BJT Aleph

[Koy]

Does anybody know why Mr. does not use BJTs on input (differential pair) instead of MOSFEts? BJTs could generally offer more advantages to MOSFEts ( except high input impedance).

 

[Holger Barske]

As said by by NP himself somewhere on the solid state forum:

I don't like to get into arguments about Mosfets vs Bipolar
because they both have their strengths and weaknesses.

Having said that, it is my experience that people arguing
the superior linearity of BJT's over Mosfets are using cases
where either the bias or the source impedance is low. For
example, the typical big Bipolar device has about 1/3 the
distortion of a Mosfet as a follower - if the current isn't
very high and if the source impedance is just a few ohms.

As the source impedance and the current rise, the Mosfet
will equal and ultimately surpass the Bipolar.

The argument is a red herring anyway, since we have already
decided that we don't base everything on THD numbers
anyway.

---
Holger

 

[PMA]

I think there is no special reason for this, it might be Nelson's policy. The other reason could be the ultimate simplicity which is easily obtainable with MOSFETs. You are completely right - you will obtain better THD results when using BJT's, but I think that it is not an issue for Nelson.

 

[Nelson Pass]

Certainly you can build these with Bipolar inputs.

I use Mosfet inputs as a matter of personal preference,
and it seems to me that use of either BJTs or Mosfets all the
way through the gain circuit gives the best results.

 

[traderbam]

Nelson,
Why do you say that using the same device type in the gain circuit is better than a mixture?
BAM

 

[Nelson Pass]

Just experience. I've never ended up caring for the sound
of any amp I built that mixed them in the gain path.

Not true of JFET's and Bipolar, by the way.

😕