13 Base Bias: CE Configuration
Learning Objective
Theory Overview
For a bipolar junction transistor to operate properly, the base-emitter junction must be forward biased while the collector-base junction must be reverse biased. This will place VBE at approximately 0.7 volts and the collector current IC will be equal to the base current IB times the current gain β. For small signal devices, the current gain is greater than 100 typically. Thus, IC>>IB and IC≈IE.
The common emitter configuration places the emitter terminal at ground. The base terminal is seen as the input and the collector as the output. Using a fixed base supply, the base current is dependent on the value of the base resistor via Ohm’s law. Consequently, any variation in current gain across a batch of transistors will show up as an equivalent variation in collector current, and by extension, a variation in collector-emitter voltage VCE.
Equipment
| (1) Dual adjustable DC power supply | model: | srn: |
| (1) DMM | model: | srn: |
| (3) Small signal NPN transistors (2N3904) | ||
| (1) 1.2 k Ω resistor ¼ watt | actual: | |
| (1) 330 k Ω resistor ¼ watt | actual: |
Link
Download the datasheet:
Schematics
Procedure
A Quick Check
- A quick and easy way to determine if a transistor is damaged is through the use of the resistance (or diode) function of a multimeter. The multimeter will produce a small current in order to determine the connected resistance value. This current is sufficient to partially forward or reverse bias a PN junction. Thus, for an NPN device, placing the red lead on the base and the black lead on the emitter and collector in turn will produce forward bias on the junctions and the meter will show a low resistance. Reversing the leads will create reverse bias and a high resistance will be indicated. If the leads are connected from collector to emitter, one of the two junctions will be reverse biased regardless of lead polarity, and thus, a high resistance is always indicated. Before proceeding to the next step, check the three transistors using this method to ensure that they are functioning. (Note: some multimeters include a “beta checker” function. This may also be used to determine if the devices are good but the beta value produced should not be considered precise as the measurement current and voltage are most likely different from the circuit in which the transistor will be used.)
Base Bias
- Consider the circuit of Figure 1 with Vbb = 11V, Vcc = 15V, Rb = 330 kΩ and Rc = 1.2 kΩ. Assume VBE = 0.7 volts. Further, assume that beta is 150 (a typical value for this device in this application). Calculate the expected values of IB, IC and IE, and record them in the “Theory” columns of Table 1. Note that the theoretical values will be the same for all three transistors.
- Based on the expected value of IC, determine the theoretical value of VCE and record it in Table 2. Also, fill in Table 2 with the typical (theoretical) beta value of 150.
- Build the circuit of Figure