FET inputs or bipolar inputs on a power amp?

[mlloyd1]

however, on that same web site is Leach's RIAA preamp which uses 2N5457 JFETs at the front end.

mlloyd1

Originally posted by ash_dac
Hi,

From the Leach amp pages...

"Because FET parameters are so unpredictable and FETs are more susceptible to flicker noise, I prefer the BJTs with emitter resistors." http://users.ece.gatech.edu/~mleach/lowtim/bckgrnd.html

Is the unpredictability a problem inherent in the manufacturing of FET's ?

http://users.ece.gatech.edu/~mleach/ece4391/noise_b.pdf

[gerhard]

Originally posted by ash_dac

Is the unpredictability a problem inherent in the manufacturing of FET's ?

It is _much_ easier to control the dimensions of vertical structures
by choosing diffusion time rather than controlling horizontal structures
with masks, exposure and etching. This was THE breaktrough of the
planar process.

FET channel length and width usually depend on horizontal structures.
This unlucky fate is shared by lateral PNPs that exist in some low cost
bipolar processes, where the extra diffusion steps needed to produce
nice PNPs are to be avoided.

Gerhard

[pooge]

Originally posted by mlloyd1
however, on that same web site is Leach's RIAA preamp which uses 2N5457 JFETs at the front end.

mlloyd1

He used them in this application to mate better with a phono cartridge. Less base current, lower noise with cartridge source, etc.

[john curl]

I am very surprised that anyone in the last 30 years, would use a 2N5457 fet for a phono input. It IS too noisy, but many fets, both American or Japanese, are better than almost any bipolar transistor, 1/f noise included.

[Fanuc]

Ho Hum...

I have never personally seen any spectrum anaylisis of outputs to the VAS of jfets/mosfet (matched/unmatched - with all it's incipient problems) cascodes versus single bjts with and without current mirrors. Just another ten dimensions are added etc....

I think the multiple Vgs spreads compared with the single Vbe of BJTs means in a differential it is a sub optimal subtractor - much more incorrect from the audio signal. Monolithic op amps are a very different technology.a

There not the same.

What the hell has input noise to do with a power amplifer anyway?

[pooge]

Originally posted by john curl
I am very surprised that anyone in the last 30 years, would use a 2N5457 fet for a phono input. It IS too noisy, but many fets, both American or Japanese, are better than almost any bipolar transistor, 1/f noise included.

It WAS designed 30 years ago, when those noisy LPs were sold.

[PMA]

Yes - low noise RIAA preamp has always been greater challenge than a simple link level stage with linear gain 2x or so.

[john curl]

An even greater problem is with the reproduce noise of analog tape recorders. This is even more difficult than RIAA reproduction, because of the absolute output vs inductance and the lack of high frequency falloff after a 50us time constant or so. In those early days, we sometimes used selected fets, but it was marginal. By 1973, however, we found solutions to this problem and dropped the input noise with fets to less than 2nV/rt Hz. Today, we can do about 1nV/rt Hz differential, and 0.4nV/rt Hz for special function inputs like moving coil.

[ash_dac]

Hi,


Butler design:-

http://www.freepatentsonline.com/5101126.pdf

The text is in patent speak but suggests that the BJT works for low input signals, and the FET works for higher input signals.

Is there any value in this approach ?

see also:-

Butler input pair

[john curl]

This design is problematic, for reasons other than noise.