Psat calculation w/ load

Dear RakeshPRG,

Let me try to add a few comments to your questions. See if they make sense to you.

RakeshPRG said:

I am trying to find out Psat for an amplifier. The model uses an input current source of variable magnitude and the output power is plotted with values taken from direct plot->main form

As the magnitude of the input current is increased, the output power is supposed to reach a saturation level yielding the value of Psat. For this, I am using an AC sweep, since my intention is to plot Psat at various frequencies

The typical metric used to characterize the non-linear behavior of an amplifier is its 1 dB compression point. This occurs when the output power falls 1 dB below the power of an ideal amplifier with the same gain. Perhaps this is what you mean by "Psat". The non-linear behavior is not a hard limit, but the amount of compression increases as the input power (or output power) is increased. In your description of the simulation to assess Psat and from your pasted ADE-L analysis, it appears you are using an AC analysis. This is not appropriate since an AC analysis linearizes devices about their DC operating point. Hence, your simulation is not capturing the non-linear device behavior that causes compression. Typically, a number of transient analyses are performed at different frequencies and power levels to assess the 1 dB compression point. A transient simulation will capture non-linear device behavior.

RakeshPRG said:

So does it mean that there is an appropriate load resistor that I need to use for calculation? like a matching load but I suppose that would not work over wide frequencies. Can someone suggest me if this is a correct way for figuring out Psat /any other alternate ways?

The 1 dB compression point will be a function of both frequency and load impedance. For any given frequency, there will be some loads that produce higher 1 dB compression points than others. Similarly, for a given load impedance, the compression point will vary with the input frequency. For a given input frequency, part of the design process is to choose the load impedance that maximizes the 1 dB compression point. 

Do these comments help at all RakeshPRG?

Shawn

In addition to Shawn's very useful points, there's a specific analysis (available for both pss shooting, and the hb anaysis) for finding the n-dB compression point efficiently (it's referred to as xdb analysis in Spectre RF). Rather than doing a simple power sweep (which can be difficult to ensure you have a suitable range over different levels), it does an adaptive search for the compression point. Yes, these are Spectre RF analyses, but they're not just for RF circuits...

Andrew.

In addition to Shawn's very useful points, there's a specific analysis (available for both pss shooting, and the hb anaysis) for finding the n-dB compression point efficiently (it's referred to as xdb analysis in Spectre RF). Rather than doing a simple power sweep (which can be difficult to ensure you have a suitable range over different levels), it does an adaptive search for the compression point. Yes, these are Spectre RF analyses, but they're not just for RF circuits...

Andrew.

Hi Andrew,

Andrew Beckett said:

there's a specific analysis (available for both pss shooting, and the hb anaysis) for finding the n-dB compression point efficiently (it's referred to as xdb analysis in Spectre RF). Rather than doing a simple power sweep (which can be difficult to ensure you have a suitable range over different levels), it does an adaptive search for the compression point.

Thank you for adding his important information and insights for RakeshPRG. I am sure that will be very useful to him as I did not include a path to finding the 1 dB compression point in a more efficient manner than a set fo transient simulations.

Thanks again,

Shawn