Understanding Loop based and Device Based Algorithms

       Two algorithms--the loop based and the device based, are available for small-signal
       stability analysis. Both algorithms are based on the calculation of Bodes return ratio.
       Loop gain waveform, gain margin, and phase margin are the analysis output.

       The probe parameter must be specified to perform. stability analysis. When it points to a
       current probe or voltage source instance, the loop based algorithm will be invoked; when
       it points to a supported active device instance, the device based algorithm will be
       invoked.

       Loop Based Algorithm

       The loop based algorithm calculates the true loop gain that consists of normal loop gain
       and reverse loop gain. The loop based algorithm requires the probe being placed on the
       feedback loop to identify and characterize the particular loop of interest. The
       introduction of the probe component should not change any of the circuit
       characteristics.

       The loop based algorithm provides accurate stability information for single loop circuits,
       and multiloop circuits in which a probe component can be placed on a critical wire to break
       all loops. For a general multiloop circuit, such a critical wire may not be available. The
       loop based algorithm can only be performed on individual feedback loops to ensure they
       are stable. Although the stability of all feedback loops is only a necessary condition for
       the whole circuit to be stable, the multiloop circuit tends to be stable if all individual
       loops are associated with reasonable stability margins.

       Device Based Algorithm

       The device based algorithm calculates the loop gain around a particular active device.
       This algorithm is often applied to assess the stability of circuit design in which local
       feedback loops cannot be neglected; the loop based algorithm cannot be performed for
       these applications since the local feedback loops are inside the devices, they are not
       accessible from the schematic level or netlist level to insert the probe component.

       With the probe parameter points to a particular active device, the dominant controlled
       source in the device will be nulled during the analysis. The dominant controlled source
       is defined as by nulling this source renders the active device to be passive. The device
       based algorithm produces accurate stability information for a circuit in which a critical
       active device can be identified such that nulling the dominant gain source of this device
       renders the whole network to be passive.