Bipolar Junction Transistors BJT The Two Types of

Bipolar Junction Transistors (BJT)

• The Two Types of BJT Transistors npn pnp

BJT Relationships - Equations

DC and DC = Common-emitter current gain = Common-base current gain = IC IB IE The relationships between the two parameters are: = = +1 1 - Note: and are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

Modes of Operation

Three Types of BJT Biasing

Electrostatic variables for a pnp BJT at equilibrium

Common-Base Circuit Diagram: NPN Transistor

Common-Base Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works. Emitter-Current Curves Saturation Region IC Active Region IE Cutoff IE = 0 VCB

Common-Emitter

Common-Collector

Eber-Moll BJT Model

Common-emitter transistor • Common-emitter transistor ampliers are socalled because the input and output voltage points share the emitter lead of the transistor in common with each other, not considering any power supplies. • ² Transistors are essentially DC devices: they cannot directly handle voltages or currents that reverse direction. In order to make them work for amplifying AC signals, the input signal must be offset with a DC voltage to keep the transistor in its active mode throughout the entire cycle of the wave. This is called biasing.

Common-collector transistor • Common-collector transistor ampliers are so-called because the input and output voltage points share the collector lead of the transistor in common with each other, not considering any power supplies. • ² The output voltage on a common-collector amplier will be in phase with the input voltage, making the commoncollector a non-inverting amplier circuit. • ² The current gain of a common-collector amplier is equal to Ø plus 1. The voltage gain is approximately equal to 1 (in practice, just a little bit less).

Common-base transistor • Common-base transistor ampliers are so-called because the input and output voltage points share the base lead of the transistor in common with each other, not considering any power supplies. • ² The current gain of a common-base amplier is always less than 1. The voltage gain is a function of input and output resistances, and also the internal resistance of the emitterbase junction, which is subject to change with variations in DC bias voltage.

TRANSISTOR MODEL • Common-Base Configuration • Common-base BJT transistor • re model • re equivalent cct.

Therefore, the input impedance, Zi = re that less than 50Ω. For the output impedance, it will be as follows;

Hybrid Equivalent Model The hybrid parameters: hie, hre, hfe, hoe are developed and used to model the transistor. These parameters can be found in a specification sheet for a transistor.

Determination of parameter

General h-Parameters for any Transistor Configuration hi = input resistance hr = reverse transfer voltage ratio (Vi/Vo) hf = forward transfer current ratio (Io/Ii) ho = output conductance

Common emitter hybrid equivalent circuit

Common base hybrid equivalent circuit

Transistor as a Switch

Power Transistor Structure

Switching Characteristics of BJT