Build Your IRF530 Mosfet Amplifier
Since i am using low power SDR transmitters (5watts), i need a driver for my EL519 and SB200 amplifiers. I am targetting about 20 up to 45 watts with 13vdc.
Some kits are sell on ebay or Aliexpress and i give a shoot to a cheat kit from Aliexpress.
The main goal and challenge is to use this kit in order to achieve a FCC compliant 20w up to 40w Amplifier using IRF530 Mosfet which are not design at all for this usage.
This amplifier GERBER and KICAD files : https://drive.google.com/file/d/1K5nc28Eh_CndHPMDvSDxyjP_qx9f9oBP/view?usp=sharing
Many test with different transistor : MRF186, IRF510,IRF530,MRF9620,IRFZ44N since this kind of kit is able to welcome different mosfet.
The final release will be with IRF530 which provide me the best results in terms of linearity ,IMD and Harmonics.
This amplifier is providing about 20 up to 30 w with only 1w input. The final result and according my needs , the amplifier is providing about 20 up to 22w with only 0.5w input.
Hereafter you find the schematic with the mods from the original
-Feedback resistor are now 47 Ohms 5w to replace the 150R provided with the Kit. i try different value and the best results are with feedback resistor between 47R up to 100R.
-I added the resistor (21 Ω 3w) across the secondary of the input transformer. It will provide a much better SWR.
-This kind of transistor shall be fed with current limiting power supply for the drain . You can just solder only one side of L1 and on the other side you can feed the it with a maximum of 14Vdc. Feeding more voltage will provide unstable transistor. for the drain a current limiter circuit is deemed to be installed.
-The amplifier is fed with 13.8vdc (Drain).
First you need to cut the parts from the PCB in order to build the transformer using the little tubes provided.
The winding for the transformers is quit simple.
The input transformer is .5/2turns
The output transformer is 3/.5 turns
Complete and final design files
Gerber and Kicad files :
Here a great video for how to build the input and output T1 and T2 transformers :
More details for the LPF
For the LPF is will depend the frequencies your are planning to use . Hereafter a table summarizing the details according the Ham bands from 80 up to 10m. This is for 5th Order Chebyshev Low pass filter. The calculation is provided for using iron power T37-2 or T37-6 toroids. The wire should be 0.6mm. For the Capacitors you should use about 500v NPO if you are targeting 100W , for QRP 100V capacitors will suffice.
This LPF should be able to handle about 100w Max
LPF 2 with SWR/PWR and SEQUENCER
Biasing (quiescent current)
The quiescent operation point will Influence the gain of the transistor, and change the
output waveform (linearity) and spectral purity.
The Quiescent current you should target is 20ma up to 40ma (for one IRF530 means about 40ma up to 80ma for a pair of IRF530) is the magnitude you should target and you need to replace the R7 resistor with a 220R to achieve this result. The biasing voltage is about 3.5Vdc up to 3.7Vdc. In order to achieve this correct biasing (40 up to 80ma for 2 IRF530), R7 and VR1 resistor are important and should be adjusted with different value in order to provide the suitable biasing (3.7v and 40ma max). One thing very important and i notice the biasing should be adjusted according the pair IRF530 you are using . My setup is about 3.73vdc in order to achieve a perfect modulation and a great spectral purity . In accordance you should pay a special attention for the biasing setup with this kind of Mosfet . To provide you an example during my test initially i was using the stock IRF530 provided with the kit and my initial biasing was about 2.5 up to 2.7vdc for a great biasing . After blowing my mosfet during many test and obviously some mistakes i swap with new IRF530 and my initial setup with 2.7vdc was simply terrible , providing a terrible AF (modulation) quality and spurious with a terrible spectral purity . With the new IRF530 my final setup is finally 3.7vdc with 60ma current . I am using this setup for more than one year and this amplifier is running really like a charm with a great modulation and a good spectral quality.
