FET inputs or bipolar inputs on a power amp? - Page 2 - diyAudio
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Old 17th June 2006, 11:43 AM   #11
ash_dac is offline ash_dac  United Kingdom
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Quote:
Originally posted by AndrewT


there's something else I didn't know.

Low pass filter with rising distortion in the pass band due to high source impedance is instructive!

Hi,

http://www.elecdesign.com/Articles/I...ArticleID=7206

I found this to be a good explanation.

I can see why few people / companies use them on the input stage. Matching the input fet's must be a real pain considering that they are relatively expensive , and used in cascaded inputs.

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Old 17th June 2006, 02:52 PM   #12
pooge is offline pooge  United States
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Here's another opinion

"FETs seem a poor idea for the input stage. The basic gm is so low compared with BJTs that there is little scope for linearisation by adding source resistors for local degeneration, so an FET input stage will be very non-linear compared with a BJTs degenerated down to the same transconductance: see 5.1.3."


From:



http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm#7
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Old 17th June 2006, 07:54 PM   #13
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Actually, there is some rationale for this opinion, but it fails in practice when you consider the ORDER of the distortion components, input noise contribution, and bias current compromises. A fet stage can be quieter, have virtually no bias current, and have essentially the same magnitude of linearity as a bipolar input stage.
If you are willing to risk FM distortion, a bipolar pair can be acceptable, so long that it doesn't have any significant resistor degeneration . Test equipment input stages are one place where this might be acceptable, if the test equipment is designed for measuring AM distortion only. For hi Z inputs, however, fets are always better by far, for more reasons than I can describe here.
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Old 17th June 2006, 09:58 PM   #14
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Quote:
If you are willing to risk FM distortion, a bipolar pair can be acceptable, so long that it doesn't have any significant resistor degeneration .
Would you please explain this statement?
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Old 17th June 2006, 10:20 PM   #15
kevinkr is offline kevinkr  United States
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Linear Systems makes matched fet pairs that would be very good in the front end of an audio amplifier as part of a long tail cascode pair.
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Old 17th June 2006, 11:59 PM   #16
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Quote:
Originally posted by john curl

If you are willing to risk FM distortion, a bipolar pair can be acceptable, so long that it doesn't have any significant resistor degeneration .
Quote:
Originally posted by traderbam
Would you please explain this statement?
Yeah. Gramatically at least. English is not my mother tongue, same as many others'.
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Old 18th June 2006, 01:32 AM   #17
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FM distortion is generated by the non-linear S curve that exists WHEN you want BOTH low noise and a bipolar transistor input. Degeneration resistor free bipolar input stages have a significant amount of FM distortion, unless the open loop bandwidth is very, very high, or the gain bandwidth is very, very high. You know: Very, very high implies qualities that higher than usually affordable by most people.
A good example of this is the AD797 op amp, or even the 5534 op amp and its relatives.
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Old 18th June 2006, 02:35 AM   #18
Leolabs is offline Leolabs  Malaysia
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Well,using JFETs or BJTs as input stages also depend on your circuit configuration,whether a conventional differential stage or a fully complementary push pull.
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Old 18th June 2006, 02:45 AM   #19
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Why?
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Old 18th June 2006, 03:09 AM   #20
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Mr Curl, how bad it is the differential input pair of bipolars without RE degeneration?
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