Tuesday, April 4, 2023

Exploring the "1-930MHz 2W RF Broadband Power Amplifier Module for FM Radio HF VHF Transmission" found on EvilBay

Figure 1:
The amplifier - with heat sink.
Click on the image for a larger version.

On EvilBay, you can find a number of sellers of a device described as:

 "1-930MHz 2W RF Broadband Power Amplifier Module for FM Radio HF VHF Transmission".  

This unit has SMA connectors for both input and output and is constructed on a circuit board and heat sink that measures just a bit under 2" square (50mm x 50mm).

As is so often the case with these sorts of things, the sellers likely have no idea what this actually is - and their listings are often sparse on details other than general operating parameters.

In the case of the device depicted in Figure 1, the parameters given in the listing are:

Type: RF Amplifier
Module: RF Broadband Power Amplifier Module
Dimensions: 50*50*15mm (L*W*H)
Working voltage: 12V (DC)
Frequency: 1-930MHz
Working current: 300--400mA (determined by output power)
Type 1: 1-930MHz 2W
Working frequency: 1-930MHz

There are a few "tells" here that this data was simply copied from some source - notably that line beginning with "Type 1" which probably means something only to the original supplier, in the original Chinese - but likely means nothing at all to anyone else.  Unfortunately, you are unlikely to get more information that this from an EvilBay listing and this hardly counts as "detailed technical information" about exactly how it works by virtue of describing its design in detail.


This same (or very similar) circuit appears to be found in other form factors - possibly including that described here:  https://yo5pbg.wordpress.com/2019/10/28/the-ultrawideband-1-1000mhz-nwdz-rf-pa-2-0-initial-tests-and-improvements/  - This device may have a description such as:  "3W 2MHZ-700MHZ RF Power Amplifier HF VHF UHF FM Transmitter Module For Ham Radio"

I do not have one of these (yet?) but based on the above web page, it looks to be pretty much identical electrically aside from the output transistor being what appears to be an older variant of the one described below.

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What is it, really?

From the picture in Figure 1, it's apparent that there are two active devices, but what are these - and will identifying these devices give a clue as to how one might really want to use one of them?

With a bit of magnification and Google-Foo, I was able to determine the nature of both of the active devices - and reverse-engineer a schematic diagram, below in Figure 2.

Figure 2: 
Reverse-engineered schematic diagram and component layout of the amplifier.  The component
designations are arbitrary as they are not marked on the PC board so a layout is provided.
Click on the image for a larger version.

This amplifier is about as simple as it gets:  A broadband MMIC with approximately 20 dB of gain is coupled into the gate of a VHF/UHF N-channel MOSFET amplifier - which itself has 10-15 dB of gain - with no matching. 

What this means is that it will take just a few milliwatts input to obtain about a watt of RF output across the intended frequency range - the precise amount of drive depending on the frequency, the supply voltage, and the desired output power.

A 5 volt regulator (U2) provides about 1.68 volts of gate bias on Q1 while supplying U1 with a stable 5 volt supply (at about 90 milliamps).  With no drive, the total current consumption is likely to be in the area of 130-150mA, but it could exceed 500mA at higher operating voltages and saturated power output levels.

Looking at the active devices:

Taking a step back, let's look at each device a bit closer - starting with U1, the MMIC on the input.  This device is the Qorvo SBB20892 MMIC (Datasheet here:  https://www.mouser.com/datasheet/2/412/SBB2089Z_Data_Sheet-1314913.pdf ).

Inspecting this data sheet we can see that its rated for operation from 50 to 850 MHz - although these types of devices typically have no problems operating at much lower frequencies (even down to DC) - and they can typically operate at quite a bit higher frequency than the specification, albeit with a bit of roll-off in gain and output power capability meaning that this stage of the amplifier should have no problem operating up to its 930 MHz stated range - or even higher.

Looking at the output stage, Q1, we see that it's a Mitsubishi RD01MUS2B RF N-channel MOSFET transistor (Datasheet here:  https://www.mitsubishielectric.com/semiconductors/content/product/highfrequency/siliconrf/discrete/rd01mus2b.pdf ) which is nominally a 7.2 volt, 1 watt transistor.  This is device has an SMD marking code of "KB861".

Right away you'll spot a bit of disparity between the EvilBay listing and the manufacturer's specifications - the former stating 2 watts at 12 volts.  Taking a close look at the specifications in the data sheet we can see that we should easily (and safely) be able to get about a watt out over the range of at least 100 to 930 MHz (and likely down to a few MHz) with a drain voltage of 7.2 volts on this device - perhaps a bit more or less, as there is no attempt at impedance matching on the output of this amplifier.