IRF530 IMD & Harmonics
Spectral purity , IMD, Harmonics and FCC or ETSI rules:
In France RF power amplifier transmitting on a frequency below 30 MHz, “spurious” and harmonics must be at least < -50 dB below the mean power of the fundamental emission for a < 25w transmitter or amplifier . If i am not mistaken for USA is < -43 dB
Achieving < -50 dB is difficult to achieve and even most of the commercial TRX cannot reach such perfomance.
In a real life for H2:
From -50 up to -40dB is considered as a good result
From -40 up to -35dB is considered as acceptable
From <-35 is considered as a non-desirable result.
50dB down for 3rd order products H3 is considered as acceptable, therefore it is recommended to reach between -55db up to -60dB
The IRF530 is not design at all for HF usage and few things shall be considered in order to have a FCC compliant amplifier.
Biasing and IMD
-Biasing : Maybe the most important part. The Biasing setup shall be tune with a spectral analyzer . Considering using genuine IRF530 Mosfet, the Biasing range is about 3.30 up to 3.8vdc to the gate with about 30 up 40ma (idq) current per MOSFET. With the initial kit it is simply impossible to achieve such value and you shall replace R7 with a different value (Mine is about 220R but you should try also with different value and some guys are using a 470k Resistor). According the measurement i perform, the IMD and distortion provided are terrible with a voltage below 3.3vdc and a current about 10 to 20ma. During my test i try with about 100ma current for the biasing but results were not satisfactory in terms of spectral purity.
-Output power : i f you are expecting to get more than 60watts output power with a good spectral purity means FCC rules at about -50db just forget it , you will never achieve it. According some test and the measurement i perform hereafter the overall results :
Biasing 3.73vdc to the gate with overall 40 up to 60ma current and 13.8Vdc for the drain :
All measurement were performed using a 2 tones and IMD capabilities provided by PiHPSDR and Quisk.
-15 to 50w output with 0.5w input : IMD and 2nd/3rd Order harmonics < -40db up to 47db –> acceptable
-50 to 60w output with 0.5w input: IMD 2nd/3rd Order harmonics< -30 up to -25db , Not so good
-65w and above watts output with 1w input: IMD and harmonics start to grow significantly and almost reach the fundamental level . IMD are only about -15 up to -20db.
-70w output : Just forget it , the IMD + 2nd/3rd Order harmonics are like a Xmas tree on the Spectral analyzer . H2 and H3 reach fundamental level .
if you are expecting a full power means about 70w amplifier compliant with FCC using IRF530 or 510 JUST FORGET IT . Instead it should be more suitable to use dedicated HF transistor likely MRF186 , RD100HHF1, MRF254 etc etc ……
The maximum acceptable input power range is about 0.5 up to 1w to reach FCC compliancy.
Below 22w output with about 0.5w input with a correct biasing the amplifier is acceptable in terms of IMD and Spectral purity.
For 15 to 20w output with 0.5w input with “PS” Pure Signal capabilities from piHPSDR i manage to reach -55db for the H3 3rd order products and about -50dB for H2 2nd order products.
Hereafter a snapshot with Pure Signal activated on 40m with about 20w output. The signal is pretty much clean nearby the carrier.
Wrong Biasing IMD screenshot
You can notice in the screenshot below the output signal is not suitable and acceptable . If i can remember this screenshot was with about 3.3v and 55ma biasing
The following one was with about 2.7v biasing and 40w output , needless to say it is a disaster in terms of spectral purtity
This is the spectral purity 2 tones and 25w output
A quick check without the LPF
IRF530 Mosfet are not dedicated for HF usage and according with that it is not realistic to provide more than 30W with 2 IRF530 without producing significant spurious ,IMD and 2nd/3rd Order harmonics at the suitable level means < -50dB.
Feeding with more than 14vdc the IRF530 is working but the MOSFET will not sustain such voltage due to Thermal Runaway (The temperature and current may increase and will be difficult to control) . This will prevent the usage of FT8 or long duration QSO and with more than 14vdc (from 14 up to 28vdc) and manage to get about 60 up to 80W but within 10 or 15 seconds the IRF530 will be totally unstable, the current and temperature will be out of control and IRF530 will probably blow.
