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close this bookAmplifier Teaching Aid (DED Philippinen, 86 p.)
close this folderLesson 2 - Bipolar Transistor
close this folderLesson Plan
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View the documentIntroduction
View the documentTransistor structures and symbols
View the documentTransistor currents
View the documentCurrent gain
View the documentTransistor connections


Title: Bipolar Transistor


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



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)

VBB 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 RC, and into the positive terminal of the VCC 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:

=> IE = IC + IB

Fig. 2-5: Transistor currents

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

IC is equal to IE

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:


IC = 10 mA

IB = 40 mA

Transistor connections

Fig. 2-6: Transistor connections


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

Fig. 2-7: CE amplifier, base biased

Base supply voltage: VBB
Collector supply voltage: VCC
Voltage base to ground: VB
Voltage emitter to ground : VE
Voltage collector to ground : VC