Electronic – How to externally protect a driver FET on a device from load short circuit

Adding as an answer because comment dialog became too long:

AG: A simple set of experiments with a DDS module (AD9850 prebuilt DDS module, less than $5 shipped internationally) should address your concerns. Creamy smooth low THD pure sine waves with an extremely fine frequency resolution of under 0.05 Hertz.

AG: No point debating in theory when practical experiments are so easy, you won't even have to make a PCB.

OP: I looked at at the AD9850 data sheet. That looks great but needs a 125MHz input clock to produce half frequency (62.5 MHz). What is the best input circuit for that? a common Colpitts/Hartely can produce the required clock? Is there any suitable IC for that? If that works, can be definite solution for my question.

AG: You would not need to look for any external "good clock source" - The device uses any standard oscillator crystal: For 10 MHz maximum output, a 20 MHz crystal is sufficient, but use a 40 MHz for some room to grow. Crystals are a few cents each.

AG: Also, the link I provided is a ready-made AD9850 signal generator board with crystal and all associated parts already assembled and tested, so you can initially experiment with it to see how well it meets your requirements.

OP: I got a little confused. the datasheet requires a clock source as CMOS level( squre 0-5 volt wave) or a sine wave with 1/2 volt bias. Are you sure a crystal alone works fine?

Yes, an XO works fine as a clock source. See the first application schematic in the datasheet:

Also see this answer to another question, for more discussion on DDS ICs and sine wave generation.

If you are making a signal generator, then the 50 ohms in the output is a good start. With a 20Vp-p sinewave and the output shorted the resistor will disipate about 1W so firstly choose this resistor to have the power rating but, what op-amp operating at maybe 30MHz can deliver 1W? Probably none so it might be worth adding a push-pull stage made from PNP and NPN emitter followers.

Because they are emitter followers and, because they can be biased (with a little care) to be on the verge of conduction (it's called class AB) taking the op-amp feedback from the output of the push-pull circuit may work OK. Be aware that it can be easy to cause ringing and oscillations so layout is a little critical but methinks you have learned this so far (did you relay the DDS and get it working better?).

The transistors should have an \$H_{FE}\$ that remains above 1 at 1GHz or higher.

Basically, my idea is to build a little power into the final drive stage and use a +1W 50 ohm resistor on the output. Be also aware that the full 20Vp-p may not be attainable using this method - maybe you can increase the power supply levels a little. Also, your choice of op-amp is worth disclosing.