The drain and Gate voltage should be provided ith a current limiting system in order to protect against any current overloading and Thermal runaway (See below the thermal regulation protection).
I will have the same comment regarding the biasing and exceeding 40ma or 100ma overall and 3.7vdc will provide also a totally unstable transistor after few seconds with the same result : Transistor blow
In addition a voltage above 14 Vdc and a biasing current above 40ma (100ma overall) are providing significant IMD and the spectral purity is not good at all. 40ma (One transistor) biasing is in my point of vue the maximum current in order to achieve a compliant spectral purity
The maximum input power for 2 IRF530 without attenuator is 1.5 to 2w , otherwise the spurious or IMD will transform your spectral analyzer screen into a Xmas Tree. With 0.5 up to 1 w i manage to have 40 to 45w output which is i was targeting.
For my amplifier the drain voltage for the mosfet is about 13.5vdc with a 40ma biasing current and 3.7vdc for the gate. i can run this amplifier permanently for one hour without any issue and the spurious/spectral purity are acceptable (My biasing current is about 60ma overall) . I use this amplifier with this setup for months now without any problem. Needless to say that you need to apply a proper cooling system with a radiator and a fan.
Thermal Runaway protection
The following schematic is populating the overall protection. To be straight forward as far the MOSFET is getting hotter , the internal resistance between the Source and the Gate will increase . This will involve an increase of the overall current for the gate biasing and since the heat increase is not instantaneous with some lag it will involve a kind of uncontrolled domino effect. This avalanche effect can be contolled with some additional circuits.
Thermal Protection principle
As mentioned above , the main issue according the IRF530 is the Thermal Runaway involving a catastrophic avalanche effect. The biasing current is usually setup at about 40 up to 50 ma for 2 x IRF530. When running the amplifier few minutes or even seconds , you will notice a significant increase of the Biasing current to few hundreds of ma and you can reach about 1 amp. When getting hotter the internal resistance is rising also . The main result is the mosfet blow out and destruction (gate/source junction). The issue is also notice with Quiescent current when no RF is applied to the gate . the Drain current current could also increase on its own and involving a catastrophic domino or avalanche effect with same the destruction of the Mosfet Source junction especially when the MOSFET is already hot. So with this kind of MOSFET we need to tackle 2 issues
To avoid this issue , there is a simple circuit you should add in the biasing path in order to regulate the biasing current against Thermal Runaway . The biasing is coming from the IC1 7805 Regulator and the thermal regulation is achieve with 1N4148 silicium diodes . The main property of this kind of diode = The current decrease , when the temperature increase. This addon is providing a temperature compensated biasing and protection
The diodes shall be connected to R7 resistor , the cathode to the ground and tackle against the radiator or against one of the MOSFET (This kind of system is widely use with pro amplifier)
For a better thermal transfer you can put some liquid tape or thermal paste to ensure they are not moving
With this system i can run the amplifier for few minutes without any issue and since my radiator is not so big , after few minutes due to the radiator heat , the biasing current is dropping significantly and the output power is dropping to less than 10watts .
This circuit is also acting as a security for your amplifier and will provide an automatic current regulation regulation for the biasing circuit.
You can install the diodes on the top of the IRF530 casing or very close and fix it with some plastic dip or Sillicone. I did some test and looks like a great solution . The Bias current swing at 45 celcius was not above 100ma. I guess i will implement and test this option very soon.
“Source junction” current limiter (Mod from ES5DOL)
Credits : Many thanks to Sergei ES5DOL for the tips .
In addition in order to prevent any surge current in the Drain junction you just add 0,1-1 Ohm resistor between ground and sources, and forget avalanche and thermal instability. Add capacitor in parallel to dump oscillations, to set amplification and frequency response correction. No additional protection, no drain-gate RC-chain required anymore (only if you need more fine frequency response tuning). My final tuning is with 0.17 ohm resistor and a 220nf capacitor.
Adding a resistor will regulate and balance the voltage and current from the drain and source. This resistor will prevent to have an infinite gain and the capacitor in // will assist against spurious oscillation.
