X amp considerations

[GRollins]

If you want to use the Fairchild equivalents to the IRFP240/IRFP9240, the N-ch is the FQA19N20C and the P-ch is the FQA12P20. But the IRF still work as advertised.

Grey

[Nelson Pass]

Originally posted by DiMenSioN
In a document on NP's site about mosfets he concludes that the irf9240 (although it should be) issen't a real good complement to the irfp240. Maybe there is a better complementary pair to be used in X-amp?

This is not correct. The IRF P channel parts have an odd
characteristic which shelves the transconductance in the mid
audio band. As such, it is somewhat flawed for use in
Common-Source applications, but it is acceptable in Common-
Drain use.

IR P channel parts made by other manufacturers, such as
Harris or Fairchild do not exhibit this.

In the case of X amps, it is not an issue.

[Zen Mod]

Originally posted by Zen Mod



you lost me there...........
rethink.......

just later I realize that you think and talk about real X , not Aleph X...
I was confused when you wrote about feedback,non-global........and that's exactly the case with real X

[DiMenSioN]

Thanks Mr Pass. Ill try to find the fairchild or harris parts then, just to be sure.

I don't know if its a passlabs secret but what devices (to3) do you use in the X amps? I do like TO3 because of their metal housing and good thermal contact with the heatsink.

Could you perhaps tell us why you switched from slow rectifiers with capacitors to fast, slow recovery rectifiers? (at least in the X-amps)

About the non linearity. I believe the formula is something like: Id = K ((Vgs-Vt)^2 - (Vgd - Vt)^2)(1+L*Vds)
Vgs = voltage Gate-source
Vgd voltage gate-drain, etc
Vt = treshold voltage
L = lapda = channel length modulation factor (dutch to englisch translation)
the (Vgs-Vt) or (Vgd - Vt) terms only contribute if they are non negative.

No matter how you look at it, you'll never have a linear piece of curve on this equation. That's why i think you should take feedback also over the source follower mosts. Because you don't want the load to influence your feedback my plan was to use one dummy sourcefollower with a resistor as a load and take feedback from that.

Jacques

[Nelson Pass]

Originally posted by DiMenSioN
I don't know if its a passlabs secret but what devices (to3) do you use in the X amps? I do like TO3 because of their metal housing and good thermal contact with the heatsink.

Could you perhaps tell us why you switched from slow rectifiers with capacitors to fast, slow recovery rectifiers? (at least in the X-amps)

About the non linearity. I believe the formula is something like: Id = K ((Vgs-Vt)^2 - (Vgd - Vt)^2)(1+L*Vds)
Vgs = voltage Gate-source
Vgd voltage gate-drain, etc
Vt = treshold voltage
L = lapda = channel length modulation factor (dutch to englisch translation)
the (Vgs-Vt) or (Vgd - Vt) terms only contribute if they are non negative.

No matter how you look at it, you'll never have a linear piece of curve on this equation. That's why i think you should take feedback also over the source follower mosts. Because you don't want the load to influence your feedback my plan was to use one dummy sourcefollower with a resistor as a load and take feedback from that.

1) We are mostly using plastics now, but we maintain a stock
of IRF244, IRF9240, and IRF250 in TO-3 cases.

2) Fast rectifiers give better EMI figures, which along with RF
filtering, allows us to pass import regulations in various countries.
Anecdotally, we think they sound better than what we had been
using.

3) It depends on what you consider linear. I have often used
banks of Source-followers without feedback, and the resulting
distortion is quite low, although not as low as you can get with
feedback. The question is - how low did you want it?

[peterpan188]

Originally posted by DiMenSioN
About the non linearity. I believe the formula is something like: Id = K ((Vgs-Vt)^2 - (Vgd - Vt)^2)(1+L*Vds)
Vgs = voltage Gate-source
Vgd voltage gate-drain, etc
Vt = treshold voltage
L = lapda = channel length modulation factor (dutch to englisch translation)
the (Vgs-Vt) or (Vgd - Vt) terms only contribute if they are non negative.

No matter how you look at it, you'll never have a linear piece of curve on this equation. That's why i think you should take feedback also over the source follower mosts. Because you don't want the load to influence your feedback my plan was to use one dummy sourcefollower with a resistor as a load and take feedback from that.

Jacques

Jacques,

The equation you wrote should be this one instead,

Id = K *((Vgs-Vt)Vds - (Vds^2/2))*(1+L*Vds)

This is the characteristic equation for MOSFET in triode region, while the following one,

Id = K *((Vgs-Vt)^2)*(1+L*Vds)

is the characteristic equation for MOSFET in saturation region. Combining both equation yields the graph attached. In saturation region, where all amplifying MOSFET works in, a small change in voltage leads to a large change in current, hence amplifying.

[Stefanoo]

Originally posted by Nelson Pass


1) We are mostly using plastics now, but we maintain a stock
of IRF244, IRF9240, and IRF250 in TO-3 cases.

2) Fast rectifiers give better EMI figures, which along with RF
filtering, allows us to pass import regulations in various countries.
Anecdotally, we think they sound better than what we had been
using.

3) It depends on what you consider linear. I have often used
banks of Source-followers without feedback, and the resulting
distortion is quite low, although not as low as you can get with
feedback. The question is - how low did you want it?

If i can, Mr. Pass, i would be curious to ask you the following questions:

speaking of sound characteristics, are these TO-3 devices that you mentioned above, better than the classic Plastic TO-247 IRFP240?
Within the 3 of them that you mentioned before, what is your favourite and, in case there is one, what is/are the reasons?



Thanks.

[GRollins]

Given that the TO-3 case is slowly being phased out, the question is moot. The favored piece is the one that's available.

Grey

[Nelson Pass]

The TO-3's are prettier, and they hold up better in corrosive
environments, but that's about it.

[GRollins]

Originally posted by Nelson Pass
The TO-3's are prettier...

I used to think so, too, but I've gotten to the point where I prefer the look of the plastic case devices.
One thing I never see mentioned is that the newer plastic cases feature three solder connections, whereas the TO-3 cases had either three mechanical connections (assuming a socket) or two solder connections and a mechanical connection (assuming that the base and emitter were soldered, with screws for the case, presumed to be the collector in this instance). I'm always wary of mechanical connections because they can come loose and I have seen them corroded before. No, I've never seen anyone else fret about this sort of thing--it's just me being silly.

Grey