DC Operating Point Analysis calculates the behavior of a circuit when a DC voltage or current is applied to it. The result of this analysis is generally referred as the bias point or quiescent point, Q-point. In most cases, the results of the DC Operating Point Analysis are intermediate values for further analysis. For example, in AC Analysis , the DC operating point is first calculated to obtain linear, small-signal models for all nonlinear components (such a diodes and transistors).
Assumptions: AC sources are zeroed out, capacitors are open, inductors are shorted, digital components are treated as a large resistor to ground.
In this example, we calculate the DC value generated at the collector, VC, of the Colpitts oscillator circuit shown in Figure 1. You can get this file, colpitts_oscillator.ms11 from the Downloads section.
Figure 1. Colpitts oscillator circuit.
Complete the following steps to configure and run a DC Operating Point Analysis.
Note: In SPICE, the command that requests a DC operating point calculation is .OP. In this case there are no analysis parameters to be set, therefore the Analysis Parameters tab does not appear.
Figure 2. Settings for the DC Operating Point Analysis.
Figure 3. Grapher View results for the DC Operating Point Analysis.
To verify this result you will analyze the Colpitts oscillator circuit shown in Figure 1. Keep in mind that when doing a DC Operating Point Analysis, inductors are shorted and capacitors are open. Figures 4a and 4b show the reduced circuit.
Figure 4. Reduced circuit to calculate the DC operating point.
Use the following expressions to calculate the collector voltage, VC:
Note that this calculation (VC = 4.81 V) matches the result obtained by the DC Operating Point Analysis in Multisim.
Since the DC Operating Point Analysis is often performed as the first step of other analyses, it is important to know how to solve convergence problems and analysis failures.
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