psrr simulation

[ghada]

hi,
when i do a psrr simulation and i need to get Vout/Vdd do I set the
Vdd to ac magnitude 1 and dc = 0 or do I give it its DC value required
which is 1.5V?

 

[Andrew Beckett]

ghada wrote, on 11/22/09 15:44:

hi,
when i do a psrr simulation and i need to get Vout/Vdd do I set the
Vdd to ac magnitude 1 and dc = 0 or do I give it its DC value required
which is 1.5V?

If you set the DC value of the supply to 0, the circuit will almost certainly
cease to work and have no small-signal transfer function to the output (because
all the devices in the circuit will be off).

If using AC analysis for this, you should ensure that the circuit is correctly
biased, and then additionally apply an AC magnitude. Since AC is a small-signal
analysis, you can specify any number you like for the input signal (AC
magnitude) because you're only going to see linear gain around the DC operating
point.

But a simpler approach (assuming you're using spectre) is to use the "xf"
analysis. This way you tell xf analysis where the output of your circuit is, and
what the frequency sweep is, and then you can find the transfer function from
every source in your circuit once the simulation has finished. So you can click
on the Vdd supply source and see the transfer function from there, or the Vss
supply source and see the transfer function from there, or the input source, and
see the desired signal transfer function - all from a single simulation.

Regards,

Andrew.