Matching RF power transistors

[kg4arn]

apologies if this has been previously discussed


How are RF power transistors matched, say for push pull service?

[dave_k]

Generally for bi-polar transistors hFE (DC current gain) is measured, and for FETs forward trans-conductance and
gate threshold voltage is measured.

[T3sl4co1l]

For BJTs, Vbe is more important than hFE.  Connect B+C together, and measure at some constant current (say 1mA, or whatever a DMM's diode test does).  Beware that, if the transistor is oscillating in this configuration (for some reason), you'll get wrong numbers.

FET threshold is measured the same way (G+D tied, apply a constant current).

Tim

[kg4arn]

Thanks. I will give it a try.

[BigBoss]

apologies if this has been previously discussed


How are RF power transistors matched, say for push pull service?

RF Power Transistor Matching is quite different than well known Complex Conjugate Matching. Because small signal conditions are not valid anymore.
You can do 2 things..
You obtain an accurate nonlinear model and you find Optimum Load Impedance ( this is not really accurate in reality) for that transistor in a professional RF simulator than you realize the matching circuit in according with..
or
You get Measured Load-Pull Data ( much more accurate if applicable) from the manufacturer and you realize  the matching circuit

[kg4arn]

Quote from: kg4arn on Yesterday at 10:26:00 AM

apologies if this has been previously discussed


How are RF power transistors matched, say for push pull service?

RF Power Transistor Matching is quite different than well known Complex Conjugate Matching. Because small signal conditions are not valid anymore.
You can do 2 things..
You obtain an accurate nonlinear model and you find Optimum Load Impedance ( this is not really accurate in reality) for that transistor in a professional RF simulator than you realize the matching circuit in according with..
or
You get Measured Load-Pull Data ( much more accurate if applicable) from the manufacturer and you realize  the matching circuit

Thanks. 
I should have been clearer.
I was wondering how to match a pair of transistors to each other, like when you buy a matched pair. 
For example, this is what I meant to ask about:  https://www.rfparts.com/catalogsearch/result/?q=MRF208
In this example they only charge a couple of extra dollars for the matched pair so the process couldn't have been too involved.
Ed

[kg4arn]

Tim,

How close do you have to be?  10%, 20%, ??

Ed

[T3sl4co1l]

Dunno, how close do you need to get?

If it's for DC biasing, Vbe needs to be within maybe <10mV, otherwise current will be off by, well, about 60mV/decade, it's pretty sensitive, that exponential.

Similarly for MOSFETs, but related to Vgs(th) and Gm.

And, as BigBoss mentioned, this is only DC parameters; whether you need tightly matched AC parameters, who knows.  If you need low THD and IMD (like a lot of radio base stations that use predistorted modulations), you might need to select through a lot of parts.  Other things (HAM radio?) probably aren't very sensitive.  YMMV.

Tim

[BigBoss]

Quote from: kg4arn on Yesterday at 10:26:00 AM

apologies if this has been previously discussed


How are RF power transistors matched, say for push pull service?

RF Power Transistor Matching is quite different than well known Complex Conjugate Matching. Because small signal conditions are not valid anymore.
You can do 2 things..
You obtain an accurate nonlinear model and you find Optimum Load Impedance ( this is not really accurate in reality) for that transistor in a professional RF simulator than you realize the matching circuit in according with..
or
You get Measured Load-Pull Data ( much more accurate if applicable) from the manufacturer and you realize  the matching circuit

Thanks. 
I should have been clearer.
I was wondering how to match a pair of transistors to each other, like when you buy a matched pair. 
For example, this is what I meant to ask about:  https://www.rfparts.com/catalogsearch/result/?q=MRF208
In this example they only charge a couple of extra dollars for the matched pair so the process couldn't have been too involved.
Ed

Himm.. You talk about "pair matching"..Sorry for misunderstanding..

The single way to match two RF Power Transistor is either to compare in terms of RF performance or request from the manufacturer-if applicable- selected devices.
It's not so easy but comparing DC Beta_Forward or measuring VBE threshold etc., wouldn't work.You cannot know how the intrinsic components will behave.
Or, you design a RF Power Amplifier with Alignment to compensate the differences ( for instance slightly different independent input-output network matching circuits,using shifted biasing etc)

Nevertheless, it's not possible to match %100 two RF Power Transistor, there will always be some imbalances..