Matchin transistors

[johns at oei.com]

     joachim (and DIY) -
     
     Thanks for the techy treatment of transistor matching.  I think this 
     might cover it all.
     
     But are you saying that the term "matched transistors" really 
     indicates 2 transistors in good thermal "sync" (using come good 
     thermal coupling) rather than 2 transistors selected for closely 
     matched VI and gain characteristics?
     
     JJS


______________________________ Reply Separator _________________________________
Subject: Re: Matchin transistors
Author:  Joachim Verghese <jocke at netcontrol.fi> at Internet
Date:    9/13/96 11:24 AM


On Thu, 12 Sep 1996 johns at oei.com wrote:
     
>      In preparing to build an ASM-1, I am considering searching for 
>      matching transistors.  I assume this means trying to find a pair of 
>      transistors that show a close match of a V-be curve for an identical 
>      I-b current ramp.  (Is this correct?)
     
The problem with using a transistor (or diode) as an exponentiating 
device is that the transfer function is heavily dependent upon 
changes in temperature.
     
In a typical linear voltage-to-exponential-current converter, the 
first transistor is connected as a voltage follower, and the second
as the exponentiator. The voltage follower adds a temperature dependent 
offset to the control voltage. This compensates for the 1st order 
thermal effects of the exponantiating transistor.
     
In other words, if the two transistors have identical thermal 
responses, then you've eliminated a major part of the converter's 
thermal drift.
     
Ideally, then, the two transistors should be matched for *thermal 
tracking*. Using a dual transistor (MAT-0x, SSM22x0) obviously
is an advantage since the two transistors are on the same silicon 
substrate. If you choose to use two discrete devices (2N390x...), 
there's no easy way to guarantee thermal tracking, but in most 
cases you'll achieve good results by just using randomly selected 
devices, as long as you provide adequate thermal coupling. Some
sort of heatsink or metal clip should be used to hold the transistors 
together.
     
As far as Vbe-Ic (or Ib) matching goes, this is generally not needed 
for exponentiator applications. Any mismatch in these parameters only 
affect the tuning offset of the oscillator/filter.
     
In systems with large numbers of oscillators/filters, it might be 
desireable to Vbe-Ic match the transistors, since this allows you
to have less coarse tuning trimmers, which makes calibration easier. 
(Remember that a change of 18 mV at the base-emitter changes the 
frequency by one whole octave.)
     
-joachim