[sdiy] Mosfet Amps?

[dan snazelle]

sorry

douglas self



not david



On Jul 10, 2011, at 9:57 PM, dan snazelle wrote:

> I think those might be issues if i was trying to design an audiophile system (this is the problem I have with David self...the same author who basically calls OTA's a historical curiosity, he is great if you are looking to build HI-END low noise power amps, etc but for guitar boosters, weird distortion boxes, etc Mosfets are great! ( i think the circuits I have been able to build with one MOSFET blow any one BJT circuit out of the water for a 
> clean hi-gain device.
> 
> I just want to get more info on them, instead of switching parts out randomly.
> 
> 
> 
> I have been wanting to buy the book you are speaking of, but it is not cheap ( i can get most electronics books used for around 1-5 dollars on amazon if i wait long enough)
> 
> so to answer your question, I am trying to make some hi-gain boxes for guitar and synth. 
> 
> I have been having fun mixing BJT's and Mosfets too
> 
> 
> thanks
> 
> 
> 
> On Jul 10, 2011, at 9:39 PM, Noah Vawter wrote:
> 
>> 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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>> 
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