Question

The DC equivalent circuit for an NPN common emitter circuit is.

I have been asked this question during an online interview.

The question is from DC Load Lines topic in division Transistor Characteristics of Electronic Devices & Circuits

Answer 1

In a common emitter (CE) circuit with an NPN transistor, the DC equivalent circuit is used to analyze the behavior of the transistor in terms of its biasing and load line without considering the AC signals. Here's how to construct the DC equivalent circuit:

Key Elements of the DC Equivalent Circuit:

1. Transistor (NPN) Model:

   • The transistor is represented by its collector-emitter junction (V_CE) and base-emitter junction (V_BE).
   • The transistor is modeled as a controlled current source, where the current flowing from the collector to the emitter depends on the base current IBI_BIB​ and the current gain β\betaβ (or hFEh_FEhF​E) of the transistor.

2. Biasing Resistors:

   • The circuit typically includes resistors for biasing, usually placed in the base (RBR_BRB​), emitter (RER_ERE​), and collector (RCR_CRC​) branches.
   • The resistor RBR_BRB​ is connected to the base of the transistor and is used to set the base current.
   • The resistor RER_ERE​ is placed in the emitter leg and is used to stabilize the operating point.
   • The resistor RCR_CRC​ is placed in the collector leg and helps determine the voltage across the collector.

3. Power Supply:

   • A DC power supply (VCCV_{CC}VCC​) is used to provide the necessary voltage for the circuit.

4. DC Operating Point (Q-point):

   • The DC operating point is determined by the base current (IBI_BIB​), collector current (ICI_CIC​), and emitter current (IEI_EIE​).
   • The load line represents the relationship between the collector-emitter voltage VCEV_{CE}VCE​ and collector current ICI_CIC​, given the values of RCR_CRC​ and VCCV_{CC}VCC​.

Constructing the DC Equivalent Circuit:

1. Remove the AC signals: In the DC analysis, all AC sources (like signal sources) are shorted, and capacitors are replaced by open circuits (since their impedance is very high for DC).

2. Biasing voltages: Use the known biasing values to find VBV_BVB​, VEV_EVE​, and VCV_CVC​. The base-emitter voltage VBEV_{BE}VBE​ is typically around 0.7V for silicon transistors.

3. Calculate the collector current ICI_CIC​: This can be done using Kirchhoff's Voltage Law (KVL) and the transistor's characteristic equations.

Example DC Load Line Analysis:

1. KVL at the collector side:

   VCC=ICRC+VCEV_{CC} = I_C R_C + V_{CE}VCC​=IC​RC​+VCE​
   • Here, VCCV_{CC}VCC​ is the supply voltage, ICI_CIC​ is the collector current, and RCR_CRC​ is the collector resistor.

2. Determine the Q-point: The Q-point is where the DC load line intersects with the transistor's characteristic curves (defined by the equation IC=βIBI_C = \beta I_BIC​=βIB​).

Conclusion:

The DC equivalent circuit for an NPN common emitter transistor typically consists of:

• The NPN transistor itself with the base, collector, and emitter terminals.
• Biasing resistors RBR_BRB​, RCR_CRC​, and RER_ERE​.
• The power supply VCCV_{CC}VCC​.
• The DC load line representing the relationship between ICI_CIC​ and VCEV_{CE}VCE​.

This circuit helps in analyzing the operating point of the transistor without considering AC signal variations.