What about FETs for Cellular Base Stations in Audio Cascode Voltage Gain Stage (line - diyAudio
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Old 11th August 2009, 12:09 PM   #1
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Default What about FETs for Cellular Base Stations in Audio Cascode Voltage Gain Stage (line

Hallo,
there are many fet devices for the GHz aera. Here three URLs:

pHEMT
http://www.triquint.com/prodserv/mor...=TGF2021-04-SD
GaAs HFETs
http://www.triquint.com/prodserv/mor...prod_id=FP31QF
GaAs MESFETs
http://www.triquint.com/prodserv/typ...As_MESFETs.cfm

the nearest threads to this are the follow:
http://www.diyaudio.com/forums/showt...p?postid=22590 (Voltages across cascode MOSFET)
http://www.diyaudio.com/forums/showt...p?postid=23146 (folded cascode puzzle)
Borbely Fet Follower SPICE modeling (Borbely Fet Follower SPICE modeling)
LD1014 SPiCE models (LD1014 SPiCE models)

are there advantages by use of such fets in audio amplifier input stages?
Thank you for your comments
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Old 11th August 2009, 12:50 PM   #2
jcx is online now jcx  United States
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speed in input stages isn't a limitation in audio amplification

1/f noise can be - I doubt these devices have an advantage at audio frequencies over Si
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Old 11th August 2009, 02:13 PM   #3
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I know GaAs MESFETs have enormous 1/f noise; the corner frequency can be round 100MHz - they are good at 1GHz up, but terrible below VHF.

I can't bring to mind data on the other types, but as rule most compound semiconductors have large amounts of 1/f noise, as there many more types of bulk and surface defect possible.
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Old 12th August 2009, 06:22 AM   #4
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tiefbassuebert,
you steadily keep wasting your time.
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Old 12th August 2009, 09:26 AM   #5
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Quote:
Originally posted by Lumba Ogir
tiefbassuebert,
you steadily keep wasting your time.
Yes, as long as I only see answers to theoretical considerations, but not about evaluation and tests, that really did performed
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Old 12th August 2009, 01:43 PM   #6
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tiefbassuebert,
these devices do a great job in wireless communications, but predictably won`t perform well in linear audio.
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Old 12th August 2009, 04:42 PM   #7
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MESFETs are too noisy for good RF clock reference oscillators.
Phase noise opinion I overheard at NTMS (North Texas Microwave
Society) meeting... I have no idea what that means for audio???
I'm not entirely sure I am even repeating the rumour correctly...

He didn't specify the noise was Schottky or GaAs or GaN or SIC?
But seemed to prefer a silicon bipolar transistor up to <=4GHz...
I would have listened in more depth had I known the question
would come up again.

I think he might have said that SiGE looked promising for lower
noise??? I'm not sure now... Can't even remember exactly who
the speaker on that topic was, but I can inquire next month.

Can't even remember if I ate breakfast? Nor how to tie my shoes...
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Old 22nd August 2009, 05:54 PM   #8
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low noise with transitor is a fault
loudspeaker get 1 our 2% distostion
i don't think most important is low distotion,all design with transistor with feedback have the same characteristics .
fet bipolar and other igbt are the same with feedback.
our ear cannot listen the difference.IMHO
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Old 23rd August 2009, 02:53 AM   #9
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by tiefbassuebertr View Post
Yes, as long as I only see answers to theoretical considerations, but not about evaluation and tests, that really did performed
The low-duty-cycle pulsed IV curves and the DC IV curves are grossly different. GaAs is notoriously noisy at DC. GaN HEMTs are likely out of your price range, and I'm not sure they're any better at low frequency. Leave them to RF duty. I design RF amps with these for a living.

OTOH, High-voltage HBT would probably do quite well. Linearity is better than GaAs HEMT or LDMOS. They're quiet enough to make low phase noise oscillators with. But I don't think they're available in small periphery discretes, because the ones good for 28V are aimed at the base station transmitter market. A 10, 30 or 250W flange-mount device is a wee bit big and expensive for a diff pair or VAS.
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