PA fail?
it becomes a 0.5W PA now?
strange
yes,it is at now a 0.5w amp! at the current of 2.5W amp!
may be there is a DC level somewhere
At what freq are you using it?
Are you testing with a dummy load at 50R?
Is the thermal layout good enough?
I would also suspect that due to a bad thermal design, the MMIC overheats and some power transistors are blown. You can confirm this by monitoring if the DC current changes when the output power is reduced.
You should make sure that the power transistor is connected to a heatsink somehow.
It could be bad thermal design if the device is made cool or by just swiched OFF for some time and the device is get back to work at 2.5W.
Is the device still works at 0.5W even after switch off /cooling then the device might burnt some of the transistors( internally) and giving poor power output.
I think it is a heatsinking problem (Heat is not dissipated) ,the MMIC works at 0.5W forever,the current is the current level of 2.5W!
I used brass for case of PA and heatsink ,brass thermal conductivity is low compared to aluminum,
what meterial do you recommend?
Aluminum will oxside,what can I do with it to prevent oxsidation?
7rots51
please answer the questions that E-design asked and then also my questions :)
maybe then we could help you more I couldnt get a datasheet for the device
off the www so please describe the device better for us as well...
is it a GaAsFET MMIC or a bipolar one ?
What is the current drain of the device?
if it is a GaAs MMIC What voltages and currents are specified for the Drain
and the Gate pins ?
incorrect Bias voltages and currents or lack thereof will cause the GaAsFET device to draw excessive
Drain current and destroy itself
There is nothing wrong with aluminium for the heatsinks 99% of commercial amplifiers and for that matter
homebrew amplifiers use aluminium
I have built 10GHz 3W amplifiers using aluminium casing and heatsinks with no long term problems
Aluminium oxidisation is not an issue and it will definately not be a problem between the
device and the heatsink. Also USE heatsink compound BUT at 1GHz up use it sparingly that is
you should almost be able to see through the thinness of the compound layer This is because RF/DC grounding
between the device is much more critical at uWave frequencies.
Dave N
Sydney
OZ
Does this happen even if you have a big fan running on it all the time and you keep the load constant (do not disconnect the load during the test)?
Link of datasheet:
http://www.amcomusa.com/downloads/datasheets/MMIC-series/AM003536WM-BM(FM)-R.pdf
To DaveN: Did you used any coating (like as silver) for aluminum?
what type of aluminum did you used? Is there any oxsidation problem between 1-2GHZ for aluminum?
Answer to Q:
-it is GaAsFET MMIC
-Id1 = 150mA Id2 = 550mA (total 700mA)
- Vgg1~=-1.3V and -Vgg2 = -1.4V
- I designed a good bias circuit and it performs bias sequence,My MMIC fail slowly after 30~60minutes continuous operation
when it is normal it draws 700mA when its output power drop to 0.5W it current is 800mA.
7rots51
No silver added just plain aluminium :)
What is the physical size of the heatsink that you are using length, width, depth ?
I would assume one side is flat for mounting the PCB/device and the other
side is finned that would be the norm.
For its intended normal use will the amplifier be in contineous transmit or will it be switching between TX and RX ?
If its going to be 100% duty cycle transmit then larger heatsinking will be needed
for a 3W TX and a ~ 50% duty cycle I would suggest a heatsink size of around
8cm wide 10cm long 5cm deep (5cm .. measured from flat surface to tip of fins)
Those are a suggested size to aim for something a bit either way wouldnt be a problem
The higher the duty cycle % the larger the heatsink will be needed
ok there's a few more questions and some suggestions I will go look at the datasheet
Dave N
Added after 1 hours 58 minutes:
Just an added thought ....
If the amplifier is NOT going to be in contineous transmit then it would be advisable to relay switch
the DC voltage supply to the Drain pins (ie the +20V rail) so that the device isnt drawing current
during times that it isnt transmitting. This also is common practice and one that I used in my 10GHz amplifier
I found if I didnt turn the drain Volts on and off as needed then the device got extremely hot even
with good heatsinking. my GaAsFET MMIC (FMM5061VF) draws 2 amps at 12V
on the Drain regardless
of if there is RF drive to the MMIC or not.
The overall effect is that if you relay switch the Drain Volts then the device has a chance to cool off
between transmit cycles :D
cheers
Dave N
continuous switch of PA may destroy it,surge current of Idd of MMIC may destroy it,thermal shock of temperature changes is a problem.
I do not accept drain switching because of the above problems.
I used circulator at output of PA before antenna.
What type of Aliminum is better for RF and heatsinking?
hi mate,
its something you should accept, it is standard practice :) if it was a regular cause of device
failure then it wouldnt be suggested and none of us would do it.
but you didnt answer the question .... Are you using it in contineous 100% TX duty cycle or is it able
to be switched off during RX periods Beacons for example have 100% duty cycle ... always transmitting
A circulator on the output is good it helps protect the TX from reflected power :)
I have never used any special aluminium in any of my commercial or amateur radio projects
there are plenty of sellers and sources of finned heatsinks
Dave
Its duty is 10% ,10% of time it is transmitting(there is RF carrier at input),but all of time the PA is fully biased.
when PA is normal(not failed):
I =350mA with no RF input
I= about 780mA with RF input.
Are you sure aluminum oxside at 1-2GHZ do not make problem?