Question

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

I got this question by my college professor while I was bunking the class.

My question comes from DC Load Lines in chapter Transistor Characteristics of Electronic Devices & Circuits

Answer 1

In an NPN common base circuit, the DC equivalent circuit focuses on understanding the biasing of the transistor and its operating point without considering AC signals. Here's a breakdown of the DC equivalent circuit for an NPN common base configuration:

Key Elements of the DC Equivalent Circuit:

1. Transistor Model (NPN Common Base):

   • The NPN transistor in a common base configuration has the emitter as the input, the collector as the output, and the base as the common terminal.
   • The base-emitter junction behaves like a diode and typically has a forward voltage drop of about 0.7V for silicon transistors.

2. Biasing Resistors:

   • Emitter Resistor (RER_ERE​): This resistor is connected to the emitter terminal and is used to stabilize the operating point.
   • Collector Resistor (RCR_CRC​): The resistor in the collector leg helps to set the collector current and the voltage drop across the collector.

3. Power Supply:

   • The circuit uses a DC supply voltage (VCCV_{CC}VCC​), which powers the circuit and determines the overall voltage levels.

4. No AC Signals:

   • In the DC analysis, all AC sources are shorted, and capacitors are considered as open circuits because they have very high impedance for DC signals.

DC Load Line:

• KVL (Kirchhoff’s Voltage Law) for the Collector Loop: The voltage at the collector is determined by the equation:

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

• The DC load line is a graphical representation that shows the relationship between VCEV_{CE}VCE​ (collector-emitter voltage) and ICI_CIC​ (collector current) for given values of VCCV_{CC}VCC​ and RCR_CRC​.

DC Analysis Process:

1. Base-Emitter Junction: The base-emitter voltage VBEV_{BE}VBE​ is typically around 0.7V for silicon transistors.

2. Collector-Emitter Loop: Using Kirchhoff's Voltage Law in the collector loop, we can calculate the collector current ICI_CIC​.

3. Calculate Q-point: The Q-point (quiescent point) is the point where the DC load line intersects with the transistor's characteristic curve. The Q-point gives the values of ICI_CIC​ and VCEV_{CE}VCE​ at zero input signal, representing the stable operating point of the transistor.

Example Calculation:

• For an NPN common base circuit, if:
  • VCC=12VV_{CC} = 12VVCC​=12V
  • RC=1kΩR_C = 1k\OmegaRC​=1kΩ
  • IE≈ICI_E \approx I_CIE​≈IC​ (since base current IBI_BIB​ is much smaller than ICI_CIC​)
  • VBE=0.7VV_{BE} = 0.7VVBE​=0.7V

The collector current can be approximated as:

IC=VCC−VCERCI_C = \frac{V_{CC} - V_{CE}}{R_C}IC​=RC​VCC​−VCE​​

The Q-point is determined by solving these equations with the given values.

Conclusion:

The DC equivalent circuit for an NPN common base configuration includes:

• An NPN transistor where the emitter is the input, the base is common, and the collector is the output.
• Biasing resistors RCR_CRC​ and RER_ERE​ to control the current flow.
• The DC power supply VCCV_{CC}VCC​.
• The load line representing the relationship between collector current and collector-emitter voltage.

This setup helps determine the transistor's operating point (Q-point) for DC analysis.