Looking further at the specifications, you might also note that the maximum drain-source voltage of this transistor is 25 volts:  If it is operated at 12 volts into a highly reactive load, it could be expected that the peak voltage could reach or exceed twice the supply voltage - and this does not take into account that the drain current, which is specified as an absolute maximum of 600mA - could also be exceeded.

What we can conclude from this is that operating at 12 volts or greater - particularly under conditions where the load to which the amplifier is connected might be mismatched (e.g. high VSWR) is probably not a good idea!

The device overall:

It should also not escape the attention of the reader the comment on the schematic relating to inductors L3 and L4 on the drain of Q1:  Together, these have a DC resistance of a bit more than an ohm.  With an expected drain current of 300-400mA in normal operation, one can expect at least a half-volt of drop across these two components which actually can work to our advantage in reducing the power supply voltage a bit.

Finally, looking closely at the data sheet you'll note that there are graphs that show operation to 10 volts drain current (or about 11 volts supply voltage, considering the drop of L3 and L4) that show outputs exceeding 2 watts.  If you feel that you really need more than 1 watt - or wish to have a bit of extra headroom for 1 watt operation (e.g. to preserve linearity) then operating at that voltage (10-10.5 volts) may be possible with the caution that you may be sacrificing reliability.

Considerations of linearity and stability:

This amplifier will generate significant harmonics, so it should never be connected directly to an antenna without appropriate filtering!  At a power output of 1 watt, if its second harmonic is 20-25 dB down (a reasonable value) that will represent several milliwatts of power on its harmonics which can easily carry for several miles/kilometers line-of-sight.

Particularly when this amplifier is operated from a supply of greater than 8 volts, care should be taken that the output is resistive (nominally 50 ohms).  Now this may sound pretty easy as antennas and filters are nominally 50 ohms, but one should consider frequencies other than that on which the amplifier may be operating:  Being an inexpensive device from EvilBay, it's hard to be sure of the quality of the components that one would use to make it operate in a stable manner (capacitors, board layout, inductors) - and since this amplifier has a rather high gain of around 30dB, it may not be unconditionally stable.

Take, for example, this amplifier being used to boost the output of an exciter on the amateur 6 meter band - around 50 MHz.  We should assure that at 50 MHz that the load (low-pass filter plus antenna) provides a reasonable match to 50 ohms.  What is not easily knowable with this sort of device is how it will behave at other frequencies:  If you move below 50 MHz, the match (SWR) will get terrible because the antenna is out of its design range - and if you move above 50 MHz, the SWR will also be terrible not just because of the antenna, but because the low-pass filter itself will start to reflect energy.  Again - in this example - the amplifier will see a match only at the antenna's design frequency - but it will be terrible everywhere else.

While an ideal amplifier wouldn't really care about off-frequency mismatches, a poorly-designed amplifier - or one that may be well-designed, but has not been designed to be intrinsically stable under all load conditions - might be prone to oscillation at some unknown frequency if it is connected to a load that presents to it just the right conditions that its built-in instability may cause oscillation.

This last point - the possibility of an amplifier oscillating at a frequency other than at where it is intended to operate - can be difficult to diagnose:  Worst case, this will cause the amplifier to die randomly and in the best case, it will output power that is lower (or randomly varying)  than expected and - possibly - have spurious outputs related to the mixing of the desired frequency and that at which it is oscillating.  If the frequency at which it is operating capriciously is above that of the low-pass filter, you may not even be able to detect that it is behaving in an untoward manner unless you were to do a broadband analysis by probing the amplifier's output directly - a process that could, itself, change the results!

This sounds like a lot of conjecture, hassle and trouble - and sometimes it is - but there are a few things that one can do to make the device work more reliably and also detect that something may be amiss.

  • Do not operate it at a higher voltage than needed to obtain the desired output power.  In the case of this device, 1 watt is about all you should reasonably expect in terms of long-term reliability.  Period.
  • If, under certain conditions, you see the power output randomly fluctuating - but the input drive and power supply voltage is constant - you likely have a spurious oscillation occurring within the amplifier.  A redesign of the filtering to change the off-frequency characteristics (e.g. the impedance well above the cut-off frequency of a low-pass filter, for example) may improve things:  Consider the use of a diplexer-type circuit with the "other" port (e.g. that which passes frequencies other than the desired) terminated.
  • A reasonable question would be:  "If I blow up Q1, where can I get another RD01MUS2B transistor to replace it?"  The answer - albeit a bit glib - is to simply buy another of these amplifier modules:  Unless you buy a lot of them at once, it will probably be cheaper to get another amplifier than just that transistor!

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This page stolen from ka7oei.blogspot.com