breakdown voltage and biasing?? - RF Cafe Forums
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Post subject: breakdown voltage and biasing?? Posted: Thu Dec 29, 2005 12:36 am
like kris i am also designing a PA but it much simpler and is single ended all the way! my thing is a on-chip PA not on a PCB. i have to worry about high voltage swing because it is a PA and transistor breaking down. in bjt the design manual of the technology i am using says that breakdown voltage of collector-emitter (BVCEO) is 4 volts although the process is 5 V process (5V power supply). i thought BVCEO is always greater than the supply voltage but here it is not the case, any idea way?
HERE IS THE MAIN QUESTION: if the VCE breakdown is 3.5 voltage does it mean that i CANNOT bias the transistor across collector-emitter higher than 3.5 V? like ac swing can go higher than BVCEO 'n i understand that but as far as DC biasing is concerned i'm very confused...if i bias at VCE = 3.5 V than the ac swing will be only 2*3.5 = 7 V instead of what i though should be (2*VCC = 2*5 = 10 V)....respond pleeeease!!
Post subject: VCEPosted: Thu Dec 29, 2005 8:21 am
It's worse than you feared.
BVCEO is the maximum allowable Voltage from the Collector to the Emitter, with the base Open - which is not normally how you use the device.
A "5 Volt" process is one which can withstand power supply voltages of 5 Volts - not all of that can necessarily appear from the collector to the emitter, as you have discovered with your "3.5 Volt VCE" rating.
So your absolute maximum swing at the collector is from 0 to 3.5 V, That's only 3.5 V peak-to-peak, not 7 V,
You'll need to work at a lower impedance level than you thought - more current, less voltage. Impedance matching will be a necessary part of such a design. If lowering the impedance can't be done then you'll need to pick a different process.
Good Luck anyway!
Post subject: Posted: Thu Dec 29, 2005 4:30 pm
so you are saying i can't bias at 3.5 VCE?? i will have to bias it as like 1.5 V? thanks for your help!!
Post subject: VCE etcPosted: Fri Dec 30, 2005 1:49 am
Yes. You should not ever have a voltage that exceeds 3.5 V between the collector and the emitter, if that's what the VCE(max) rating of the transistor is. For class A operation, that implies a DC bias point near 1.5 Volts.
What happens if you exceed 3.5V from collector to emitter?
If you exceed it too much, you get instantaneous failure (we've all done it!) However, if you exceed it by a little, you're gambling on process statistics and long-term reliability.
Post subject: Posted: Fri Dec 30, 2005 1:56 pm
hi...thanks a lot. but let me clarify something. i have couple of books that cover PAs and in all of them they show that the output voltage waveform (vc or vce waveform) is centered around VCC (because of the RFC inductor at the collector, i think) which would mean that they are biasing the transistor at the supply voltage!!! doesn't make sense to me...maybe they are assuming that their VCE breakdown is higher than the supply voltage, could that be the case?
but if the above is true and i also use a RFC then my 5V supply will go across collector-emitter and since my BVCEO is only 3.5, this means breakdown. do i not use RFC?? please clarify if possible...thanks
Post subject: Posted: Tue Jan 03, 2006 8:44 pm
BVceo is a very conservative number because as guest says it is not how you normally use the device.
BVces is the maximum allowable Voltage from the Collector to the Emitter, with the base Shorted. BVces is usually much larger than BVceo and approaching that limit requires sinking all consequent base current to the emitter.
What you can get away with is between those two numbers. Also, pushing these limits can cost you some noise floor because break down is very zener like.
The news gets much worse for a PA when VSWR is taken in to account. If you are pushing the limits with a well matched Class A, there is a mistermination phase that will likely push the peak to peak voltage towards twice its terminated value.
Use the RFC but not the 5V process.