Electronic – Very high frequency FET


I am trying to amplify a very high frequency square wave. The waveform is 3.3Vpp (biased so it's going from 0 to 3.3v), around 145MHz. I'd like to amplify it to around 10Vpp (doesn't matter if it's DC biased or not, but I'd prefer to stick with a single-ended power supply).

The simplest approach seems to be a simple FET switch, but I've had trouble finding the right one. Most have rise times on the order 8-20ns, way too slow.

Ideally, it'd be capable of this sort of speed and be enhancement mode, with a threshold voltage below 3.3V. The only things I've found this fast are some Cree Gan FETs which would be perfect, but they seem to all be depletion mode, with negative threshold voltages.

Doesn't need to be able to handle much power at all, 500mW at most, so something small and SMD would be perfect.

Does what I want exist? Or am I barking up the wrong tree entirely?

ETA: The output of this FET would go into a impedance matching network to match a 50ohm transmission line. I don't know the exact parameters of the network as, as far as I understand, it would depend on the FET.

The source is, as stated, a square wave betweeon 0 and 3.3v, coming into the FET on a 50ohm impedance transmission line.

Best Answer

So you want to drive a resonant circuit at 145MHz? Just do that, perhaps with a unity-gain buffer to provide power gain, at 0.5 watts. Using power = Freq * C * Vrms^2, you have C = power/(Freq * Vrms^2). With Vpeak of 1.6v, Vrms of 1.1v, the C = 0.5 / (145MHz * 1.1 * 1.1) = 3,000 picoFarad. Current will be (using P = I * V) = 0.5/1.1 = 0.4 amps at 3.3 volts peakpeak Power dissipation in such cases has the local power equaling the delivered power, or 0.4 watts.

Just use a class-C circuit, over groundplane, to convert the digital swing into a 0.5 watt output. Motorola/ONNN certainly sells such beasts, and has lots of application notes to guide you to success.


This may help. Or not.


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