##

### 1. Introduction

**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.

### 2. Running DC Operating Point Analysis

In this example, we calculate the DC value generated at the collector, V_{C}, 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**.

- Select
**Simulate»Analyses»DC Operating Point**.

**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.

- Select the
**Variables in circuit** list, select **All variables** from the drop-down list, and then highlight **V(vc)** from the list.

- Click the
**Add** button to move the variable to the right side under **Selected variables for analysis**, as shown below.

**Figure 2.** Settings for the DC Operating Point Analysis.

- Click
**Simulate** to run the **DC Operating Point Analysis**. The **Grapher View** window opens and presents the result for **V(vc)**:

**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.

(a)

(b)

**Figure 4. **Reduced circuit to calculate the DC operating point.

Use the following expressions to calculate the collector voltage,** **V_{C}:

Divider current:

Base voltage:

Emitter voltage:

Emitter current:

Collector current:

Collector voltage:

Note that this calculation (V_{C }= 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.