This article is own by Grant Hodgson. .
Eagle-eyed readers may have noticed the reference to an ATF-54143 in John G3XDY’s notes on the pre-amplifier testing results at the November Adastral Park round table, published in the March 2002 Newsletter. Here are some more details of this pre-amp which has some rather interesting properties :-
The ATF-54143 is the first of a new breed of low noise GaAs FETs. Released by Agilent Semiconductors (formerly HP) in mid-2001, it is less than one year old and offers some remarkable properties. It was designed for the mobile phone base station market, where low noise and good strong signal handling ability have to be achieved at the same time. However, it can be used up to at least 6GHz (although the gain is starting to roll off at this point), and is ideally suited for amateur microwave use in the lower and middle bands.
Agilent call the device an Enhancement-mode Pseudomorphic High Electron Mobility Transistor, or E-PHEMT. HEMTs have been around for some time now, and will be familiar to anybody who has built a microwave low noise amplifier (LNA). ‘Pseudomorphic’ is a development of the basic HEMT, and refers to the way the Gallium Arsenide is doped during device manufacture; the channel of the FET being made from many thin layers, which form a lattice structure. It sounds very impressive but is only of relevance to the semiconductor physicists involved in the details of the design and fabrication of the device itself. (Incidentally, PHEMT is pronounced ‘pee-hemt – not ‘femt, but there doesn’t seem to be a universally accepted way to pronounce E-PHEMT’!)
The really interesting bit is ‘Enhancement’. Until now, all low noise and high power GaAsFETs for RF and Microwave use were of the ‘depletion’ type, which means that the device has to be biased into the operating region by ensuring that the gate is at a more negative potential than the source. The two most usual ways of achieving this are by grounding the gate at DC and using resistors in the source lead(s), or by grounding the source lead(s) and using a separate negative voltage generator.
Enhancement mode FETs require a positive voltage to be applied to the gate, which obviously makes things far easier in terms of circuit design and construction.
The second interesting feature of the ATF-54143 is it’s strong signal handling performance. Traditionally, low noise GaAsFETs have had significantly worse performance in terms of being able to handle either in-band or out of band strong signals than bipolar transistors or MOSFETS. The parameter most often used to describe strong signal handling performance is the 3rd order intercept point IP3, either referred to the input (IIP3) or the output (OIP3) ,,.
Note that the difference between the IIP3 and the OIP3 is simply the gain or loss of the device, so that an amplifier (or transistor) with an IIP3 of +10 dbm and 6dB gain will have an OIP3 of +16dBm. The ATF-54143 has an OIP3 of up to 37dB at 2.3GHz, with an associated gain of 16dB, giving an IIP3 of +21dBm which is a very impressive figure indeed. The IP3 is a function of the bias conditions, and this allows a trade-off to be made between IP3, gain and noise figure.
On the higher microwave bands, strong signal handling is not usually an issue, but there are at least two cases where good strong signal handling could be used to good effect, particularly at 1.3GHz :-
i) The 1.3GHz band has a close proximity to the frequencies used by the huge Civil Aviation Authority radars in some parts of the country. Some 1.3GHz receivers are consequently overloaded.
ii) Repeater builders have to go to great lengths to ensure that the repeater output does not de-sensitise the receiver. Traditionally repeaters have required a low loss, high Q cavity duplexer in order to separate the Tx and Rx signals. A receiver front end with a very high IP3 may not be as susceptible to self-desensitisation, allowing the possibility of a lower-specification receive filter.