How it works :
The gate to source voltage, Vgs, controls the transistor. As this voltage increases, the channel becomes more conductive, and more current flows through the drain and source.
The voltage across a resistor is given by Ohm’s law: V=IR
So, as more current flows the drain and the source, more current flows through Rs, and, by Ohm’s law, the voltage across Rs increases. Since the gate voltage isn’t changing, the gate-source voltage decreases, since the source is now closer to the gate.
There will be one point at which this is stable. If the transistor is too much off, there won’t be enough current in Rs, and Vgs will be high enough to turn the transistor on more, increasing the current in Rs.
If the transistor is too much on, there will be too much current in Rs, and Vgs will be low enough to turn the transistor off more, decreasing the current in Rs.
If the gain of the transistor is infinite, then as long as the input is not pushing the output into the supply rails, then the gate-source voltage will be constant, right at the transistor’s threshold voltage: Vgs=Vth. Since the transistor’s gain is infinite, it has unlimited ability to correct any deviation from this through the feedback mechanism mentioned.
The gain of the circuit approaches Rd/Rs as the transitor’s gain increases. Or put another way, as Rd/Rs decreases, the transistor’s gain becomes less relevant to the gain of the whole circuit. That is, almost all of the change in gate voltage appears as a change in Vrs, and only a negligible amount as a change in Vgs.
–> Vg is constant so, if Id increases, Vrs decreases
Thermal runaway outcome
This overall system (Diodes + 0.17 ohm resistor with 220nf capacitor in // + Drain current limiter) is acting mostly as a 3 stages protection .
The main stage is the couple 0.17 ohm resistor with 220nf capacitor in // in order to prevent (Mostly a kind of control) an infinite gain for the Source junction
The second stage is the thermal protection with the diodes.
The Third protection is the Drain current limiter thanks to the step down voltage regulator
This protection system is really recommended to prevent the Drain/Source destruction due to surge current involved by thermal runaway issue. The amplifier will have less gain but an efficient protection.
NTC resistor as a protection study
Obviously adding a NTC resistor as a protection and compensator against Thermal runaway is something i was consisdering initially . I did some test with different value for NTC (From 1K up to 5K).
The system is reacting really well against thermal runaway , therefore is really too sensitive and the Biasing voltage was fluctuating too much and too quickly in a magnitude range of 3.7vdc down to 1 or 2vdc . This swing is involving very quickly a terrible modulation and really bad IMD3. The protection was really efficient but the modulation and amplifier efficiency were really fuzzy and not consistent.
In fact the main difficulty with NTC is where to install it . Too close from the MOSFET , the temperature will climb to fast and away from the MOSFET the temperature will rise too slowly.
Using diodes instead are less sensitive and the Bias Voltage swing is better without any IMD3
Drain Voltage and Current limiter
I am using a current limiting system (https://www.ebay.fr/itm/DC-CC-CV-Buck-Converter-Step-down-Power-Supply-Module-300W-20A-6-40V-to-1-2-36V-/172718963804) to feed the transistor Drain. For this kind of MOSFET used as a power amplifier it is a Mandatory feature. You can build you own one (Many schematics on internet) but since i have already this cheap power supply with a current limititing setting , i used it .
The power supply should be connected as below:
The step down converter is connected to the main power supply. The overall current is about 7 amp.
Final Design with overall Thermal Runaway Protection
Some additional pictures
80 m filter.
The filter has cut-off at about 4,5 Mhz
40 m filter.
The filter has cut-off at about 9 Mhz.
20 m & 17 m filter.
This filter has a cut-off at 19,9 Mhz. and is capable of 30 m to 17 m.
15 m, 12 m & 10 m filter.
This filter has a cut-off at 35,5 Mhz. and is capable of 15 m to 10 m.
To tune properly this amplifier you need the following equipment :
-Lab power supply with current and voltage control.
