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[Amps] Re: AB1 v. AB2

To: <amps@contesting.com>
Subject: [Amps] Re: AB1 v. AB2
From: garyschafer@attbi.com (Gary Schafer)
Date: Sat, 30 Nov 2002 18:42:41 -0500
BEAR wrote:

> Gary Schafer wrote:
>
> > BEAR wrote:
> >
> > > Rich wrote:
> > >
> > > > >
> > > > >----- Original Message -----
> > > > >From: Rich <2@vc.net>
> > > > >To: <bearlabs@netzero.net>; AMPS <amps@contesting.com>
> > > > >Sent: Friday, November 29, 2002 1:40 PM
> > > > >Subject: Re: [Amps] Re: AB1 v. AB2
> > > > >
> > > > >
> > > > >>
> > > > >>
> > > > >> >amps-request@contesting.com wrote:
> > > > >> >
> > > > >> >>
> > > > >> >>
> > > > >> >> Message: 8
> > > > >> >> To: amps@contesting.com
> > > > >> >> Reply-To: ad5gb@myway.com
> > > > >> >> From: "ad5gb" <ad5gb@myway.com>
> > > > >> >> Cc:
To: <amps@contesting.com>
> > > > >> >> Date: Thu, 28 Nov 2002 22:18:13 -0500 (EST)
> > > > >> >> Subject: [Amps] AB1 vs AB2 ???
> > > > >> >>
> > > > >> >> Greetings Amps gurus,
> > > > >> >>
> > > > >> >> Happy Holidays and special thanks to all who've been so helpful 
> > > > >> >> to me
> > > > >over
> > > > >> >the past year!  It means a great deal to me.
> > > > >> >>
> > > > >> >> I'm wondering if someone can explain why AB2 GG service is more 
> > > > >> >> linear
> > > > >than
> > > > >> >AB2 grid-driven service?
> > > > >> >
> > > > >> >I dunno about that one... check a handbook.
> > > > >> >
> > > > >> >> I've been working with my latest project ( still on paper ) which 
> > > > >> >> is a
> > > > >pair
> > > > >> >of 4-400As and it appears that all other things being equal, about
> > > > >200-250
> > > > >> >watts more is available in AB2 operation than is available in AB1.
> > > > >> >
> > > > >> >This is because in effect the load line is "longer" in AB2 than 
> > > > >> >AB1. AB2
> > > > >> >permits you to draw grid current, whereas
> > > > >> >AB1 does not. That means you can drive the grid *positive* past "0"
> > > > >> >whereas in AB1 your drive stops at "0"
> > > > >> >since current can not be delivered to the grid in AB1.
> > > > >
> > > > >> >
> > > > >> **  AB2 doesn't permit anything.  AB2 neans the operator Chose to 
> > > > >> drive
> > > > >> the tube into its less-linear grid-current region.  Up until that 
> > > > >> point,
> > > > >> the tube is operating partly in Class A and then in Class AB1 -- as 
> > > > >> the
> > > > >> grid-V sinusoidally decreases as it approaches 0V .
> > > > >
> > > > >By *definition* AB2 means that the grid is driven positive with 
> > > > >respect to
> > > > >the cathode - and regardless *is* in the grid current region. AB1 by
> > > > >definition is up to
> > > > >"0" volts with respect to the cathode, and NEVER draws any grid 
> > > > >current.
> > > > >
> > > > >An amp set up for AB1 can NOT be driven into grid current - if it is 
> > > > >able to
> > > > >be driven into grid current then it is an AB2 amplifier, regardless of 
> > > > >how you
> > > > >chose to run the input levels.
> > > > >
> > > > **  increase drive to the point that grid current flows and the AB1 amp
> > > > becomes an AB2 amp.
> > > >
> > > > >In the case of most circuits, the AB1 amps are AC coupled and the AB2 
> > > > >amps are
> > > > >DC coupled to the grids - which permits current to flow. This is 
> > > > >especially
> > > > >clear  in audio applications...
> > > > >
> > > > **  with AC coupling, grid-current flows into the grid bias supply.
> > > >
> > > > >You can *say* that an amp is "in" AB1 until it recieves a signal that 
> > > > >drives
> > > > >it into grid current, but I don't know of anyone that can control 
> > > > >peaks that
> > > > well.
> > > > >So, ultimately
> > > > >the difference in how an amp is classified is not where you run it, but
> > > > >*how* it is
> > > > >designed.
> > > >
> > > > **  so increasing drive will not eventually cause grid current in a well
> > > > designed AB1 amp?
> > >
> > > it can't. If it can then it is an AB2 amp.
> > >
> > >                _-_-bear WB2GCR
> > >
> >
> > So how would you "design an AB1 amp that could never go into grid current" 
> > with
> > increased drive?
> >
> > 73
> > Gary  K4FMX
>
> At least at audio frequencies that is done by AC coupling to the grids...

Well designed AB1 RF amplifiers are almost always AC coupled to the grids too 
in a grid
driven amplifier. They are no different than audio amplifiers. Either can be 
driven into
grid current  with increased drive.


>
>
> At RF I'm not sure at the moment - but by definition the difference is grid 
> current vs.
>
> no grid current.
>
> For sure ALC type feedback, or a clamp circuit would keep an AB2 circuit from
> drawing grid current, one would think...

When an AB1 amp draws grid current it is then operating in AB2.


>
>
>         73,
>
>                 _-_-bear
>
> --
>   _-_-bearlabs
>
>    http://www.bearlabsUSA.com
>     - Silver Lightning Interconnects & Custom Mods -

73
Gary  K4FMX


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