How to Determine the DC Operating Point of the Circuit
The DC operating point of a circuit refers to the DC operating point of a solid state circuit such as a diode circuit. The DC operating point of a diode circuit is defined as the current that flows through the diode and the voltage across diode terminals when just DC voltages are applied to the solid state circuit through a resistor or resistor network. The determination of a diode's DC operating point of a diode is usually done graphically using the diode's current-voltage characteristics.
Other People Are Reading
Instructions
-
-
1
Subtract the DC power supply voltage from the "on voltage" of the diode. For silicon diodes use 0.7 Volts, and for geranium diodes use 0.3 volts. Divide this result by the resistor value that is in series with the diode. For this example, use a DC power supply voltage of 10.7 volts, a diode "on voltage" of 0.7 volts and a resistor value of 1000 ohms to obtain 0.01 amperes (10 milliamperes) since 10.7 volts minus 0.7 volts divided by 1000 ohms is 0.01 amperes.
-
2
Examine the graph of the current-voltage characteristics (I-V curve) for the diode you are using. Locate the current axis (the vertical axis, often labeled "Id" for diode current) and mark the point where the diode current equals 10 milliamperes on this axis. This would be the point with coordinates (0V, 10 mA) on the I-V curve. Call this point the "saturation point."
-
-
3
Locate the diode voltage axis (the horizontal axis, often labeled "Vd") and mark the point where the voltage on this axis equals the supply voltage. For this example, since a supply voltage of 10.7 volts was used, you would mark the point with coordinates (10.7V, 0mA) on the I-V curve. Call this point the "cutoff point."
-
4
Draw a line from the cutoff point to the saturation point. Call this line the "load line." Mark the point where the load line intersects the I-V curve of the diode. Call this point the "DC operating point."
-
5
Draw a line vertically down from the DC operating point such that the line intersects the horizontal voltage axis. The voltage on the horizontal axis at the point of intersection is the DC operating voltage.
-
6
Draw a line horizontally left from the DC operating point such that it intersects the vertical diode current axis. The current on the vertical axis at the point of intersection is the DC operating current.
-
1
Tips & Warnings
For different diode circuit arrangements, you determine the short circuit current (the current that would flow through the diode if the diode was shorted) and then you determine the open circuit voltage (the voltage that would appear across the diode or transistor if the diode or transistor was an open circuit). You use these points to plot the load line. You can obtain closer approximations to the actual DC operating point's current and voltage values by using the equation for the current through a diode. This equation requires that you know how to calculate algebraic expressions with exponents in them.
Determining the DC operating point of transistor circuits is similar to the above procedure. However, you will often have to take into account the values of several biasing circuits that may be used in the circuit.
References
- Photo Credit Comstock/Comstock/Getty Images
