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[AMPS] AB1 vs AB2

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Subject: [AMPS] AB1 vs AB2
From: (Rich Measures)
Date: Wed, 11 Jun 97 15:35:57 -0700
>A better way is to use a HV NPN transistor, with the collector 
>grounded. The base goes to a pots wiper, the hot end of the pot goes 
>to a regulated source higher than the grid requires. ...snip...

NPN and PNP HV transistors suffer from 'secondary breakdown effect' when 
medium voltages and currents co-exist (as is the case in regulator 
service).  One design solution to 'secondary breakdown effect' is to 
derate the dissipation to about 1/8 and parallel additional transistors.  
Another solution is to use a power FET transistor.  They are virtually 
immune to 'secondary breakdown effect'.  Motorola and International 
Rectifier make a 6A, 600v, 150w FET that appears to perform  
satisfactorily in HV regulator service. 

R. L. Measures, 805-386-3734, AG6K   

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