Transistor: Common Emitter Configuration (or) Characteristics of a Common-Emitter

COMMON EMITTER CONFIGURATION (OR) CHARACTERISTICS OF A COMMON-EMITTER

A transistor, in a common-emitter configuration, has also two important characteristics namely input characteristics and output characteristics.

Input Characteristics:

These curves give the relationship between the base current (I_B) and the base to emitter voltage (V_BE) for a constant collector-to-emitter voltage (V_CE).

Output characteristics:

The curves give the relationship between the collector current (I_C) and collector- to-emitter voltage (V_CE) for a constant base current (I_B).

The potentiometer R₁ and R₂ is used to vary the voltage V_CE and V_BE The DC milli ammeters and DC voltmeters are connected in the base and collector circuit of a transistor to measure the voltage and current.

Input Characteristics of a Common-Emitter Configuration

These characteristics obtained by using the circuit arrangement shown in fig.3.48. The following procedure to obtain the characteristics.

(i) Collector-to-emitter voltage (V_CE) set to 1 volt.

(ii) Increase the base to emitter voltage (V_E) in small steps and record the base current (I_B)

(iii) Plot the graph between V_BE and I_B. It is shown in fig.3.48

From the input characteristics we observed the following information.

(i) There exists a threshold (or) knee voltage (V_K) below which the base current is very small.

(ii) Beyond the knee, the base current (I_B) increase with the increase in base-to-emitter voltage (V_BE) for a constant collector to emitter voltage (V_CE).

(iii) As the collector to emitter voltage (V_CE) is increased above 1V, the curves shift downwards.

(iv) The input characteristics may be used to determine the value of common- emitter transistor a.c input resistance (R_i).

It value is given by the ratio of change in base to emitter voltage (ΔV_BE) to the resulting change in base current (I_B) at a constant collector-to-emitter voltage (V_CE).

R_i = ΔV_BE /ΔΙ_Β | V_CE = Constant

Output Characteristics of Common Emitter Configuration

The characteristics are obtained by the following settings are made:

(i) Adjust the base current (I_B) to 40 μA

(ii) Increase the collector-to emitter voltage (V_CE) in a number of steps and record the collector current I_C.

(iii) A similar procedure may be used to obtain characteristics at I_B = 80 μA, 120 μA, and so on. It is shown in fig.3.49

From the characteristics we observed the point:

(i) The output characteristics may be divided into three regions saturation region, active region and cut-off region. The saturation region and cut-off regions are shown by the shaded.

(ii) As the collector to emitter voltage (V_CE) is increased above zero, the collector current (I_C) increases rapidly to saturation value.

(iii) When the collector to emitter voltage (V_CE) is increased further, the collector current slightly increases. This increase in collector current is due to the fact that increased value of collector-to-emitter voltage (V_CE) reduces the base current and hence the collector current increases. This effect is called an early effects.

(iv) When the base current is zero, a small collector current exists. This is leakage current.

(v) The characteristics may be used to determine the common-emitter transistor a.c output resistance it is given by

R₀ = ΔV_CE /ΔΙ_C

(vi) The characteristic may be used to determine the small-signal common-emitter current gain (or) a.c beta (β_o) of a transistor.

β_o = ΔΙ_C /ΔΙ_Β

Example: 6

Determine the value of I_C and I_E for the circuit shown in fig.

Solution:

I_B = 125 μA, β = 200

We know that (β)

β = I_C/I_B

200 = I_C /125×10^-6

I_C = 200 × 125 × 10^-6

I_C = 25 mA

I_E = I_B + I_C

I_E = 125 × 10^-6 +25 × 10^-3

I_E 25.125 mA

Example: 7

Find the value of I_Cand I_B for the given circuit.

Solution:

I_E = 14 mA, β = 150

I_B = I_E/1+ β

I_B = 14/1+150

I_B = 0.0927 mA

Collector current

I_C =I_E - I_B

I_C = 14 - 0.0927

I_C = 13.907 mA

Example: 8

A BJT has β = 140, calculate the approximate collector and base currents. If the emitter current is 12 mA.

Solution:

I_E = 12 mA, β = 140

We know that common base current path

α = β / β+1 = 140 /140+1 = 0.9929

Common-base current gain α

α = I_C / I_E

I_C = α × I_E

I_C = 0.9929 × 12

I_C = 11.91 mA

We also know that emitter current

I_E = I_B + I_C

I_B = I_E - I_C

I_B = 12 - 11.91

I_B = 0.09 mA

Example: 9

A BJT has a typical β = 110, if the collector is 40 mA. What is the value of emitter current.

Solution:

β = 110, I_C = 40 mA

β = I_C / I_B

110 = 40mA / I_B

I_B = 40 /110 = 0.3636

Ι_B = 0.3636 mA

I_E = I_B + I_C

I_E = 0.3636 + 40

Ι_E = 40.36 mA

Example: 10

In a common base configuration, the emitter current is 6.15 mA and the collector current is 6.05 mA. Determine the common-base current gain.

Solution:

I_E = 6.15 mA, I_C = 6.05 mA

α = I_C / I_E = 6.05 / 6.15 = 0.9837

α = 0.9837