Transistor Configuration

Transistor Configuration
if base is free→input
if collector is free→output
The transistor can be operated in 3 region
1)cut off voltage
the region for $I_E=0$ is called cut off region .This is a high voltage low current region in which the transistor is perfectly off. Hence the both input and output junction are reverse biased
2)Saturation region
The region to left of $V_{CB}=0$ is called saturation region .this is low voltage high current region in which the transistor is perfectly ON .Hence both input and Output junction are forward biased
3)Active region
It is the normal region of operation of a transistor .In active region input junction is forward biased and output junction is reversed biased .The transistor can be used as a switch  operating in saturation and cut off region ,In the active region the transistor can be used as as amplifier


Common Base Configuration
In this,base is common to both input(Emitter) and output(collector)

Input charcteristics

Input current $I_E$ VS Input voltage $(V_{EB})$ for different values of output voltage $(V_{CB})$

After the cut in voltage ,normally 0.7V for silicon and 0.3v Germanium ,$I_E$ increases rapidly with very small increase in $V_{EB}$



$$\bbox[5px,border:2px solid red] { Dynamic input resistance \left. r_i=\frac{ΔV_{EB}}{ΔI_{E}}  \right |_{V_{CB}=constant }  }$$

Output characteristics

It is the curve betweeen collector current $I_c$ and collector base voltage $V_{VB}$ at constant Emitter current $I_E$
      
$$\bbox[5px,border:2px solid red] { The dynamic output resistance \left. r_o=\frac{ΔV_{CB}}{ΔI_{C}}  \right |_{I_{E}=constant }  }$$

Common collector configuration

 Here the emitter is common to both input (base) and Output (collector)
Input charcteristics

$$\bbox[5px,border:2px solid red] { Dynamic input characteristics \left. r_i=\frac{ΔV_{BE}}{ΔI_{B}}  \right |_{V_{CE}=constant }  }$$
Output Characteristics

$$\bbox[5px,border:2px solid red] { The dynamic output resistance \left. r_o=\frac{ΔV_{CE}}{ΔI_{C}}  \right |_{I_{B}=constant }  }$$

Common collector configuration(Emitter flower buffer)

Here the collector is made common to both input (base) and output (Emitter)
Common collector configuration has very high input impedence and very low output impedance
Due to this it is coomonly used for impedance matching
As the current gain is high and voltage gain is nearly unity ,it is also used as buffer
$\frac{v_o}{V_i}=1 ==> V_o=V_i$