single ended common source amplifier with current source load, high Z node

[analog_chip]

Hi,
A single ended common source stage with PMOS current source load has a high Z O/P node. Is it OK to drive a load (say cap) ? Since it is a high Z node the voltage may jump to gnd or vdd due to process variation and either can go to triode.

Also is it OK to have a high Z node in a circuit? or some mechanism to stabilise. I understand cmfb but that is for differential,

 

[godfreyl]

Hi,
A single ended common source stage with PMOS current source load has a high Z O/P node. Is it OK to drive a load (say cap) ?

Yes, but voltage gain will be lower. If you add a source-follower at the output, you can drive lower load impedances and still have high gain.

Since it is a high Z node the voltage may jump to gnd or vdd due to process variation....

That is a problem with any high gain circuit. Normally negative feedback is used to stabilise the quiescent output voltage.

 

[analog_chip]

Thank you godfreyl! Can you please give an example of feedback in a single ended design like CS with current source load?

 

[SIDDHARTHA HAZRA]

Hi,
Also is it OK to have a high Z node in a circuit?

Ya its okay to have high impedance node in a circuit because without it high gain can not be achieved and closed loop operation can not be done.
For Gain: gm (ron || rop) will be high if both rop and ron are high. Replacing the current source PMOS with a diode connected PMOS will make the output node deterministic but will reduce gain to gm/gm (approx)
Closed loop operation: In any loop if a node voltage is well defined by a device, say like the drain of a diode connected MOS, then the loop has no control on it. That voltage is defined just by the device equation.
But all high impedance nodes are loop controlled. So in any control network (negative feedback systems in general) all nodes that gets adjusted by the feedback information is high impedance.

Since you have raised a point, CMFB technique can be applied in the single stage amps (but its a foolish method). We can use a diff amp whose one input is the single stage amp's output and the other input is a reference and the output of this diff amp is connected to the PMOS current source load of the single stage amp. Practically it will work but designing a whole diff amp for a single stage amp sounds foolish. I would rather move completely to a diff amp.

I would like to know if any thing simpler exists...