10W 70cm(435Mhz) amplifier. - Page 1
Forum section: electronics / rf-microwave

author: Fflint
author_rank: Frequent Contributor
timestamp: April 02, 2025, 07:36:30 am
content: I have an amplifier based on single 10W ldmos transistor (BLF6G21-10G ) that claims a very wide band from 50Mhz to 1100mhz. I need to use it on 430mhz (+-5mhz) Actually measuring with a vna it is more like 120Mhz to 970Mhz still a very wide band. The transistor is described as 10W (although various places in the datasheet making claims 2W or even 0.7W is "typical use"). But after fixing the bias to the datasheet value it maxes the rf power at 2W (33dBm) while it runs at 400mA @28V. That's 12W of heat so efficiency of only 18% which actually beats the datasheet efficiency of 15% they give.It seems this efficiency is horribly low. My 45 years old vacuum tube amp in class AB runs at 65% (but in a narrow band). So I'm thinking, is this horrible efficiency a result of trying to make a wideband amplifier and having a pretty bad but still acceptable match over entire 100Mhz-980Mhz spectrum therefore if I match it to 430MHz I can actually get efficiencies in range of 50% ? The datasheet says the device tolerates 10:1 SWR so it is not unreasonable to think a designer could match the entire range to 8:1 etc. Which I assume would result in 70% power loss that with 50% true efficiency would look like 15% overall efficiency.Does this make sense? My "big" amp on mrf186 on the same frequency of 435mhz has efficiency of about 50-60%.

author: ftg
author_rank: Frequent Contributor
timestamp: April 02, 2025, 02:54:00 pm
content: The datasheet biaspoints and efficiencies are for WCDMA, so very linear service.The 15% value is for -50 dBc ACPR 2-carrier WCDMA.They claim 31% efficiency with -39 dBc ACPR at higher power.As for the widebandness, what it usually means is the transistor does not have internal narrowband matching, like most LDMOS transistors meant for base station use have.So less about it inherently covering 50MHz to 1100MHz without any matching, but that it is possible to use it to build an amplifier for any frequency in that range with it.What kind of input and output matching are you using with it?Annoyingly Ampleon has no simulation models or any real information about the input and output impedances of those devices.So some of the design work might include a decent amount of shots in the dark.

author: Fflint
author_rank: Frequent Contributor
timestamp: April 02, 2025, 04:50:48 pm
content: Thanks for answering.Quote from: ftg on April 02, 2025, 02:54:00 pmYes, somehow I missed that detail. The 31% is at 2W load power.Do you think I should shift that bias point? Is it likely to be way too high? I do need linear service,but it doesn't have to be a record breaker.Also there is a chart showing input return loss vs load power and that chart goes to 12W (CW).I'm running it with CW at 450MHz at up to 20dBM. The gain is not too high (probably due to input matching).Quote from: ftg on April 02, 2025, 02:54:00 pmIt's a Chinese amplifier so whatever parts they had in a drawer that day I guess. (The avNA seems to support it showing 1.5 up to 2dB of return loss on the input and similar on the output (cold).It has the mmic amplifier as the first stage then 3 capacitors arranged into a T on the input. The output is a series inductor followed by shunt capacitance and final series cap. I haven't measured the values.I'm thinking about measuring the response looking from.the front and especially rear with a vna and trying to cone up with component values to match.Quote from: ftg on April 02, 2025, 02:54:00 pmYes, also I just have one transistor now. So I can't just pull.one from s drawer toeasure it.But that 18% efficiency was very bad IMO. If I had a spare I could measure just the transistor and that could be much easier. Any suggestions on the current efficiency?

author: Fflint
author_rank: Frequent Contributor
timestamp: April 02, 2025, 05:12:37 pm
content: One more thing I noticed, chatlrt in the datasheet showing CW efficiency in function of load power.2W does have a bit under 20%, but then we have 8W at 60% and I don't understand something about this. Lets say indeed we have it running at 18% so we have2W at load, and 11.5W input power (or 421ma)Then8W at load and 13.3W input power.(475mA)So it is supposed to increase the current by 50mA and increase the power 4X at the time? Isn't that weird?Edit: Actually I have to correct what I said. Outside looking in from the rear the impedance is very good 49ohm + tiny numbers. Input is also not that bad, 30ohm - j7, not horrible for the input....Perhaps it is my bias. I'll try lowering maybe.

