**Bipolar Junction Transistor:**

The notations utilized here for voltages and currents correspond to the following conventions : DC bias values are mentioned by an upper case letter with upper case subscripts, for example I_{C ,} V_{DS}. Instantaneous values of small-signal variables are mentioned by a lower-case letter with lower-case subscripts, e.g. υ_{s}, i_{c}. Total values are indicated by a lower-case letter with upper-case subscripts, foe instance. υ_{BE}, i_{D}. Circuit symbols for independent source are circular and symbols for controlled sources have a diamond shape. Voltage sources have a ± sign within the symbol and current sources have an arrow.

Figure illustrated the circuit symbols for the NPN and PNP BJTs. The collector-base junction is reverse-biased and the base-emitter junction is forward biased in the active mode. For the NPN device, the active-mode collector and base currents are specified by

i_{C} = I _{S }exp(υ _{BE} /V_{T}) ; i_{B} = i_{C }/ β** **

**Figure: BJT Circuit Symbols**

where V_{T} refer to the thermal voltage, I_{S} refer to the saturation current, and β is refers to the current gain. These are given by

V_{T} = kT/ q = 0.025 V for T = 290; K = 25.86 m V for T = 300 K

I_{S} = I_{SO} (υ_{CE} /V_{A})

and

β= β_{0} (1 + (υ_{CE} /V_{A }))

where V_{A} is the early voltage and I_{SO} and β_{o}, respectively, are the zero values of I_{S} and β.

Because I _{S}/ β = I_{SO}/ β_{0 }, it follows that i is not a function of υ_{CE}. The equations apply to the pnp device if the subscripts BE and CE are reversed.

The emitter-to-collector current gain α is defined as the ratio i_{C }/ i_{E} .To solve for this, we

may write

i _{E} = i _{B} + i _{C }= ((1/ β )+ 1) i _{C} = (1 + β/ β) i _{C}_{ }

It follows that

α= i_{C}/ i_{E } = β/1 + β

Therefore, the three transistor currents are associated by the equation

i_{C} =β i_{B} = α i_{E }