Best jfet for hybrid cascode

[zintolo]

Hi,

I'm planning to build the amplifier here attached, and I already tested LSK170 and U440 on the bottom of the hybrid cascoded phase inverter, but I would like to know if there are other options with more gain and linearity that I can check to improve the design. More gain and linearity could permit to add some gnfb to the design, improve THD and dampening too.

Thank you in advance,
Roberto

 

[zintolo]

I've read some threads about it (EG this one Complementary diff input with JFET and BJT cascode and this one Cascodes- the truth is out there... ), and the J113 seems one of the best candidates.

Any other suggestion?

 

[BesPav]

but I would like to know if there are other options with more gain

Yes, a pair or more JFETs.

 

[U130421]

I advise you this mosfet. It has very high gm and perfect square law up to 10ma.
Unfortunately no spice model.

 

[tikiroo]

Spice model attached

 

[zintolo]

Kokoriantz, tikiroo,

thank you very much! I will try it and report the results.

 

[KlausB]

The new IF9030 from Interfet also looks promising, although it is very expensive at the moment and I could not find a spice model.

Another option to increase gain might be a Sziklai configuration with a jFET and a BJT at the input as shown in the schematic below.

 

[KlausB]

Sorry, I forgot to upload the file

 

[zintolo]

Spice model attached

I've a newbye issue, but LTSPICE tells me that he cannot find the definition of SSM3K339R. I've saved the lib file and called the the lib file...

 

[zintolo]

The new IF9030 from Interfet also looks promising, although it is very expensive at the moment and I could not find a spice model.

Thank you KlausB, it is indeed, also because I'd need two almost matched.

Another option to increase gain might be a Sziklai configuration with a jFET and a BJT at the input as shown in the schematic below.

A BJT instead of the 12AX7? I must supply the cascode at around 450V due to the feedback configuration for the output tubes, is there anything that can be safely used and well sounding?

 

[tikiroo]

Hi Zintolo, example file attached. You have to edit the jfet (CTRL right click) to change "Prefix" from "JN" to "X", then change "Value" to the name of the model (NMOS_SSM3K339R) as it is a .subckt model. Note that this fet needs a positive bias voltage to conduct.

 

[zintolo]

Thanks Tikiroo!

The new file is in attachment: 91 Wrms at 0,38% THD (3rd and 7th harmonics mainly).

Harmonic	Frequency	 Fourier 	Normalized	 Phase  	Normalized
 Number 	  [Hz]   	Component	 Component	[degree]	Phase [deg]
    1   	1.000e+03	3.823e+01	1.000e+00	    0.03°	    0.00°
    2   	2.000e+03	4.356e-03	1.139e-04	   86.84°	   86.81°
    3   	3.000e+03	1.373e-01	3.592e-03	   -2.68°	   -2.71°
    4   	4.000e+03	1.093e-03	2.859e-05	   12.75°	   12.72°
    5   	5.000e+03	4.655e-03	1.218e-04	  148.06°	  148.02°
    6   	6.000e+03	4.368e-04	1.142e-05	  -89.08°	  -89.11°
    7   	7.000e+03	4.304e-02	1.126e-03	   -0.90°	   -0.93°
    8   	8.000e+03	1.353e-04	3.539e-06	  129.20°	  129.17°
    9   	9.000e+03	1.190e-03	3.114e-05	   89.71°	   89.68°
Total Harmonic Distortion: 0.376791%(0.378500%)

 

[U130421]

Although I bought 100pcs/4$ the complimentary, the cheapest I found this mosfet is on ebay from UK 5pcs/4$.

 

[zintolo]

Thanks kokoriantz,
how many of them do you suggest to buy, in order to match a quartet (two per channel)?

 

[U130421]

With transistors, I always get a matched pair from five same lot. If Mouser shipment is affordable 10pcs cost 2.6€ https://www.mouser.it/ProductDetail/Toshiba/SSM3K339RLF?qs=vPP9GyyTAo2h4IM%2B%2BLvR%2BA==.

 

[KlausB]

A BJT instead of the 12AX7? I must supply the cascode at around 450V due to the feedback configuration for the output tubes, is there anything that can be safely used and well sounding?[/QUOTE]

The BJT mentioned is not to replace the 12AX7. What I meant is a jFet (N) and a BJT(pnp) as a compound of two transistors BELOW the cascoding tube to enhance gain. The spice file I posted shows a nicely working example.

All the best for 2021

KlausB

 

[zintolo]

With transistors, I always get a matched pair from five same lot. If Mouser shipment is affordable 10pcs cost 2.6€ SSM3K339R,LF Toshiba | Mouser Italia.

Thank you very much kokoriantz, and sorry for the late reply! Great hint!

 

[zintolo]

The BJT mentioned is not to replace the 12AX7. What I meant is a jFet (N) and a BJT(pnp) as a compound of two transistors BELOW the cascoding tube to enhance gain. The spice file I posted shows a nicely working example.

Thanks KlausB, I will try it. What if I would flip tube and SS, having the tube below and the SS on top? Are there some suggested BJTs?

All the best for 2021
KlausB

Thank you, I wish the same to you and the whole forum!