-a TinySA to check the specterum purity , IMD and harmonics level
My heathsink is not as big but i limit the output power to 25w for FT and digital mode and about 25 up to 45w for SSB and CW . For the output power limitation the key and it is mandatory you really need a current limitation power supply due to the avanlanche effect with this kind of MOSFET. In fact you need 2 power supply source to power up this amplifier:
-One output for the device and this also will be used for the biasing : about 12vdc up to 14vdc max, this is the main voltage
-In parallel you will connect to the step down power supply module with current limiting capabilities to feed L1 (L1 is only connected to one side to feed the source through the output transformer) This part is the key.
To setup the amplifier :
-To setup the biasing you need to disconnect the source voltage and just feed the amplifier with the main voltage
-Apply the main voltage, trigger the PTT and setup the bias to 3vdc with the variable resistor
When done you can apply the Source voltage and setup the current limitation to the minimum, the source voltage is adjusted to about 12 up to 13vdc
-You will apply with your transmitter about 500mw maximum
-Trigger the Ptt and apply the power up to 500mw
-Check the Biasing voltage and current (The current should be below 100ma and the perfect one should be from 40 up 60 or 70ma)
-Check the source voltage and it should null since the current is null
-Increase bit by bit the source current until the output power is starting to increase. When you reach about 30 up to 40w you can stop to increase the source current. meanwhile double check the biasing and it should be stable to 3vdc.
If you have a 2 tone generator you can now finalize the biasing and check the spectrum with a spectrum analyzer like TinySA
-apply 2 tones or use your microphone and increase bit by bit the biasing voltage until you reach a perfect modulation and spectrum.
You need now to finalize the Drain current and when applying some power (Tune or CW carrier) , the current should drop a bit and for exemple if the drain voltage is 12vdc when operating the amplifier the voltage should drop to about 8vdc . This drop of the drain voltage is not critical at all and you will notice during SSB , the voltage will remain stable. For this kind of amplifier this is the key and your amplifier will last for ever avoiding any catastrophic avalanche effect with blowing your MOSFET.
At this stage you amplifier should be setup with about 30 up to 40w output power with a great modulation and spectrum. You should notice the Source voltage will drop on the peak of the modulation which is normal and this will protect the amplifier from some catastrophic avalanche effect. Meanwhile the biasing is protected by the diodes in case of increase of the heat.
-For this technology of MOSFET (IRF510, 520 or 530) the maximum input power is 1w for one MOSFET and 1.5 max if using 2 MOSFET . Therefore the best results in terms of spectral purity is with 0.5w up to 1w input.
Building this kind of amplifier for you QRP rig will teach you a lot in terms of spectral purity, IMD , biasing , Fourier Transform, Harmonics and many more . It remains a great experimentation platform and it will provide the basis if you are moving for a more QRO amplifier using LDMOS technology.
If you are using an IRF510 during experimentation with this mosfet i notice, the minimum drain voltage you should use is about 24vdc, 28vdc will be perfect . With 13 up to 22vdc, the IRF510 is a bit unstable with a high impedance at the input and a significant capacitance at the output and will produce IMD and non desirable spurious . Feeding the drain with 28vdc for an IRF510 is providing a stable amplifier with reasonnable spurious.
-Biasing : for IRF530 the maximum idq current is about 30 to 40ma (Voltage will be between 3.5v and 3.7v) and i adjust mine to 3.7vdc for 40ma current. A proper adjustment of the biasing is really the key to achieve a clean amplifier. Never use the the Bias to adjust the output power. A wrong biasing will provide spurious , IMD , modulation clipping and distortion. With an unproper biasing the 2nd/3rd Order harmonics will be non acceptable.
-Thermal Runaway protection is MANDATORY with this kind of MOSFET not only for the Mosfet protection but also for the Spectral purity. With or Without it is day and night.
-A protection controller is highly recommended to protect the Amplifier (SWR, Bias and Gate/drain current/voltage)
-Current limiting circuit for the Drain and Gate voltage is mandatory
-Drain Voltage for IRF530 should not be above 14vdc;
-Gate biasing voltage should not exceed 4v for IRF530 MOSFET (Mine is 3.7vdc)
-Biasing Voltage and current should have about 3.7v with about 40ma current per transistor . My setup is 3.73vdc for about a total of 60ma biasing current.
