BJT Transistor Biasing: Operational Process & Analysis

Posted on May 1, 2024 in Physics

OPERATIONAL PROCESS

Experiment Steps

1. Check the transistor terminals and status.
2. Assemble the circuit in Fig. 1.1 and measure the following values:
   • V_E = 0V
   • V_C = 5.29V
   • R_3.9K = 3.85KΩ
   • V_BE = 0.65V
   • V_CE = 5.29V
   • R_1.5M = 1.5MΩ
   • I_E = I_C = 0.97mA
   Measure the exact resistor values used in each experiment.
3. Place a bulb near the transistor for 30 seconds, then observe and record:
   • V_CE = 5.18V
   • V_E = 0V
   • I_C = 0.98mA
   • V_C = 5.18V
   Note the transistor’s temperature.
4. Assemble the circuit in Fig. 1.2 and repeat Step 2.
5. Assemble the circuit in Fig. 1.3 and repeat Step 2.
6. Assemble the circuit in Fig. 1.4 and repeat Step 2.
7. Assemble the circuit in Fig. 1.5 and repeat Step 2.
8. Assemble the circuit in Fig. 1.6 and repeat Step 2.

Observations

• The transistor’s temperature increases due to the heat from the bulb.
• V_CE decreases and I_C increases with higher temperatures.

QUESTIONNAIRE

Comparison of Experimental and Theoretical Data

Compare the experimental data with the theoretical analysis and plot the load line and Q points. A table of experimental and theoretical values for each circuit is provided, along with a table of absolute errors.

Effect of Change in β

A change in β causes a shift in the Q point because I_C = βI_B. If β changes while I_B remains constant, I_C and V_CEQ will vary, causing the Q point to shift on the load line.

Load Line Determination

Cutoff and saturation regions are used to determine the load line because they are easy to calculate. In cutoff, I_C = 0, providing the x-coordinate. In saturation, V_CE = 0, providing the y-coordinate. The line drawn through these points defines the operating point (Q) and belongs to the transistor’s characteristic curve.

Biasing Stability

The voltage divider bias configuration (Fig. 1.5) offers the best stability against temperature variations. The fixed bias configuration (Fig. 1.1) provides the worst stability due to its sensitivity to changes in I_CO and β with temperature.

Advantage of Emitter Resistor

An emitter resistor improves stability by keeping DC bias currents and voltages closer to their set points, even under changing conditions like temperature, supply voltage, and β.

Collector Feedback Bias vs. H-Type Bias

Both bias types are stable, but H-type bias is more effective against variations in β and V_BE with temperature.

Universal Bias Circuit Behavior

In a universal bias circuit (voltage divider bias), an increase in β causes I_B to decrease significantly. This is because V_BE = V_B – V_E, and V_B is constant. As β increases, I_E and V_E increase, leading to a decrease in V_BE. This decrease in V_BE offsets the increase in β, making the circuit stable against β variations.

Circuit Design Example

A detailed example is provided for designing the circuit in Fig. 1.5 with I_CQ = 1mA, V_CC = 12V, 50 < β < 150, and I_CBO = 0. The design process involves selecting appropriate resistor values and ensuring stability against temperature variations.

Transistor Characteristics

The experiment used a BC548 transistor with the following characteristics: V_CE = 30V, V_CEOmax = 20V, I_Cmax = 200mA, P_totmax = 300mW, T_jmax = 150°C, V_CBO = 30V, V_BEO = 5V, h_FE = 125-500 (at I_C = 2mA, V_CE = 5V, f = 1kHz).

CONCLUSIONS

The experiment demonstrated the impact of temperature variations on BJT biasing and the importance of choosing appropriate biasing configurations for stability. The voltage divider bias configuration was found to be the most stable, while the fixed bias configuration was the least stable. Using an emitter resistor further enhances stability. The experiment successfully analyzed and designed a BJT amplifier circuit with specific operating point requirements.