cascode vs asymmetrical cathode coupled

[oldeurope]

#40

... However if you use the same formula for a low µ power triode as a 6C33C the result is very inaccurate. A 6C33C has a µ of typically 2.7 and a Rp of 80 ohm which gives a gm of 33.75mA/V, with the faulty formula this would give an output impedance of 1/gm which in this case is 29.6 ©. If we instead use the correct formula Zo = 1/[(1/Rp)+gm] we get 21.6 © a 37% difference which can't be seen as insignificant. ... Regards Hans

gm = Rp / (µ+1) = 80Ω / (2,7+1) = 21,6Ω
There is 0% difference.

I got gm from the data book.
What do you want to suggest here?

No comments to the other text, this is a moderators job.

D.

 

[revintage]

Sorry Hans,
Can not cope with your all but friendly viktigpetter-attitude.
Will keep from going into threads that you participate in hereafter.

 

[tubetvr]

gm = Rp / (µ+1) = 80Ω/ (2,7+1) = 21,6Ω;

Gm for a triode is µ/Rp, not Rp/µ+1

Regards Hans

 

[oldeurope]

Re: #40

Originally #41 posted by oldeurope


gm = Rp / (µ+1) = 80Ω / (2,7+1) = 21,6Ω
There is 0% difference.

I got gm from the data book.
What do you want to suggest here?

No comments to the other text, this is a moderators job.

D.

1/gm = Rp / (µ+1) = 80Ω / (2,7+1) = 21,6Ω
There is 0% difference.

Darius

 

[oldeurope]

#42

Thank you very much Lars.

BTW:For an ECC83 [µ = 90...110] I don't care about +1 .

Kind regards,
Darius

 

[SY]

Re: Re: #40

1/gm = Rp / (µ+1) = 80Ω / (2,7+1) = 21,6Ω
There is 0% difference.

Darius

I think you missed the point. The problem is that 1/gm is NOT the correct formula for output impedance, it is only an approximation in the limit of mu>>1. If you look at the output impedance, this formula gives a 30% error for Hans's example.

And, once again, by definition, gm = mu/rp, NOT (mu + 1)/rp.

 

[oldeurope]

no problem ...

Originally #46 posted by SY


I think you missed the point. ... If you look at the output impedance, this formula gives a 30% error for Hans's example.

And, once again, by definition, gm = mu/rp, NOT (mu + 1)/rp.

Hi SY,
now I am totally confused. I'll think about it.

Most important is this :

Originally #46 posted by SY


... The problem is that 1/gm is NOT the correct formula for output impedance, it is only an approximation in the limit of mu>>1. ...

The ECC83 12AX7 has definitely µ>>1. Thus my calculation is correct.

Back to topic please, thanks!

Kind regards,
Darius

 

[oldeurope]

Looks not good for the cascode.
Pentode distortion characteristic, negative current feedback
or a low load resistance must make the gain << µµ.

Kind regards,
Darius