[sdiy] Mosfet Amps?

[Noah Vawter]

fwiw,

Douglas Self in "Audio Power Amplifier Design Handbook" recommends  
against FETs in audio amps for various reasons, but nevertheless  
includes examples and information on them as small-signal amplifiers  
in Chapter 4 and power output stages in Chapter 12.

In his Disadvantages section, he cites low transconductance, and  
variability of conduction as issues with FETs.
He also mentions that in the small-signal amplifier input stages,  
noise performance will be inferior with FETs when used with devices  
with low output impedance.  His guesstimate is 5kOhms.

In chapter 12, the list of disadvantages to using FETs is quite  
large.  7 Major items.  Here are some highlights:

poor linearity, higher gate voltage, high minimum channel resistance  
(I'm not sure that I buy this one.)  liability to parasitic  
oscillation, difficult to place devices in parallel.

however, he sums up all of the differences between FET and BJT  
amplifiers (power audio amplifiers) simply by saying that it's matter  
of cost and FETs will be 1.5-2.0 times as expensive.


*By the way, this book is excellent.  I recommend it for anyone  
interested in advanced electronics and amplifiers.  Experts and  
experienced designers may already know everything, but it filled in a  
huge gap of knowledge for me, and nearly every designer needs to  
consider the metal inside of an amplifier at some point.

After studying Self's book, I'm cocky enough to ask you: How many  
watts are you looking to dissipate and into what?

-Noah!


On Jul 10, 2011, at 6:09 PM, dan snazelle wrote:

> I have lots of electronics books. But basically none of them show  
> how to use MOSFETS as linear amplifiers. they usually just show  
> using Mosfets as INVERTERS (ala CMOS)
> or as switching devices.
>
> I know you can make amplifiers with them.
>
>
> Can anyone recommend a book or a website that covers them in more  
> depth?
>
> thanks
>
>
>
>
>
>
>
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