MRF101 VHF Amplifier (100W) - Page 1
Forum section: electronics / rf-microwave

author: rheb1026
author_rank: Regular Contributor
timestamp: February 05, 2021, 05:18:21 pm
content: I'm looking for another set of eyes on my design of a 100W amplifier for the 2m ham band, I'm specifically targeting the 144-146 MHz range. Attached is a PDF of the schematic. I'm mostly looking for comments in the areas of the input and output matching since this is where I'm least confident, but all comments are appreciated.My main concern is that I have the right input and output impedance for the MOSFET. I found the values listed on my schematic in the NXP 136-174 MHz reference design. But doing the calculations from the .s2p file to convert to impedance gives a very different number. The bias conditions listed in the .s2p file are also very different from the conditions in the reference design (quiescent drain current of 1A vs 100mA).I should note that this will be powered by a 48V power supply (VDD).

author: langwadt
author_rank: Super Contributor
timestamp: February 05, 2021, 05:20:57 pm
content: schematic is a blank page?

author: rheb1026
author_rank: Regular Contributor
timestamp: February 05, 2021, 05:38:25 pm
content: Hmmm looks okay when I open it, but I've attached a JPEG of it. Let me know if you can't see this either

author: richard.cs
author_rank: Super Contributor
timestamp: February 05, 2021, 06:22:01 pm
content: The PDF works for me.Some comments, not about the matching as it's not really my area either:What packages are decoupling capacitors C4 and C5 in and what kind of dielectric? C5 might benefit from being duplicated.Same question for the input and output capacitors C1 and C6Are you sure you want the input DC coupled? The bias will come back out of the input which may or may not matter depending what your signal source is. If your signal source is lo-Z to DC then your amp won't work.R5 seems rather low to meHave you assessed how the bias network will behave over temperature? Presumably D2 is intended to give it a negative temperature coefficient? How does that compare with the threshold temperature coefficient of Q1?Are you intending to thermally couple the two together?Oh yes I see the note.When RV1 is near the upper end of its range D2 will have no effect and you loose temperature compensation.Q2's gate is exposed to the outside world. Some ESD protection would be good. How about a series resistor (connected after R6), a shunt capacitor and zener.Your bias setting (and it's temperature coefficient) will be influenced by the resistors you fit to R1 and R3I would put capacitors to ground right next to J4 and J4 to try and strip off any RF leaking out of the power and enable inputs.A nice additional feature would be some protection against an open circuit or grossly mismatched RF output, but it adds some complexity.

author: rheb1026
author_rank: Regular Contributor
timestamp: February 05, 2021, 07:04:43 pm
content: Quote from: richard.cs on February 05, 2021, 06:22:01 pmThank you for the critique, it is appreciated.The decoupling caps C4 and C5 would probably be some sort of ceramic cap, depending on what I could find in the voltage rating.C1 and C6 I was planning to use a C0G. I have picked out C6 as an ATC 600F series capacitor ( 600F430FT250XT )Thank you for catching this one... I do not want the input to be DC coupled. This will be driven from low level RF from a transverter, about a watt give or take. Looking at the NXP reference design a watt in gives ~100W out, which is what I'm aiming for.I admit that I have not done much detailed analysis for the bias network yet, but I will.Good call on the protection components for Q2. I will definitely add these.I am building this up with a few prototype boards at first, then eventually combining the boards depending on how I will package it all. A low pass filter board and a directional coupler for SWR and power out will be other boards

author: Weston
author_rank: Regular Contributor
timestamp: February 05, 2021, 07:25:22 pm
content: To get the efficiency they mention in the datasheet you will need to run quite heavily in class C, which introduces a lot of distortion. You will probably need some more filtering to reduce the harmonics. Your current output match is high-pass.What is wrong with the reference design for the 136–174 MHz amplifier on page 30 of the MRF101AN datasheet? It fits all your requirements and is fully verified. It has a few filter stages to get rid of the harmonics.Its been a year or so since I designed an amplifier with the MRF101AN, but I recall from when I characterized the reference designs that they were almost like switch-mode amplifiers, where they use the matching networks to provide the correct impedance for the harmonics to control the shape of the drain waveform and reduce the voltage peak.

author: rheb1026
author_rank: Regular Contributor
timestamp: February 05, 2021, 08:05:32 pm
content: Quote from: Weston on February 05, 2021, 07:25:22 pmThis project is more of a learning opportunity, and I'm totally happy with it being an inefficient class A amp (for now). I could just purchase the parts and board from the reference design, but I wanted to go through the process of designing my own, learning about impedance matching, filter design, etc.I specifically left off the output filter from this and will spin a separate board for that to allow easier experimentation and characterization

author: Co6aka
author_rank: Unknown
timestamp: February 06, 2021, 05:46:31 am
content: THIS...Quote from: rheb1026 on February 05, 2021, 08:05:32 pm...and maybe do the same with the bias/temp control circuitry. (I particularly like the modular approach when hacking out new designs.)