please help: source follower

[v_naren]

hello friends
I have designed a CD circuit as below.

The W/L = 150um/0.18um
its an RFMOSFET

The bias current is 1.221mA. At this value of the current the real part of the Zout is 50Ohms. I am using an ideal capacitor of value 100F!!! for bypassing conceptually...please dont bother about this. I have changed its value to pF range values too but the problem still persists.

The DC bias value of the input voltage source is 1.4V. For tran case I have set the source as
Vi= 250mV * sin(wt) + 1.4V

where w=2.45GHz


**broken link removed** Folder/CD.jpg


I am also showing the DC o/p at the source vs. DC i/p at the gate of the MOSFET for letting u know whhy I biased at 1.4V..for sake of swing
**broken link removed** Folder/DCvsDC.jpg

here is tran o/p at the source of the MOSFET and gate of the MOSFET and across the 50ohm respectively.
**broken link removed** Folder/o1.jpg

**broken link removed** Folder/o2.jpg

**broken link removed** Folder/o3.jpg


can someone please explain why is the negative swing bad??

The mosfet is in the active region always.....and there is an ideal current source at the bottom and not MOSFET.

can someone please explain why is the negative swing bad??


please help...[/img]

 

[rajath]

The decoupling capacitance is definitely too big! I guess the primary reason for that non-linearity is the low biasing current. Maybe the non-linearity persists with a lower decoupling capacitance, as the node capacitance is still large due to the large MOS parasitic capacitance. Increase the current and then check it out.
And please don't post links with spaces in them: either use a hyphen or an underscore.

 

[v_naren]

plaese read your basics properly and my post again and reply

 

[rajath]

If you find anything specifically wrong with my suggestion, mention it. Please don't give "holier than thou" replies. It doesnt help anybody.

 

[Humungus]

I think that the load is too low for the combination of transistor and current source.

When Vin goes high, your transistor can supply enough current to drive the load high. Nevertheless when you try to have a negative swing of 150mV, this means that you must draw a current of 3mA out of your load. As you have an ideal current source of 1.22mA, this is the maximum current that you can obtain from the load, your transistor goes very deep into weak invertion tryin to cut off and then the system is no more a follower.

Anyway, you should not mistreat the others when they are trying to help you, even more when they do it for free. Up to you to think and to distigguish if the answer is right or not.

Regards