lundi 31 janvier 2011

Transistor configuration

We know that, transistor can be used as an amplifier. For an amplifier, two terminals are required to supply the weak signal and two terminals to collect the amplified signal. Thus four terminals are required but a transistor is said to have only three terminals Therefore, one terminal is used common for both input and output.

This gives rise to three different combinations.
1.      Common base configuration (CB)
2.      Common emitter configuration (CE)
3.      Common collector configuration (CC)

1. CB configuration


A simple circuit arrangement of CB configuration for pnp transistor is shown below.

In this configuration, base is used as common to both input and output. It can be noted that the i/p  section has an a.c. source Vi along with the d.c. source VEB. The purpose of including VEB is to keep EB junction always forward biased (because if there is no VEB then the EB junction is forward biased only during the +ve half-cycle of the i/p and reverse biased during the –ve half cycle). In CB configuration, IE –i/p current, IC –o/p current.

Current relations


1.current amplification factor (α)

It is defined as the ratio of d.c. collector current to d.c. emitter current

2. Total o/p current

We know that CB junction is reverse biased and because of minority charge carriers a small reverse saturation current ICO flows from base to collector.

IC = IE + ICO

Since a portion of emitter current IE  flows through the base ,let remaining emitter current be αIE .

IC = αIE  + ICo

Characteristics

  1. Input characteristics


I/p characteristics is a curve between IE and emitter base voltage VEB keeping VCB constant. IE is taken along y-axis and VEB is taken along x-axis. From the graph following points can be noted.
  1. For small changes of VEB there will be a large change in IE. Therefore input resistance is very small.
  2. IE is almost independent of VCB
  3. I/P resistance , Ri = ΔVEB / Δ IE       VCB =constant

2. Output characteristics

o/p characteristics is the curve between IC and VCB at constant IE. The collector current IC is taken along y-axis and VCB is taken along x-axis. It is clear from the graph that the o/p current IC remains almost constant even when the voltage VCB is increased.

i.e. , a very large change in VCB produces a small change in IC. Therefore, output resistance is very high.


O/p resistance Ro   = ΔVEB / Δ IC       IE = constant

Region below the curve IE =0  is known as cut-off region where IC is nearly zero. The region to the left of VCB =0 is known as saturation region and to the right of VCB =0 is known as active region.

2. CE configuration



In this configuration the input is connected between the base and emitter while the output is taken between collector and emitter. For this configuration IB is input current and IC is the output current.

1. Current amplification factor (β)

It is the ratio of d.c. collector current to d.c. base current.
i.e., β = IC / IB

2. Relationship between α and β

We know that
divide both numerator and denominator of RHS by IC, we get

Derivation  of Total output current IC

We have

Transistor Characteristics

1. i/p characteristics



Input characteristics is a curve between EB voltage (VEB ) and base current (IB ) at constant VCE. From the graph following can be noted.

  1. The input characteristic resembles the forward characteristics of a p-n junction diode.
  2. For small changes of VEB there will be a large change in base current IB. i.e., input resistance is very small.
  3. The base current is almost independent of VCE.
  4. Input resistance , Ri = ΔVEB / Δ IB   V CE = constant

2. Output characteristics



It is the curve between VCE and IC at constant IB. From the graph we can see that,
  1. Very large changes of VCE produces a small change in IC i.e output resistance is very high.
  2. output resistance Ro = ΔVCE  / ΔIC  │IB = constant


Region between the curve IB =0 is called cut-off region where IB is nearly zero. Similarly the active region and saturation region is shown on the graph.

3. CC configuration



In this configuration the input is connected between the base and collector while the output is taken between emitter and collector.
Here IB is the input current and IE is the output current.

Current relations

  1. Current amplification factor (γ)


2. Relationship between α β and γ


Derivation of total output current  IE

            We know that IC  =

                                   IE = IB + IC

                                               IE = IB  + αIE + ICBO

                                   IE(1-α ) = IB +  ICBO

3.5 Comparison between CB, CC and CE configuration



Characteristics
CB
CE
CC
1. Input reistance (Ri)

2. Output resistance (Ro)

3. Current amplification    
    factor

4. Total output current

5. Phase relationship between input and output



6. Applications



7. Current gain

8. Voltage gain


low

high



In-phase


For high frequency applications


Less than unity

Very high
low

high


Ic =


Out-of phase


For audio  frequency applications


Greater than unity

Grater than unity
high

low


IE = γIB + γICBO


in-phase


For impedance matching


Very high

Less than unity

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