-A good quality LFP filter is MANDATORY
Without applying the conditions as above , your amplifier will generate non desirable harmonics level and the spectral purity will not fit the FCC rules we should cope as amateur radio in all respects.
This tiny amplifier is really great and cost-effective (14 Euros for a kit). i can use it even for FT8 or RTTY long QSO without any issue. The output power provided is able to drive nicely my 2 EL519 amplifiers and with 18 up to 20w i can achieve about 300w and 600w
I am driving my tube amplifier (EL519) from my SDR transceivers with only 0.5 watts and i get 15 up to 20 watts output from this amplifier. My overall output power including my tube amplifier is about 270 up 310w which is sufficient.
The controller is monitoring the amplifier current and obviously the SWR and output power
- The amplifier is biased with 3.7vdc with about 70ma quiescent current –> input power is 1w for about 40 up to 55w output.
- The controller is providing the following capabilites:
- Temperature monitoring with dedicated menu to adjust the threshold to trigger a 12vdc Fan
- Current monitoring with a dedicated menu to adjust the max amp (Limitation) in order to cut-off the biasing with an opto relay when the current is too high
- A dedicated menu for the Max SWR adjusment –> It will trigger the opto relay to cut-off the biasing
- A dedicated menu to adjust the output power range you want to monitor . Since the power is about 50w , my adjusment is 100w which is tailoring the bargraph display accordingly.
- Using FT50-43 toroids and 5w 50ohm resistor, the SWR bridge able to handle up to 1Kw
The full package : Gerber , BOM, kicad , pictures, and arduino Sketch for the main controller and the SWR bridge : https://drive.google.com/file/d/1OAA3AwHDP3Jk_RUIIhog9YnVMdybFQ75/view?usp=sharing
The Arduino Sketch : https://drive.google.com/file/d/1E4R0gcUoR4Z5WwKWr_Zc6-ottOrRJNIo/view?usp=sharing
Currently i am using MOSFET amplifier for 3 years with this kind of controller with another amplifier and working perfectly.
For VD Diodes , i am using BAT41 diodes or 1N5711 . For R1 and R4 since i do not have 50R Resistor , i am using 47R 5w resistor . The Menu is offering to adjust the Resistor Value.
The gerber file for the SWR bridge : https://drive.google.com/file/d/1i1FAbeb4EyGl6EbUAo1TmMHviFV_yVLi/view?usp=sharing
Alternative SWR bridge
The link to download the SWR Bridge Gerber file : https://drive.google.com/file/d/1OKnSJLDMavRQmTEpTh-a3gz-aksZtfnM/view?usp=sharing
Prior the integration with the LPF , the final tailoring was to test with some different resistor value at the secondary of the primary transformer . i manage to get a perfect input SWR with a 24ohm 5w resistor. The output power with 13.8vdc is about 50w on 7Mhz and about 30w on 10m.
The main extent for an Amateur radio is to build , experiment , setup and use its own build equipement . Using IRF530 transistors is a challenge since this MOSFET is not designated to be used as a HF Linear Amplifier (It was the same regarding my EL519 Tube amplifier). The main challenge is to achieve a great device especially for amplifier with a good spectral purity and make it compliant. I spent hours , blow (Few blow was like a gunshot!!!!) about 15 transistors (Surely more hahahahaha !!!!!) but at the end of the road the result is great.
I am running this amplifier for 3 years now without a single glitch or issue and use it mostly for Digital modes likely PSK31, RTTY and FT8 but also for CW QSO.
Obviously the IRF530 is not the best choice and best performer , therefore with some patience and testing you can achieve a very decent and cheap tiny amplifier. As mentioned above building this kind of amplifier using IRF530 is a cheap and easy introduction for newbies. The IRF530 is tricky and you will learn a lot about capacitance (Especially at the input for the IRF530), biasing , quiescent current , IMD ans spectral purity. It will be a great introduction for LDMOS amplifier
Some details regarding the IRF530 Amplifier: