Bipolar Transistor Configurations | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

Bipolar Transistor Configurations

As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responds differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement.

• Common Base Configuration - has Voltage Gain but no Current Gain.
• Common Emitter Configuration - has both Current and Voltage Gain.
• Common Collector Configuration - has Current Gain but no Voltage Gain

COMMON-BASE CONFIGURATION

Common-base terminology is derived from the fact that the : base is common to both input and output of the configuration. Base is usually the terminal closest to or at ground potential. Majority carriers can cross the reverse-biased junction because the injected majority carriers will appear as minority carriers in the n-type material. All current directions will refer to conventional (hole) flow and the arrows in all electronic symbols have a direction defined by this convention.

Note that the applied biasing (voltage sources) are such so as to establish current in the direction indicated for each branch.
 

To describe the behavior of common-base amplifiers requires two sets of characteristics:

1. Input or driving point characteristics.
2. Output or collector characteristics

The output characteristics has 3 basic regions:

• Active region –defined by the biasing arrangements.
• Cutoff region – region where the collector current is 0A.
• Saturation region- region of the characteristics to the left of V_CB = 0V.

The curves (output characteristics) clearly indicate that a first approximation to the relationship between I_E and I_C in the active region is given by

I_C ≈I_E

Once a transistor is in the ‘on’ state, the base-emitter voltage will be assumed to be V_BE = 0.7V

In the dc mode, the level of I_C  and I_E due to the majority charge carriers are related by a quantity called alpha, a= α_dc

I_C = aI_E + I_CBO

It can then be summarized to I_C = aI_E (ignore I_CBO due to small value)

For ac situations where the point of operation moves on the characteristics curve, an ac alpha defined by α_ac.

Alpha is a common base current gain factor that shows the efficiency by calculating the percentage of current flow from emitter to collector. The value of a is typical from 0.9 ~ 0.998.

Biasing:Proper biasing CB configuration in active region by  approximation I_C ≈ I_E (I_B ≈ 0 uA)