of current mirrors. what i would like to know
is if in the basic two transistor curent mirror, do the transistors need to
be matched?

B


    

  

-- 
Scott Bernardi
sbernardi at comcast.net
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