[SI-LIST] Re: Op-Amp
- From: "o. laney" <olaney@xxxxxxxx>
- To: a.ingraham@xxxxxxxx
- Date: Thu, 4 Mar 2010 17:29:48 -0800
Yes, I've done that too. The double emitter follower has some advantage
in that similar power dissipation in each means some degree of tracking
as Vbe changes with temperature. You could also make the diode thermally
intimate, but the absolute value of Vbe is determined not only by
temperature but also by current density in the junction. This is
nominally the same for complementary transistors such as the 2N3904/3906
pair using the same resistor values and rail voltages and driving a
reasonably light load. It's a means to keep the offset error within a
few tens of millivolts. Whether it is worth the trouble is a judgement
call that depends on the application. It certainly wouldn't matter for
an AC coupled signal, but then neither would an emitter follower without
any compensating diode or transistor. It's the DC coupled assumption
that drives the whole discussion, and some applications are far more
tolerant of offsets than others. The choice depends on that.
Orin
On Thu, 4 Mar 2010 19:36:34 -0500 "Andrew Ingraham" <a.ingraham@xxxxxxxx>
writes:
> > There are many schemes that are the responsibility of EEs such as
> > ourselves to invent, but one I use is to simply cascade a PNP
> emitter
> > follower into an NPN one (or vice versa).
>
> Another one I've seen is to put a forward-biased diode before the
> emitter
> follower. It's the same idea except the Vbe compensation uses the
> diode
> instead of a second follower. And you could use another NPN for the
> diode,
> thus avoiding a PNP.
>
> Andy
>
>
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