author: Fflint
author_rank: Frequent Contributor
timestamp: April 03, 2025, 06:01:42 am
content: It doesn't appear to be caused by bias...I lowered the bias to 60mA and then increased it to 120mA. It is there wasn't a huge difference.Also I added a piece of aluminium that gets pressed against the transistor from the top and I've put everything on a big radiator so I can run it at 600mA. These are the results:1 - bias a t100mA , as described, further test 567mA at 3.53W2 - bias at 60mA, got to 561mA at 3.53A3 - bias at 120mAh, got to 576mA at 3.59w.Almost the same efficiency.

author: ftg
author_rank: Frequent Contributor
timestamp: April 03, 2025, 09:15:18 am
content: That shown construction with that feedback arrangement is wideband and will take some work to retune to narrow band use.I first encountered it in an article by W1GHZhttps://www.w1ghz.org/small_proj/Simple_Broadband_Power_Amplifiers.pdfThere is even a short thread about it here:https://www.eevblog.com/forum/rf-microwave/w1ghz-wideband-ldmos-amplifier/The construction works for making wideband amplifiers from random telecom LDMOS.So a random 1800MHz or 2100MHz LDMOS makes a perfectly working HF+VHF laboratory amplifier.But one will not get the maximum power the transistor is capable from that.So, for narroband use the input and output matching will need to be modified.Do you know what model the driver MMIC is?As it's output power will also affect the maximum output power available.

author: Fflint
author_rank: Frequent Contributor
timestamp: April 03, 2025, 04:23:56 pm
content: Quote from: ftg on April 03, 2025, 09:15:18 amVery interesting! I haven't expected that. I read the article and the short thread and I wonder what is the mechanism of the output power being limited, but the transistor still consuming the supply power as if it was delivering rated output.The match seems very good even when measured cold so it doesn't seem to be the case of power being lost in a bad match.On the first glance it seems this feedback method affects the gain available and this is indeed supported by my measurements and the other thread (I get 12 to 14 dB, instead of 18 "promised").If this is the case my assumption is it should simply produce lower power output, not have low power output and very low efficiency as well... I'm not sure what mechanism affects that efficiency. The lower gain I can understand through negative feedback.Quote from: ftg on April 03, 2025, 09:15:18 amYes I do know what the mmic is, because it burned the moment I plugged the amp (the smps chip was set to 8.5V instead of 5V) and I compared the shape under a microscope to my other collection of failed Chinese amps. It matched yg602020. It could possibly also be ecg055b maybe, but all i have have slightly less rounded corners. I'm not sure why the people that sell them go to the great lengths to laser the tops of the chips and grind them. As you can see on my photo the mmic is lasered away (this photo was taken before it was swapped). And the power transistor has been ground down but I have my ways to read the ceramics under a microscope with clever Illumination). So I have to guess at the mmic(it works with the one I fitted), but I'm sure of the drive transistor. I could understand wanting to "protect the Ip" if the things worked as sold, but no. Every single one I bought had deadly flaws that had to be correctedComing back to the amp, the way I understand it is we basically have almost no matching as that feedback takes care of it. If I wanted it to give me full power narrowband I'd have to essentially make a new amplifier from scratch.I recently built a mrf186 based 70cm narrow band (kit) amplifier (with an order of magnitude more power). That other amp uses a dual ldmos device and there is a quarter wave coax on the front in the role of balun followed by a single capacitor in series, then one(adjustable) in shunt. On the output it is pretty much the same thing.Sadly quarter wave coax will.be way to big to fit into this (but at the same time it is a single device so no balun required - the coax also helps in transforming the match but I could achieve this a different way most likely, by changing the place the shunt cap is at alongside the microstrip line). Can you or someone else post some links to articles or other resources about such matching? I think I have a fairly good general grasp on it, but would rather not have to reinvent the wheel.