DC analysis calculates the DC operating points of circuits. You can use a DC simulation to generate data on plots; for example, the IV curves of a transistor and annotating DC values on a schematic. It is also the first simulation step when performing other simulations such as linear, harmonic balance, or transient simulations. Any circuit with a nonlinear model or a source with a DC component automatically requires a DC simulation. For example, you might have a nonlinear transistor model that is biased up and you are doing a linear measurement such as (S(2,1)). A DC simulation must occur first to get the linear characteristics of the nonlinear transistor model.

The following are the most common measurements used for the DC simulator alone:

- IDC and VDC are available under the Nonlinear > Current and Nonlinear > Voltage categories when adding or editing measurements. These are DC currents with all AC sources turned off. If you are sweeping power, these values are flat versus power.
- IVCurve and IVCurve2 are available under the Nonlinear > Current category when adding or editing measurements. These are DC measurements used specifically with IVCurve sources to easily plot IV curves of transistors.
- Any schematic annotations beginning with "DC" require the DC simulator only.

One of the most common mistakes users make is to use the IDC measurement in a power amplifier and not understand why the value is constant versus input power. Due to self-biasing effects, the DC current should vary as input power is increased. However, when performing the IDC measurement, none of the AC sources are on; so, self-biasing effects are not included. The proper way to look at the DC value of a circuit under AC drive conditions is to use the Icomp measurement with a harmonic index of 0. For example, see the following graph that shows the IDC and Icomp measurements and the DC value of a power amplifier power sweep.

The measurements agree at low power and then diverge quickly, and the amplifier goes into saturation.

Team SimTech

Cadence Design Systems