Author Topic: Cascode voltage drop  (Read 1205 times)

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Offline TheBaconWizardTopic starter

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Cascode voltage drop
« on: September 07, 2018, 09:28:07 pm »
Can I assume that a pair of mosfets as a cascode amplifier can each have a smaller voltage drop than the top rail is at? (ie half what one would use in a single fet)

Also, I guess the top fet has to be able to take the entire current flow?
 

Offline coppercone2

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Re: Cascode voltage drop
« Reply #1 on: September 07, 2018, 09:55:10 pm »
your name makes me happy
 

Offline Cerebus

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Re: Cascode voltage drop
« Reply #2 on: September 08, 2018, 01:55:28 am »
Can I assume that a pair of mosfets as a cascode amplifier can each have a smaller voltage drop than the top rail is at? (ie half what one would use in a single fet)

Yes and no. Yes, kind of obviously, the drop across the individual transistors has to add up to the drop across both of them (plus any passives). They don't however share the voltage drop equally. The whole point of a cascode is to fix the voltage at the drain (collector) of the input transistor to remove (as far as possible) the effects of capacitance and other variances due to changing VDS or VCE. So the cascoding, output transistor has to do all the work of varying voltage drop to leave a fixed drop across the input transistor.

Quote
Also, I guess the top fet has to be able to take the entire current flow?

Yes, it has to pass as much current as the bottom transistor wants to. Obviously in doing this the changing Is of the cascoding transistor will slightly change the Vgs of the cascoding transistor and so slightly change VD for the input transistor, thereby removing a small fraction of the cascoding effect.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 
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Offline TheBaconWizardTopic starter

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Re: Cascode voltage drop
« Reply #3 on: September 08, 2018, 03:54:59 am »
 :-+ Thanks, probably saved me some blue smoke!
 

Offline T3sl4co1l

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Re: Cascode voltage drop
« Reply #4 on: September 08, 2018, 04:36:35 am »
The voltage can be whatever you want, but understand that less voltage means less transconductance or more capacitance, give or take exact specifics of the transistors in question.  On the other hand, lower voltage means higher current for the same power limit, if operation is limited by power dissipation.  That can be helpful.

Transconductance is usually ~flat until saturation, and the bottom transistor sees little change in voltage thanks to being constrained by the top one; so it can have low headroom, and fairly high capacitance, without significant impact on operation.  This in turn leaves more voltage swing for the top transistor, and it will have lower [average] capacitance over that range.

A dramatic example of this was from some CRT monitors I once salvaged: bottom transistor PN2369, a fairly large (200mA), low voltage (15V), reasonably fast type; followed by a BFQ225 (100V 100mA TO-202 5W 1GHz) to deliver the required swing (>30Vpk at >50MHz).

Tim
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Electronic design, from concept to prototype.
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