Transistor As an Amplifier | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

TRANSISTOR AS AN AMPLIFIER

Common-Emitter Configuration

It is called common-emitter configuration since : emitter is common or reference to both input and output terminals. Emitter is usually the terminal closest to or at ground potential.

 Amplifier design is using connection of CE due to the high gain for current and voltage.

Two set of characteristics are necessary to describe the behavior for CE i.e. input (base terminal) and output (collector terminal) parameters.

Proper Biasing of common-emitter configuration in active region.

I_B is in microamperes compared to milliamperes of I_C.

 I_B will flow when V_BE > 0.7V for silicon and 0.3V for germanium.

Before this value, I_B is very small.

 Base-emitter junction is forward bias. Increasing V_CE will reduce I_B for different values.

Output characteristics for common-emitter npn transistor.

For small value of V_CE (V_CE < V_CESAT, I_C increases linearly with increasing value of V_CE 

and if  V_CE > V_CESAT, I_C does not totally depend on V_CE → constant I_C

 I_B(µA) is very small compared to I_C (mA). Small increase in I_B causes big increase in I_C

 I_B=0A → I_CEO occurs.

Noticing the value when I_C= 0A. There is still some value of current flowing.

 Beta (β) or amplification factor

The ratio of dc collector current (I_C) to the dc base current (I_B) is dc beta (β_dc ) which is dc current gain where I_C and I_B are determined at a particular operating point, Q-point (quiescent point).  It is defined by the following equation:

30 < β_dc < 300 → 2N3904   (IC no.)

On data sheet, β_dc=hfe where h is derived from ac hybrid equivalent circuit. FE are derived from forward-current amplification and common-emitter configuration respectively.

For ac conditions, an ac beta has been defined as the changes of collector current (I_C) compared to the changes of base current (I_B) where I_C and I_B are determined at the operating point. On data sheet, β_ac=hfe  It can defined by the following equation:

From output characteristics of common emitter configuration, find β_ac and β_dc with an

Operating point at I_B=25 mA and V_CE =7.5V

Relationship analysis between α and β