Have been working with a RF-1110 amplifier that is built around the MRF-150 Power FET devices. The basic construction is four blades or amplifier sub-assemblies with four MRF-150 devices on each blade, all of the blades together gives you around 1 kW of output. Each of the MRF-150 devices is its own amplifier with its own bias and feedback and operate independently with the only APC and key line control being common. They are all feed by two 50-volt power supplies and each MRF-150 has the ability to have its bias set by a pot on the front of the blade along with a microswitch that allows you drag the gate voltage to ground for that device.
I do not have a lot of experience working with the MRF-150 but have been working with broadcast transmitters for years and have been told that all of the individual blades or amplifiers are not field repairable so you just end up pulling bad amplifiers and sending them back to the manufacturer for repair, that’s not going to be happening with the RF-1110
I do have several questions about working with the MRF-150 devices:
First- is there any issue in running these devices without drive? Each device has its own bias circuit and assuming that they have no issue running with full voltage applied and normal bias and will just be sitting there eating up current with no ill effect? Looks something like they have to be biased way on to work linear but did not know if you keep the bias in place regardless of drive. Think the key line control removes any bias until the drain voltage is applied, can bias without drain destroy the device?
Second – how important is it for all the devices to have the same gate voltage? Dose that matter or is it more important to look at output of each amplifier and use the bias to adjust each device so it’s the same gain as the others?
They provide a microswitch that drags the bias to ground and am going to assume that you can use them to shut down all the devices and look at output and then one at a time switch on a device and set its bias for gain or current?
Third – they have a big deal about the devices being matched pairs, why? Each amplifier is its own separate deal. Gains or currents can be trimmed by the bias pot for each device so why go thru the added expense of buying matched pairs? So what if you have matched pairs because there are sixteen devices so in theory you would need sixteen matched devices.
Forth – I intend to build up a device so I can run any one of the blades external of the amplifier itself. This would be a test harness that will connect to the plug on the back of the blade and feed the blade under test fifty volts, APC and Keyline but I am thinking that any RF testing would be at low level, maybe 0.1 or 1 watt of drive. The normal gain of the amplifier is 15 dB and can I expect to see the same gain at low levels or do you have to use a minimum amount of drive before the devices start to work? Assuming not being they are supposed to be linear but thought I would ask.
Fifth – will it affect the device if it has RF applied to its drain when its biased off? With the drain voltage removed? When you have all these devices parallel think you will always have some leakage from the ones that are working over to ones that are cut off?
Ok, so there are a ton of questions on the care and feeding of high power MOS devices, lets see what everyone has to say. Just for reference you can see the data sheet for the MRF-150 at:
https://cdn.macom.com/datasheets/MRF150.pdf
