This is a High power LDMOS device meant for LTE Band13. In most of the devices which i have come across yet, grounded stubs are usually not used. Even if they are, I am curious What can be the probable reason for the use of that shunt capacitor?
Without having seen I'd guess a resistance matching circuit.
The capacitor will be some way "down the line" - acting as a short circuit. The "real short" will act as a limiting element giving a minimum (or maximum) impedance.
The "trick" is that shorted stubs are more stable than open stubs - less influenced by cleanliness, humidity etc.
Thank You for the answer. The stub is in the input matching circuit of a high power RF transistor (LDMOS).
Can you tell me why they put a stub anyways? Cause in most of the designs I have seen till now, they don`t prefer stub, rather go with lumped elements.
"Input matching" is just a synonym for "impedance (aka resistance) matching".
As discrete components (aka "lumped elements") exhibit quite a number of "parasitic
effects" (parasitic inductance, capacitances etc.) - especially at higher frequencies:
Thus wave guides, stubs etc. are preferred by the more experienced designers.
If you can get access to designs from Agilent/Keysight/Hewlett Packard/ Rhode&Schwarz etc. (the traditional high.frequency equipment companies) you will find little discretes (aside from transistors/amplifiers) and a lot of stubs, wave guides etc. Current "cheap electronics" may show lumped elements but will exhibit inferior performance. Another welcome effect of stubs etc. is that losses are reduced to the unavoidable (losses in the PCB) whilst discrete components will add significantly higher losses.
Thank You....
Yah, i have noticed agilent and the likes, that they prefer stub matching. But i have used some high power transistors in my design in GSM band from freescale and infineon and they even used the recommended design from the datasheet, but saw this stub the first time!
Morover Sir, can you explain me how the shunt stubs reduces the losses in a PCB?
Thank You in Advance :)
Hello,
the stubs do not reduce the losses in the PCB, but the only create the resistive (copper) losses and the dielectric losses in the PCB (which can be reduced by using HF materials like those from Rogers Corp. instead of "plain old FR4").
Discrete components add inductive, capacitive and resistive losses. Regarding the capacitive losses HF PCB materials are be superior to "medium priced" (ceramics prevailing) capacitors.
In theory - but seldom in SMD - lower-loss capacitor dielectric materials like MICA, metalized paper or metalized foils are available.
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