Question about LNA stability resistor

To unconditionally stabilize the amplifier, I put a shunt resistor of 25 ohm after the transistor. Since my Vce is 3V, the resistor is going to consume about 0.2W of power. Is this resistor gonna induce very much heat? I just found that the smaller this resistor, the better the stability. But I don't have any concept about the heat generated by this resistor. I saw that people usually put 100 ohm here. So is it ok to put 20 or 25 ohm here?



Correction: Since it's DC power, I think it should be about 0.4W

Don't know what's happening to my last thread but it seems to be disappeared.

I'm trying to design a LNA at GPS L1 band frequency. To stabilize the amplifier, I put a 20 ohm resistor after the transistor. The stability factor is about 1.4, which I think is close to 1.0. So should I give it more budget for real fabrication? (for example make it larger than 2) Or is it okay to just keep it like this?(Cuz I found that If I want to make the stability factor larger, I have to put a smaller resistor here and the maximum gain will drop a lot.)
Also, for a 3V Vce, I think this resistor is going to consume about 0.4 W of power, is it going to generate too much heat?(I saw people usually put 50, 100 or larger here. I don't have any idea about the heat in real case.)

http://www1.oan.es/reports/doc/IT-CDT-2018-10.pdf
check fig.7,8 maybe it help
Also, stability probably is worse in reality, you may try to ground emitter using via component model with some real substrate.

Thanks a lot.

I used via ground and it still satisfied the stable condition. So is my design okay? Or should I make the stability factor larger?(like larger than 2.0?)

Where are your Matching Circuits ? They may help to improve the stability so no resistor might be needed..
A small amount of series or parallel feedback may also improve the stability.
Design your circuit with matching circuits and -if it's applicable- feedback circuit then play with stability factor if it's still out of spec.

I suggest to start prototyping on FR4 substrate, it helps a lot, and affordable at these frequencies. Best results may be obtained with vector network analyzer or some DIY measuring equipment. As I understand you are using simplest biasing (base bias), I think it may shift s-parameters to unwanted region for some BJTs of same type because of BJT parameters variation (not sure, almost did not used BJTs, mostly use FET transistors)

1. Why did you choose output shunt stabilization (shunt resistor)?
2. Is it the only available stabilization option?
3. Where is the unstable region on the Smith chart?
4. Can shunt stabilization be achieved by connecting R2 in parallel with DC_Feed2? So, with DC_Feed2 shorting R2 at DC, there will be no power dissipated in R2.
5. Is this project for work, hobby or school? If it is the latter, why didn't u consult your lecturer who is paid to help u?



6. Is there any requirement for the input & output to be matched to 50 ohm?

20ohm 0805 resistor will be hot at room temperature (150ma). Google for resistor wattage.

Instead of a shunt 20 ohms resistor use a series 20 ohms resistor placed in the collector.
In this situation the IP3 of the stage will decrease about 1dB but the stability improvement will be better than using a shunt resistor (approach which is never used because charge a lot of DC current).

Thanks a lot for your reply!
1.2. I tried different simulations and found that this is the most effective way.
3. I'm trying to make it unconditionally stable from 0 to 6 GHz, so I don't think I need to look at the stability circles.
4. I tried but I found that it was not effective enough and gain sacrificed a lot.
5. This is a course project. I'm going to ask my instructor later but I want to proceed as much as possible. I should finish the project within about 1 month and 10 days. So I think I may be able to do two fabrications. I don't care about the price very much. I think my instructor will cover some of it but I'm willing to pay it by myself. It's more important to learn.
6. This picture is just a starting point. Matching network should be done after I choose the stabilization method.



I've changed another transistor. This time I'm going to use a 50 ohm resistor and I think I've found the resistor that has proper package and power rating. But I still have a question. If the power rating is 500 mW and the dissipated power is 180 W, how hot will the resistor be in room temperature?

1&2. see 3
3. The output stability circle will tell you which stabilization option is available, i.e. series or shunt? For example, if the unstable region encloses the Smith chart's short circuit point, then the shunt resistor trick is not available; in this case, you will need to use a series resistor.
4. Did I ask whether connecting R2 in parallel with DC_Feed2 will be effective / reduce the gain? I wanted to know if connecting R2 in parallel with DC_Feed2 will obviate the problem of DC flowing thru the resistor, but you didn't answer.
5. If u don't approach your instructor first, how could he realize that this project requires prerequisite courses in amplifier stabilization & matching?
6. Will the stability condition change after the input & output are conjugately matched?

Use a series 0 ohm or 5 ohm resistor at the output of transistor since transistor has a low resistance it will have low consumption.