How to set Vgs to make the  RF MOSFET reach full power - Page 1
Forum section: electronics / rf-microwave

author: teatime
author_rank: Contributor
timestamp: March 12, 2024, 08:47:23 am
content: Please see the datasheet of MRF171A.The datesheet shows:45W, 150MHz, 28V,Efficiency = 50% (typ.)according to the datesheet, the Idq set to 25mA or 100mA,MRF171A almost can output 30Watt, but28v x 0.1A X 50%=1.4(watt)If you want increase the ID, you must increase the VGSIf the MRF171A output full power, my comprehend as below:28V X 3.214A X 50%=45(watt)So I really cannot understand the curve in the datesheet.Would you please clear up my doubts,Thank you very much!

author: RFDx
author_rank: Regular Contributor
timestamp: March 13, 2024, 06:36:09 am
content: Quote from: teatime on March 12, 2024, 08:47:23 am100mA is the quiescent current with no input signal at all. The current depends on the chosen mode of operation. Like the datasheet says, with 100mA you get linear operation in class-AB. With (very) small input signals the amplifier works in pure class-A but as the input signal increases the mode changes to class-B.Quote from: teatime on March 12, 2024, 08:47:23 amThe current goes up once you apply an input signal. Vgs is modulated, depending on the peak amplitude of the input signal, from an initial bias voltage value of ~2,6V (@ Id=100mA) to something like 4.5V where the transistor draws ~3A, which would be enough for an output power of at least 40W.

author: teatime
author_rank: Contributor
timestamp: March 13, 2024, 07:02:49 am
content: Thanks alot for your reply.I want make the MOSFET work at CLASS A mode.If we want get 40W output power, then the ID should be around 3A ?28V X 2.85A X 50%=40WattAm I right?

author: teatime
author_rank: Contributor
timestamp: March 13, 2024, 07:13:40 am
content: Quote from: RFDx on March 13, 2024, 06:36:09 amActually, the circuits design to Class A mode.if the Vgs too small, once the input signal increase,the output waveform will distortion.(It's means the Id not increase according to input signal, I checked some Class A design, Id is very big, the MOSFET very hot）I'm adjust the Vgs to make Id to about 1.8A, then the distortion will be better.

author: RFDx
author_rank: Regular Contributor
timestamp: March 13, 2024, 07:19:16 pm
content: Quote from: teatime on March 13, 2024, 07:02:49 amAn output power of 40...45W PEP in class-C/B/AB would be feasible. Because of the limited device dissipation (max. 115W @25°Ccase temp.) I wouldn't go above 30W in class-A. A 30W single ended power amplifier with a supply voltage of 28V would need a quiescent current of ~2A and expects a (resistive) drain load of ~14Ohm, which is coincidentally perfect for a 1:4 output transformer with 50 Ohm load.

author: iMo
author_rank: Super Contributor
timestamp: March 14, 2024, 10:30:56 pm
content: Quote from: teatime on March 13, 2024, 07:13:40 amThe mosfet final transistors in PAs (power amplifiers) are usually set by setting the Vgs such you get the Idq (quiescent current) with none input signal, in your PA for example those 100mA.Then with the "input signal power" as depicted in the graphs you will get those 30W at your 28V (for example). No need to get perfect signal "without any distortion", as the higher harmonics shall be filtered out by the low pass filter at the PA's output.Also the quiescent current is usually switched off when the PA is not activated.