Laboratory - Bipolar Transistor Biasing

Laboratory: Bipolar Transistor Biasing In this experiment, we will measure construct a base bias and a voltage-divider bias circuit and compare the stability of the two bias circuits with different transistors. Also we will select appropriate bias resistors for each type of bias circuit. Experimental Apparatus ● Resistors – 470 Ω, , 2.0 kΩ, 6.8 kΩ, 33 kΩ, 360 kΩ, 1.0 MΩ ● 3- 2N3904 Small signal npn transistor ● Breadboard Digital Multimeter Theory Transistor Biasing is the process of setting a transistors DC operating voltage or current conditions to the correct level so that any AC input signal can be amplified correctly by the transistor. A transistors steady state of operation depends a great deal on its base current, collector voltage, and collector current and therefore, if a transistor is to operate as a linear amplifier, it must be properly biased to have a suitable operating point. One of the most frequently used biasing circuits for a transistor circuit is with the self-bias of the emitter-bias circuit where one or more biasing resistors are used to set up the initial DC values of transistor currents, ( IB ), ( IC ) and ( IE ). This self biasing collector feedback configuration is another beta dependent biasing method that requires only two resistors to provide the necessary DC bias for the transistor; The circuit shown is called as a “fixed base bias circuit”, because the transistors base current, IB remains constant for given values of Vcc, and therefore the transistors operating point must also remain fixed; This type of transistor biasing configuration, often called self-emitter biasing, uses both emitter and base-collector feedback to stabilize the collector current even more as resistors RB1 and RE as well as the base-emitter junction of the transistor are all effectively connected in series with the supply voltage, VCC. The common emitter transistor is biased using a voltage divider network to increase stability. The name of this biasing configuration comes from the fact that the two resistors RB1 and RB2 form a voltage or potential divider network across the supply with their center point junction connected the transistors base terminal as shown. Procedure 1. Measure and record the values of the resistors, RB, RC. 2. Build the base bias circuit using two resistors (RB =1.0 MΩ and RC =2.0 kΩ) and 2N3904 transistor. (Refer to schematic on pg.) Connect the circuit to a 12V power supply. 3. Measure the voltage across VRB, VRC, and VC. Record all measurements. Remove the transistor and test the other two transistors in the same circuit. Record the measured values. 4. Compute the values of VRB, VRC, VC, IB and IC. Assume βDC is 200, use βDC to find the collector current IC, the voltage across the collector resistor VRC and the voltage from collector to ground VC. Record all computed values. 5. Measure and record the values of the resistors 470 Ω, 2.0 kΩ, 6.8 kΩ, 33 kΩ. 6. Build and test the voltage-divider bias circuit using four resistors and 2N3904 transistor. (Refer to schematic on pg.) Connect the circuit to a 12V power supply. 7. Measure the voltage across VB, VE, VRC, and VC. Record all measurements. Then remove the transistor and test the other two transistors in the same circuit. Record the measured values. 8. Compute the values VB, VE, VRC, VC and IE using the corresponding equation. Record all computed values. Data Questions Conclusion To conclude, after measuring the voltage drop, we concluded that the voltage across the collector resistor and the voltage from collector to ground for the three transistors varied in the base bias circuit however the voltage-divider bias circuit the voltage drop is the same for the three transistors. In addition, we computed the values of the given DC parameter using the corresponding equation and Ohm’s Law and compare it to the measured values. To compute IE for the voltage-divider bias circuit, we can assume that IE is equivalent to IC since IB is a very small value, therefore IB is open and R1 and R2 are in series.