allFET circlotron

HI, cascode in vas increase bandwidth but require more compensation, do you try AC sim ?

Do you have books to learn how to spot the points to use?

thanks

What spots? I have learn LTspice in time and asking on LTspice tread, and still I do now now 30 procent of it.

Cascode needs almost always some compensation maybe through feedback.

have you schematic what you done.

regards
 
I did set some caps for compensation.

The old way do not work save, special in the Mhz region, oscillating here will blow the amp. Do also now, laterals can also oscillate on there own, I have a small cap between drains and gates in the hybrid, and do work now for already 13 years

But reading docu,s about compensations is maybe a good idea.

regards
 

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jfet biased like UGS/UP
15mA for vas

Thermally stable, still improvable with real CSS

You need enough current to drive the mosfets, with lateral things go easy but with verticals the driver need to be class A al the time to charge discharge gate capacitances and have low distortions.

You can use BJT if you want you let see with last schematic that works fine.

I have the vas working on 60mA or such, that give good driver with some balls. But maybe I am to negative thinking here. using a current siurce loaded vas then it follows always the 15 mA route.

regards
 
I can measure mine working amp, tomorrow because now it is dark and there are high voltages for the tubes present.

get the autobias current feedback not to work, the simulation stops timestep to small, moestly because of oscillation I think.

voltage feedback version does work, low HD even wit 220 mA idle and with low or high output voltage..

regards

regards
 

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Yes

But that is not a mosfet version vas, who has much more capacitances on input that is why I use a cascode to prevent this miller effect who can multiply capacitances, a BJT is lower in capacitance and it is a current driven device, mosfets are voltage.

But schemtic looks nice only the 510 ohms resistors makes a very low current drive possible so bandwidth is limited and distortion higher, you need to get enough voltage and current to charge discharge input capacitances of the lateral mosfets 20N20 that is why I have a extra driver for that in vertical version who idle on 60mA.

regards
 

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Last edited:

All you have send are just normal complementary amps, A circlotron is in fact two half ended amplifiers in bridge, when you seek on schematics who have only Quasi-complementary amplifiers, these vas are fase inverted, that is why you need a differential vas or a other way to make inverted signals who can drive the mosfets, enough current to overcome the gate capacitances and get enough bandwidth.

https://www.electronics-notes.com/a...n-sziklai-quasi-pseudo-complementary-pair.php

regards