Why is high load resistance good? - RF Cafe Forums
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Post subject: Why is high load resistance good?
Unread postPosted: Fri Jul 08, 2005 3:21 pm
This question is probably too basic so I feel a little foolish asking it here. I'm not an RF engineer but I'm reading a thesis where they're talking about GaN transistors and everyone is bragging about how it has a large breakdown voltage and can take large load resistances. They say the large load resistance helps in matching.
Why does the large load resistance aid in matching?
I see it mentioned all over the place but even introductory texts I've looked at mention it as if "everyone already knows this".
Unread postPosted: Sat Jul 09, 2005 12:58 pm
I believe this has to do with the fact some power amplifiers require isolators at he output to improve IP3 and to protect the the amplifer from reflections due to an antenna failure, for example,
GaN is less susceptible to these issue compared to GaAs and LDMOS.
But don't take my word for it, I have no idea what I'm talking about.
Unread postPosted: Mon Jul 11, 2005 3:36 am
Joined: Tue Mar 15, 2005 11:43 pm
I'm trying my best to elaborate this issue. Yes, GaN and SiC are well-known for their high impedance. Maybe you could look at the point of view, due to they have wide bandgap, so it's possible to operate under high-voltage operation with small current, that's why the impedance is higher.
Secondly, regarding the "They say the large load resistance helps in matching. " it refers to Bode-Fano theorem.
Because when the impedance is high, means the loss in input and output matching circuit will not be significantly affect the overall circuit performance, as it won't turn the impedance phase too much.
That is one significant property that enable " single-stage wideband power amplifier design"
I hope it helps.
Let me know if you need any help.
I'll try my best to elaborate.