Re: gEDA-user: TO-92 Best Practices

[Gabriel Paubert <paubert@xxxxxxx>]

On Tue, Mar 02, 2010 at 10:35:22AM +0000, Peter TB Brett wrote:
> On Mon, 1 Mar 2010 22:50:29 -0800, Donald Tillman <don@xxxxxxxx> wrote:
> 
> > This particular project uses some analog IC design styles implemented  
> > with hand-matched discrete transistors; diff amps, current mirrors and  
> > so forth.  So I'd need an efficient way to hand-match surface mount  
> > transistors.  With TO-92's I can just slap them into a rig and collect  
> > them into batches.  Surface mount?  I dunno.  Do they even make SOT-23  
> > sockets?
> 
> The usual approach is to buy SMT packages containing 2 or 4 transistors on
> a single piece of silicon (i.e. literally back-to-back on the wafer).
> They're invariably well-matched enough for all but the most ultra-precise
> applications, in my experience.

Indded. Another important point is that you won't end up with devices
operating at different temperatures which is crucial for differential
circuits (Vbe of a bipolar transistor drops at about 1.5 to 2mV per
degree, even cheap op-amps have offset drifts in the few µV/° range,
thanks to the input transistors being close on the same substrate).

	Gabriel


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