(introduction...)

Title: Bipolar Transistor

Objectives:

- Know the structure and symbols of bipolar transistors
- Able to calculate the current gain
- Understand how the currents in a transistor are split

Figure

Introduction

Amplifier Principle

Fig. 2-1: Amplifier- principle

Small- and Large Signal Amplifier

Fig. 2-2: Pre- and power amplifier

Amplifier circuits provide power gain.

Ex: P input- = 5 mW, P output = 50 W

Transistor structures and symbols

Fig. 2-3: NPN and PNP structure

Transistor currents

(see Fig. 2-4)

V_BB forward biases the emitter diode, forcing the free electrons in the emitter to enter the base. The thin and lightly doped base gives almost all these electrons enough time to diffuse into the collector. These electrons flow through the collector, through R_C, and into the positive terminal of the V_CC voltage source. In most transistors, more than 95% of the emitter electrons flow to the collector, less than 5% flow out the external base lead.

Fig. 2-4: NPN Transistor

Recall Kirchhoff's current law:

=> I_E = I_C + I_B

Fig. 2-5: Transistor currents

Because I_B is very small, for circuit analysis, we can do the following approximation:

I_C is equal to I_E

Current gain

Transistor circuits provide the power gain that is needed in electronic applications. They also provide voltage gain and current gain (bdc). Current gain bdc of a transistor is defined as:

Ex:

I_C = 10 mA

I_B = 40 mA

Transistor connections

Fig. 2-6: Transistor connections

CE CONNECTION

The common emitter (CE) connection is the most widly used transistor connection.

Fig. 2-7: CE amplifier, base biased

Base supply voltage: V_BB
Collector supply voltage: V_CC
Voltage base to ground: V_B
Voltage emitter to ground : V_E
Voltage collector to ground : V_C