1. Bipolar Transistor Review
Transistors are three terminal devices that acts like electrically controlled switches or amplifier controls.
It has either NPN or PNP configuration.
An npn bipolar transistor uses a small input current and positive voltage at its base
(relative to its emitter) to control a much larger collector-to-emitter current
Conversely, a pnp transistor uses a small output base current and negative base voltage (relative to its emitter) to control a larger emitter-to-collector current.

2. Bipolar Transistor Review
Types of Bipolar Transistor
Used to amplify low level signals. Gain is 10 to 500. IC max 80 to 600ma, operating frequency 1 to 300Mhz
Used primarily as switches. Gain is 10 to 200. Max IC rating 10 to 1000mA. Max switching rates between 10 to 2000MhZ
Used to amplify high frequency signals. Gain is 10 to 500 IC max 10 to 600mA, max operating frequency 2000Mhz
Used in high power amplifiers or power supplies. They have heat sinks. Power rating 10 to 300W. Gain is 10 to 500 IC max 1 to 100 A, operating frequency 1 to 100Mhz
Two transistors in one which has more stability at high current levels. Gain is much larger than a single transistor
Light sensitive transistors. When base exposed to light it flows a base current which operates the transistor.

3. Bipolar Transistor Review
An npn bipolar transistor is made by sandwiching a thin slice of p-type semiconductor between two n-type semiconductors.
When no voltage is applied at the transistor base electrons in the emitter are prevented from passing to the collector side because of the depletion region at the p-n junction.

4. Bipolar Transistor Review
When a positive voltage is applied to the base of an npn transistor, the pn junction between the base and emitter is forward biased.
Electron enters into the very thin base region from the emitter.
This process is a trick by which most of the electrons are driven into the collector region and hence heavy current flows from emitter to collector region.
The amount of emitter-to-collector current can be varied by varying the base current.

5. Bipolar Transistor Review
The figure shows a typical characteristic curve for a bipolar transistor.
It describes the effects the base current IB and the emitter-to-collector voltage VEC have on the emitter/collect currents IE and IC.
IE is practically equal to IC

6. Bipolar Transistor Review
Saturation Region: It is the region of operation where the maximum collector current flows and the transistor works like a closed switch from collector to emitter.
Cutoff Region: It refers to the region of operation near the voltages axis of the collector characteristic curve where the transistor acts like an open switch. Only a very small leakage current flows in this mode of operation.
Active mode/region: It describes transistor operation in the region to the right of saturation and above cutoff, where a near-linear relationship exists between terminal currents (IB, IC, IE).
Bias: Bias refers to the specific dc terminal voltages and current of the transistor to set a desired point of active-mode operation or quiescent point (Q-point)

7. Bipolar Transistor Review
Formulae

8. Bipolar Transistor Review

9. RC RB

10. Relay Driver
+12V
120Vac RB