Understanding Loop based and Device Based Algorithms
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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
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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 generalmultiloop 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
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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.