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 V_i along with the d.c. source V_EB. The purpose of including V_EB is to keep EB junction always forward biased (because if there is no V_EB 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, I_E –i/p current, I_C –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 I_CO flows from base to collector.

I_C = I_E + I_CO

Since a portion of emitter current I_E  flows through the base ,let remaining emitter current be αI_E .

I_C = αI_E  + I_Co

Characteristics

1. Input characteristics

I/p characteristics is a curve between I_E and emitter base voltage V_EB keeping V_CB constant. I_E is taken along y-axis and V_EB is taken along x-axis. From the graph following points can be noted.

1. For small changes of V_EB there will be a large change in I_E. Therefore input resistance is very small.
2. I_E is almost independent of V_CB
3. I/P resistance , R_i = ΔV_EB / Δ I_E       V_CB =constant

2. Output characteristics

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

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

O/p resistance R_o   = ΔV_EB / Δ I_C       I_E = constant

Region below the curve I_E =0  is known as cut-off region where I_C is nearly zero. The region to the left of V_CB =0 is known as saturation region and to the right of V_CB =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 I_B is input current and I_C 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 = I_C / I_B

2. Relationship between α and β

We know that

divide both numerator and denominator of RHS by I_C, we get

Derivation  of Total output current I_C

We have

Transistor Characteristics

1. i/p characteristics

Input characteristics is a curve between EB voltage (V_EB ) and base current (I_B ) at constant V_CE. 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 V_EB there will be a large change in base current I_B. i.e., input resistance is very small.
3. The base current is almost independent of V_CE.
4. Input resistance , R_i = ΔV_EB / Δ I_B   V _CE = constant

2. Output characteristics

It is the curve between V_CE and I_C at constant I_B. From the graph we can see that,

1. Very large changes of V_CE produces a small change in I_C i.e output resistance is very high.
2. output resistance R_o = ΔV_CE  / ΔI_C  │I_B = constant

Region between the curve I_B =0 is called cut-off region where I_B 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 I_B is the input current and I_E is the output current.

Current relations

1. Current amplification factor (γ)

2. Relationship between α β and γ

Derivation of total output current  I_E

            We know that I_C  =

                                   I_E = I_B + I_C

                                               I_E = I_B  + αI_E + I_CBO

                                   I_E(1-α ) = I_B +  I_CBO

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