Subject: Logic Design

Topic: Biasing of BJT

Difficulty: Hard

Input characteristics

• The input characteristics describe the relationship between input current or base current $(I_B)$ and input voltage or base-emitter voltage $(V_{BE})$.
• First, draw a vertical line and a horizontal line. The vertical line represents y-axis and horizontal line represents x-axis. The input current or base current $(I_B)$ is taken along y-axis (vertical line) and the input voltage $(V_{BE})$ is taken along x-axis (horizontal line).
• To determine the input characteristics, the output voltage $V_{CE}$ is kept constant at zero volts and the input voltage $(V_{BE})$ is increased from zero volts to different voltage levels. For each voltage level of input voltage $(V_{BE})$, the corresponding input current $(I_B)$ is recorded.

• A curve is then drawn between input current $I_B$ and input voltage $(V_{BE})$ at constant output voltage $V_{CE}$ (0 volts).
• Next, the output voltage $V_{CE}$ is increased from zero volts to certain voltage level (10 volts) and the output voltage (VCE) is kept constant at 10 volts. While increasing the output voltage ($V_{CE}$), the input voltage $(V_{BE})$ is kept constant at zero volts. - After we kept the output voltage ($V_{CE}$) constant at 10 volts, the input voltage $(V_{BE})$ is increased from zero volts to different voltage levels. For each voltage level of input voltage $(V_{BE})$, the corresponding input current ($I_B$) is recorded.
• A curve is then drawn between input current $I_B$ and input voltage $(V_{BE})$ at constant output voltage $V_{CE}$ (10 volts).
• This process is repeated for higher fixed values of output voltage ($V_{CE}$).
• When output voltage ($V_{CE}$) is at zero volts and emitter-base junction is forward biased by input voltage $(V_{BE})$, the emitter-base junction acts like a normal p-n junction diode. So the input characteristics of the CE configuration is same as the characteristics of a normal pn junction diode.
• The cut in voltage of a silicon transistor is 0.7 volts and germanium transistor is 0.3 volts. In our case, it is a silicon transistor. So from the above graph, we can see that after 0.7 volts, a small increase in input voltage $(V_{BE})$ will rapidly increases the input current ($I_B$).

Output characteristics

• The output characteristics describe the relationship between output current $(I_C)$ and output voltage ($V_{CE}$).
• First, draw a vertical line and a horizontal line. The vertical line represents y-axis and horizontal line represents x-axis. The output current or collector current $(I_C)$ is taken along y-axis (vertical line) and the output voltage ($V_{CE}$) is taken along x-axis (horizontal line).
• To determine the output characteristics, the input current or base current IB is kept constant at 0 μA and the output voltage $V_{CE}$ is increased from zero volts to different voltage levels. For each level of output voltage, the correspondingoutput current $(I_C)$ is recorded.

• A curve is then drawn between output current $I_C$ and output voltage $V_{CE}$ at constant input current $I_B$ (0 μA).
• When the base current or input current $I_B$ = 0 μA, the transistor operates in the cut-off region. In this region, both junctions are reverse biased.
• Next, the input current ($I_B$) is increased from 0 μA to 20 μA by adjusting the input voltage ($V_{BE}$). The input current ($I_B$) is kept constant at 20 μA.
• While increasing the input current ($I_B$), the output voltage ( $V_{CE}$) is kept constant at 0 volts.
• After we kept the input current ($I_B$) constant at 20 μA, the output voltage ( $V_{CE}$) is increased from zero volts to different voltage levels. For each voltage level of output voltage ( $V_{CE}$), the corresponding output current ($I_C$) is recorded.
• A curve is then drawn between output current $I_C$ and output voltage $V_{CE}$ at constant input current $I_B$ (20 μA). This region is known as the active region of a transistor. In this region, emitter-base junction is forward biased and the collector-base junction is reverse biased.
• This steps are repeated for higher fixed values of input current $I_B$ (I.e. 40 μA, 60μA, 80 μA and so on).