RF Transistor Selection

if you use a high mobility, high current amplifier, stability may be difficult near the nearly DC frequencies you mention ;-) . You might find that, in general, if a transistor is well charactarized at 6 GHz, its going to be tough as nails to get a stable input match at 200 MHz. If you tell us the linearity (IIP3), current, input match and gain you are striving for, it will be easier to make a more specific recommendation. Try bipolars, JFETs, even large geometry GaAs and SiGe from Toshiba, NEC(CEL) IBM, Infineon Philips and Agilent. I've had varying degrees of success with each in different applications. The toughest challenge will be to try to find a device with NF min at optimum near the other parameters above. If your input match doesn't fall within the required noise circle on the smith chart, you may need to use negative feedback (at the expense of gain and perhaps stability) to change the input parameters.

Best of luck

Frank Raffaeli AOM Wireless

IIP3 - 3 order Input interception point.

This is a non-linear parameter. For low-power input, for each dB you add at the input you get +1dB at the output (of course) and +3dB third order distortion. This translates in a graphic like this:

- x-axis: input power;

- y-axis: output power;

- two lines: - one for "signal" (1st order), "growing" at 1 dB/dB - one for "distortion" (3rd order), "growing" at 3 dB/dB

These two lines cross at a given point. The abciss of this point is known as IIP3, i.e., IIP3 *would* be the input power you would apply in order to get at the output the same power for both 3rd order distortion and signal.

I wrote *would* because for such a high input power these lines are no longer straight lines. But for low power they are. One should extrapolate the lines from low-power in order to achieve the cross point and hence the IIP3.

Hence IIP3 is related with non-linear 3rd order distortion, which is the predominant in-band distortion.

Ricardo

Cost is going to be an issue here... Generic Gasfets often used in preamps are your best bet here... but there are some newer devices by Agilent that are smoking good quality.

Under $25 per device, go with the Gasfets...

Under $160 per device, go with the Aglient Devices. As you can figure, the IP3 for the Agilent Devices are much better.

But the IP3 for the common ARR used type Gasfet is not that bad. When you get into "busy places" the better devices pay off. Contact me through the sonic server Email icons if you need additional information.

cheers skipp

: In sci.electronics.basics sthim wrote: : Hi,

: I am working on a rf preamp project and I've been having trouble : finding a transistor. Some of the transistors are not stable for the : frequencies that I'm working on (2 different preamps for 200MHz and : 500MHz) or others do not have the NF I'm looking for (0.5db). I was : hoping if somebody could suggest some transistors... fets, hempts, : anything ...

: Thanks : sthim