User Guide

Constrained Optimization 4

-5

The power consumption of the cell is the goal (the characteristic

to be minimized) and the gain-bandwidth product is the

inequality constraint.

To continue the example, consider the dependence of power

consumption and gain-bandwidth product on bias current in one

of the amplifier stages. Power consumption is proportional to

the bias current, while the gain-bandwidth product is

proportional to the square root of the bias current. Bias current

must be reduced in order to reduce power consumption. Below

some critical bias current the minimum gain-bandwidth

requirement will be violated. This critical value is the

constrained minimum for this problem.

Feasible and Infeasible Points

The starting point for an optimization can satisfy all the

constraints (a feasible point) or it can violate one or more of the

constraints (an infeasible point). Depending on the feasibility of

the starting point, the PSpice Optimizer does the following:

• From an infeasible point, it attempts to reduce the goals and

to reduce the amount by which the constraints are violated.

• From a feasible point, it attempts to reduce the goals while

keeping the constraints satisfied.

Note

Because the PSpice Optimizer sometimes trades

off reduction of the

oals a

ainst violation of the

constraints to make pro

ress, an iteration can

produce an infeasible point even thou

h the initial

startin

point was feasible.

The PSpice Optimizer can also

handle

equality constraints

where a performance measure is

required to be equal to some

defined value.