Junction Transistors

Junction Transistors

Junction transistors consist of two junctions made from N-type and P-type semiconductor materials and are called bipolar transistors (two polarities). They have three connections, emitter, base and collector. Transistor Operation  The forward biased base/emitter junction causes electrons to be attracted from the emitter area towards the base. Arriving in the base area, most of the negative electrons come under the influence of the more positive collector and are attracted by it. This is shown in the left hand drawing, where the base current plus collector current equals the emitter current. Alpha gain is collector current divided by emitter current, and is always less than 1. Beta gain is collector current divided by base current and can be a fairly high number. Therefore, causing a small base current to flow makes a much larger collector current flow. A small base current controls a large collector current. There is 0.6 volts across the base/emitter junction, when it is forward biased. (0.3 volts for germanium). Biasing a Transistor  Choose a general purpose transistor with a beta gain higher than 100. Decide on the collector current. The base bias voltage is be 1/3 of the supply voltage. The current through the base bias potential divider is to be 1/10 of the collector current. Calculate the two base resistor values, R1 and R2. The emitter voltage is 0.6 volts lower than the base voltage. The value of the emitter resistor R4 is the emitter voltage divided by the collector current. The value of the collector resistor R3 is the supply voltage divided by three times the collector current. The values of the capacitors depend upon the application. Study a few circuits. For common collector and common base some of the capacitors are connected differently.