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I've browsed through DigiKey and there are these RF FETs. Are they for final stage amplification? Say you have an opamp that doesn't go for the final desired voltage, so one uses these FETs as Common amplifiers?

Is that why they are rated with gain and frequency in the page, rather than Transconductance and rise/fall times?

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  • \$\begingroup\$ Are you aware that a lot of RF fets are JFETs and not MOSFETs - is it this that is confusing you? \$\endgroup\$ – Andy aka Oct 30 '15 at 8:30
  • \$\begingroup\$ I suppose you're from good 'ole Eengland, so you don't visit the site. But all MOSFETs and JFETs (that are not in "RF FETs") are under "FETs - Single". So with that classification, I'm asking, "what exactly is the difference with MOSFETs and JFETs that are not "RF FETs", than those that are". \$\endgroup\$ – Derpy_Merp Oct 30 '15 at 12:40
  • \$\begingroup\$ OK, you know, what??? I'm sorry.... I'm a bit rude. I'm just a bit pissed tonight, is all. I still highly regard your input, Andy. \$\endgroup\$ – Derpy_Merp Oct 30 '15 at 12:41
  • \$\begingroup\$ I believe a lot of the RF fets are built differently, i.e. laterally as opposed to vertically, I think it helps with switching speeds whereas vertical (aka power MOSFETs) are optimized for minimum RDSon and by extension, maximum power handling (but have way higher parasitic capacitances). \$\endgroup\$ – Sam Mar 12 '17 at 7:09
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There are hundreds of types of RF FETs all with their specific (sometimes niche) application. There are RF FETs for high, medium and low power, switching operation or linear. High, medium or low transconductance. Many frequency ranges.

That a certain RF FET is made for a certain application does not mean it cannot be used in a different application. You will have to look at the datesheet to find out if it suits your needs. But some specialist RF FETs can be very expensive so in general you would only use it for it's intended purpose.

"Are they for final stage amplification?"

Some are, some are not.

"Say you have an opamp that doesn't go for the final desired voltage, so one uses these FETs as Common amplifiers?"

You could but as opamps in general aren't considered "RF" using an RF FET with an opamp would be silly. General purpose FETs are good enough for such a task and probably a lot cheaper as well.

"Is that why they are rated with gain and frequency in the page, rather than Transconductance and rise/fall times?"

Yes, in RF the "analog" parameters are important like gain (which is actually the transconductance under specified load conditions), cutoff frequency, input impedance, biasing requirements, noise, etc.

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They are for use as RF amplifiers. If you look at the frequency column, many go up to many GHz.

They are rated with gain and frequency because that's the best model to use when designing the circuit RF function.

They are still FETs, they still have a DC transconductance, which may be handy when checking the DC stability of the bias circuit.

Once you look at the price, power handling and frequency, you'll see why there are different classes of FET for different jobs.

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RF fets are specified for RF .Power mosfets are not so you are on your own when you use them .Power mosfets are optimised for low on resistance at the expense of much greater input capacitance .This can make gate drive more difficult .In fact the product of gate resistance and Cgs makes many of the newer low on resistance cheap power devices useless at RF .The newer low RDS powermos devices are not suitable for analog operation which could be a showstopper in some RF applications.The RF fet has higher RDs on and much lower capacitances and is totaly suitable for analog mode.Lateral construction is a popular way of achieving this.The RF fet costs more to produce and is made by few manufacturers in relatively small numbers these days.The characteristics are more linear which allows good performance with little or no feedback.Decades ago the differences between power and RF fets were not so pronounced but cost and on resistance meant that the BJT was dominant in power conversion.An old school example of such a FET is the VN88AF which I have seen being used for RF and for Audio and for power . The complimentry audio fets that have been available for decades at a price are more like RF fets than power fets.

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