Complementary JFET-JFET cascode input, BJT VAS-Drivers, Lateral Output

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mlloyd1

mlloyd1

Member
Joined 2001
Elsewhere on this site, EUVL has mentioned J111 (N channel) and J174 (P channel) as possible JFETs to use for cascoding of the 2SK170 and 2SJ74. In my limited experiments, J111 and J174 apply at least 2v across the 2SK/2SJ devices at operating currents discussed here. I've never been able to get my hands on (known good) 2SK246 and 2SJ103 that gave me even 1v across the cascoded devices.

mlloyd1

dodo said:
Isn't true the rule of thumb ( based on Borbeley jfet infos)
Vgs of the uper j-fet cascoding device (sk246 ) must show at least 2V at the bias level complementary differential input jfet pairs (sk170/sj74)
in your case sk246 /sj103 must show at least 2V vgs on Id=2,5mA ?
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A

AndrewT

R.I.P.
Joined 2004
to generate at least 2*Vpinchoff across the lower high gm device one must use a higher Idss device that is lower gm.

The bigger the difference in gm and the bigger the difference in Idss then the higher is the cascode voltage.

For a 8mA k170 I would use a 16mA to 30mA Idss with a gm 2mS to 5mS
Have a look at the bf244c. But there are very many others.
 
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