Matching RF power transistors - Page 1
Forum section: electronics / rf-microwave

author: kg4arn
author_rank: Unknown
timestamp: August 05, 2017, 08:26:00 pm
content: apologies if this has been previously discussedHow are RF power transistors matched, say for push pull service?

author: dave_k
author_rank: Frequent Contributor
timestamp: August 06, 2017, 12:33:22 am
content: Generally for bi-polar transistors hFE (DC current gain) is measured, and for FETs forward trans-conductance andgate threshold voltage is measured.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2017, 02:11:35 am
content: 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

author: kg4arn
author_rank: Unknown
timestamp: August 06, 2017, 02:38:02 am
content: Thanks. I will give it a try.

author: BigBoss
author_rank: Regular Contributor
timestamp: August 06, 2017, 08:58:00 am
content: Quote from: kg4arn on August 05, 2017, 08:26:00 pmRF Power Transistor Matching is quite different than well knownComplex 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..orYou get MeasuredLoad-Pull Data( much more accurate if applicable) from the manufacturer and you realize the matching circuit

author: kg4arn
author_rank: Unknown
timestamp: August 06, 2017, 11:18:58 am
content: Quote from: BigBoss on August 06, 2017, 08:58:00 amThanks.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=MRF208In this example they only charge a couple of extra dollars for the matched pair so the process couldn't have been too involved.Ed

author: kg4arn
author_rank: Unknown
timestamp: August 06, 2017, 11:22:13 am
content: Tim,How close do you have to be? 10%, 20%, ??Ed

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2017, 12:14:15 pm
content: 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

author: BigBoss
author_rank: Regular Contributor
timestamp: August 06, 2017, 06:56:03 pm
content: Quote from: kg4arn on August 06, 2017, 11:18:58 amHimm.. 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..