Since high voltage BJTs may be hard to find, I have been thinking about using cascode ladders with low voltage BJTs to use them in high voltage railed amplifiers. Mine is only an idea which I did NOT use in practice. I simulated the idea under LTSpice but found the VAS formed out of ladder cascodes limits the permissible voltage swing which can be obtained by using high voltage BJTs.
I am attaching the simulated schematic using 80V DC rails and entirely low voltage BJTs. Since, this is only a simulation I did not design a power stage, but stopped with the VAS.
The major issue of swing limitation logically seems to have a lot to do with the VAS.
I am attaching the simulated schematic using 80V DC rails and entirely low voltage BJTs. Since, this is only a simulation I did not design a power stage, but stopped with the VAS.
The major issue of swing limitation logically seems to have a lot to do with the VAS.
Attachments
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If lowV bjt's are favoured or available and has to be used in high voltage amplifiers (+/-80V -> 160V max-swing), many of us will use a single mosfet as 'cascode-topping', going up to 800V without any problem.
And, also a musing, a cascode consists of a common emitter and a common base configured bjt, which work and behave differently. So the lv-bjt is superior as 'ce-bottom' and a hv-bjt/mos is superior as the 'cb-topping'!
So why use only lv-bjt's - as an outcome of your thinking?
Granted: never seen before, so that's a plus.
And, also a musing, a cascode consists of a common emitter and a common base configured bjt, which work and behave differently. So the lv-bjt is superior as 'ce-bottom' and a hv-bjt/mos is superior as the 'cb-topping'!
So why use only lv-bjt's - as an outcome of your thinking?
Granted: never seen before, so that's a plus.
BJT are not good for high voltage because of second breakdown.
It makes sense to cascode several 800V stages with MosFets to reach really high voltages and it is done frequently, but why low voltage parts? Did you buy an "assortment" and want to use it up?
It makes sense to cascode several 800V stages with MosFets to reach really high voltages and it is done frequently, but why low voltage parts? Did you buy an "assortment" and want to use it up?
Thanks for your replies.
This means, a high voltage MOSFET can readily implement a cascode in high voltage amplifiers. A single high voltage MOSFET is a much more elegant and efficient solution compared to what I proposed.
If I may ask, can you suggest MOSFET models which can be used as a high voltage cascode in an input stage and in a VAS?
This means, a high voltage MOSFET can readily implement a cascode in high voltage amplifiers. A single high voltage MOSFET is a much more elegant and efficient solution compared to what I proposed.
If I may ask, can you suggest MOSFET models which can be used as a high voltage cascode in an input stage and in a VAS?
https://www.diodes.com/assets/Datasheets/ZVP4424A.pdf
https://www.diodes.com/assets/Datasheets/ZVN4424A.pdf
https://www.diodes.com/assets/Datasheets/ZTX458.pdf
https://www.diodes.com/assets/Datasheets/ZTX558.pdf
Mos are quite expensive, they are good for output stages like your class A
https://www.diodes.com/assets/Datasheets/ZVN4424A.pdf
https://www.diodes.com/assets/Datasheets/ZTX458.pdf
https://www.diodes.com/assets/Datasheets/ZTX558.pdf
Mos are quite expensive, they are good for output stages like your class A
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A couple of things limit how far you can push this idea. First, the voltage drop in the composite transistor at saturation is far higher than the sum of the Vce(sat)’s. They all stay slightly in the active region. Some current must be maintained in the string of resistors that set the base bias - and that means added voltage drop. The other is maintaining the voltage division under dynamic conditions. At low frequency it’s no issue. When slewing, it is. In the power stage, the practical limit is about three in series, because of both of these issues. You can always run your VAS off a higher rail than the outputs to deal with the extra drops, but that obviously is of no help in the output stage. And overdriving the output stage contributes to the dynamic balance issues which lead to inexact voltage division. You will have to keep the supply well under the breakdown voltage divided by N.
This approach IS used in low voltage CMOS (digital and RF IC technologies) to get higher voltage swings out of stacks (even very large stacks of a dozen or more) of 2.7 volt transistors. But it does have its limits, due to the reasons I cited above.
This approach IS used in low voltage CMOS (digital and RF IC technologies) to get higher voltage swings out of stacks (even very large stacks of a dozen or more) of 2.7 volt transistors. But it does have its limits, due to the reasons I cited above.