Re: MOS switch
yawijaya said:
Hi, thanks a lot, the last one really helps!anyway, what should you do if the datasheet doesn't tell the minimum VDS, it says that typical is 10 or 12.5 V, but that's high though...
the minimum Vds will depends on how much distortion you will tolerate. most mosfets are designed for switching applications, and they want their devices to be in saturation.
for linear application (where you are now), that's not what you want to do. instead, you want to have ample headroom to control the output voltage so it doesn't clip. the 4-5v number I provided you is quite good even for high power applications and is conservative for small signal applications (like yours).
you can design for higher Vds(min) but you may not gain enough to justify the added cost (inefficiencyy, heat dissipation, high voltage components, etc.)
i need to find out how many Id i want and then check on the current-volt curve to see which vds i need and then i can calculate for the Vgate..is that correct?
generally no. for a single-ended small signal amplifier like yours, you want to figure out what the worst load it will be driving. it looks like you are designing an audio amplifier, and your load is likely a preamp. They tend to have a minimum input impedance of 470ohm and usually much higher than that (4.7k or higher).
so if you design to drive a 470ohm load (worst case scenario), you want your output impedance (R1) to be no greater than the load. I typically design so that R1 is 50% - 10% of the load. that means R1 is 220ohm or lower.
once you have that, you will determine the gain of your amp. It is (R1//Rload) / Rs where Rs is the resistor between the source of the mosfet and the ground. say that you want to achieve a gain of 2x, that means (220//470)/2x=68ohm (rough math). that math assumed a mosfet with infinite gain so in real world we will likely have a lower gain thus may require a lower Rs to compensate.
the input impedance is roughly R2//R5. if you are running at a 9v rail and your Vd sits at about 7v. since the mosfet needs Vgs=3-4v to open so the R2/R5 divider will attenuate Vd by a factor of 2x so R2=R5. That means your input impedance is 1/2*R2.
I usually design my amp to have input impedance of no higher than 47k and no lower than 4.7k and typically around 10k so R2=R5=22k.
R4 is there for stability and I will keep it at 1k. you can choose the capacitors to suit your needs - mostly frequency response related. I usually use 0.22u - 0.47u for C2 and 4.7u - 47u for C4.
so here is the final design.
as you can see, it provides reasonably good performance. the gain is slightly less than 2x, due to an imperfect mosfet as well as attenuation caused by R4 (in your schematic).
after this, you will need to figure out its frequency response (ac analysis) to make sure that a) it doesn't oscillate and b) it fits your needs.