Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!rutgers!mcdhup!mcdham!stever
From: stever@mcdham.UUCP (Steve Reinhardt)
Newsgroups: sci.electronics
Subject: Re: Design idea: PA and Music Amp questions
Summary: MOSFETS cannot be paralleled with impunity in a linear system.
Keywords: high current amp
Message-ID: <1773@mcdham.UUCP>
Date: 23 May 91 14:53:37 GMT
References: <72314@microsoft.UUCP> <4785@osc.COM>
Organization: Motorola Semiconductor, Wallingford, Ct.
Lines: 59

In article <4785@osc.COM>, jgk@osc.COM (Joe Keane) writes:
> In article <72314@microsoft.UUCP> gordonl@microsoft.UUCP (Gordon LETWIN)
> writes:
> >My idea is basically to take any amplifier that I please, but just feed it's
> >output into a 8 ohm dumy load and build a high current voltage follower
> >that follows the amp output.  The follower would use some very high current
> >MOS transistor.  The theory is that this would impress the "high musical
>
> I've seen lots of schematics for high-power amplifiers.  Typically they use a
> current feedback configuration, and there is one stage of bipolar transistors
> between the op-amp output and the output MOSFETs.
> 
> One nice feature they note is that, unlike bi-polar transistors, you can
> parallel MOSFETs arbitrarily to get as much power as you want.  So if you're
> running out of power and you want to add more speakers, you can just drop in
> some more output transistors, with heat sinks of course.
> 

Unless you have money to burn, don't do this.  MOSFETs do have the requisite
temperature coefficient to permit paralleling, but only in switched mode
systems. When they are used in the linear region, they may not react quickly
enough to save themselves; that is, the transconductance will not decrease
from the increase in temperature rapidly enough to promote current sharing
across the group of devices.

In addition, the VGS(th) (threshold voltage for turn on) can vary by a factor
of two for a given type of device, and failure can occur very quickly. Imagine
driving an 8 ohm load from 100 volt supply rails, when only one device turns
on. Messy stuff. Of course, if the device doesn't short ( unlikely), then the
remaining devices will start to share, unless one of them is higher gain, etc.

There are lots of designs published out there, in applications notes from
Motorola, Siliconix, PMI and others. In addition, the Audio Amateur
publication has many full construction articles that will help avoid these
pitfalls.

You might want to ring up your local Motorola sales office, and ask for
DL135, the Power MOSFET transistor data book. The first section is a
good tutorial for learning how these beasties work.

> Note that even though MOSFETs have very high input resistance, power MOSFETs
> have can have a hefty input capacitance, so it takes a decent amount of
> current to drive a bunch of them in parallel.  As long as the previous stage
> is designed with this in mind, it shouldn't be a problem.
> --
> Joe Keane, amateur electronics hacker
> jgk@osc.com (...!uunet!stratus!osc!jgk)

----

Pereant, inquit, qui ante nos nostra dixerunt.
"Confound those who have said our remarks before us"- Aelius Donatus
Steve Reinhardt, Motorola, Wallingford, Ct. 06492          (203) 284 0810
UUCP: stever@mcdham.UUCP or stever@ct.mcd.mot.com
-- 
Pereant, inquit, qui ante nos nostra dixerunt.
"Confound those who have said our remarks before us"- Aelius Donatus
Steve Reinhardt, Motorola, Wallingford, Ct. 06492          (203) 284 0810
UUCP: stever@mcdham.UUCP or stever@ct.mcd.mot.com