Transistor Configuration

When a transistor to be connect in a circuit will require 4 terminals, 2 for the input and two for the output. We will share one terminal for both the input and output terminals and we will overcome this problem. So a transistor can be connect in three type configuration.

1. Common base ( CB ) configuration
2. Common emitter ( CE ) configuration
3. Common collector ( CC ) configuration

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Common base configuration of transistor

In this circuit arrangement input is apply between emitter and base. And output is taken from collector and base. Here the base and transistor is common both input and output circuit and hence the name is common base connection.

Current amplification factor ( α )

Here the output current is I_C and input current is I_E .                                                                                                                                            It is the ratio between the change in collector current to the change in emitter current at constant V_CB collector base voltage is know as current amplification factor α.

So current amplification factor α = ΔI_C / ΔI_E   at constant V_CB .The value of α ranges from 0.9 to 0.99.

Common base input characteristics

It is the curve between emitter current and emitter base voltage at a constant collector base voltage. I_E is taken along y – axis and V_BE taken along x – axis.

The emitter current I_E increases rapidly with very small increasing emitter base voltage that means input resistance is very small. The emitter current is almost independent of collector base voltage V_CB .                                                                                                      So Input resistance  r_i = ΔV_BE / ΔI_E   ,  at constant V_CB .  

Common base output characteristics

It is the curve between collector current I_C and collector base voltage V_CB at constant emitter current I_E .Collector current is taken along y – axis and collector base voltage along x – axis.

The collector current varies with  only at very low voltages and the transistor is never operate in this region. When the value of V_CB is resist above 1 to 2 volt the collector current becomes constant. That means now I_C is independent of V_CB and depends upon I_E only.

A very large change in collector base voltage produces only a small change in collector current. This means the output resistance very high.

So output resistance r_o = ΔV_CB / ΔI_C    , at constant I_E .

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Common emitter configuration of transistor

In this circuit arrangement input is apply between base and emitter. And output is taken from the collector and emitter. Here the emitter is common to both in input and output.                                                                                                                                                The common emitter connection has two characteristic.

1. Input characteristic
2. Output characteristic

Common emitter input characteristics

It is the curve between base current and base emitter voltage a constant collector emitter voltage.

Keeping V_CE is constant the base current value note for various values of V_BE .then plotting the readings taking I_B along y – axis and V_BE along x – axis. This characteristic same as the forward bias diode curve. As compare to common base I_B increases less rapidly with V_BE .

Therefore input resistance of a common emitter circuit is higher than common base circuit.

Input resistance r_i = ΔV_BE / ΔI_B   ,  at constant V_CE . The value of input resistance is the order of a ( few 100 Ω )

Common emitter output characteristics

It is the curve between collector current  and collector emitter voltage  at constant base current I_B .                                                Keeping the base current I_B constant and then collector current is note for various value of V_CE .Then plotting the readings taking I_C along y – axis and V_CE along x – axis .

The collector current I_C varies with V_CE for V_CE between 0 and 1Volt only after this I_C becomes almost constant and independent of V_CE .This value of V_CE up to which collector current I_C changes with V_CE is called the knee voltage ( V_knee ).The transistor are always operated in the region above knee voltage. 

However a small increase in I_C with increasing V_CE is cause by the collector depletion layer getting and capture in a few of more majority charge carrier before hole.

For any value of V_CE above knee voltage collector current I_C ≅ β × I_B .Where β is the current amplification factor for common emitter connection.

 Output resistance r_o = ΔV_CE / ΔI_C  at constant I_B .So the output resistance is the order of 50 kΩ.

current amplification factor ( β )

In Common emitter connection input current is I_B and output current I_C .                                                                                           Current amplification factor β is defined as the ratio between change in collector current to the change in base current.

 So the current amplification factor is β = ΔI_C / ΔI_B

What is CC configuration (Common Collector Configuration)

In this configuration input signal is apply between base and collector. And output is taken across the emitter and collector. Here the collector is common to both input and output circuit. So the name common collector configuration.                                                     The common collector configuration also has two characteristic.

1. Input characteristic
2. Output characteristic

Common collector input characteristics

It is the curve between base current  and base collector voltage  a constant emitter collector voltage _. Here in this curve the input variable is I_B , V_CB and the output variable V_CE is present.

I_B = Input Current , V_CB = Input Voltage , V_CE = Output Voltage

Keeping V_CE is constant at 2V and the base current value is note for various value of V_CB  Then plotting the readings taking I_B along y – axis and V_CB along x – axis. We again keeping V_CE is constant at 4V and note down the base current value for different values of _VCB, then plot the readings.

Common collector output characteristics

It is the curve between emitter current I_E and emitter collector voltage V_CE at constant base current I_{B .} So in this curve we get 3 regions you can see in the graph.

So first we will keep the base current I_B as constant and note the emitter current I_E for various value of V_CE . Now we will increase the base current from 0.  

When the base current is 0 then the transistor also 0 no current will flow, so we call it cut off region. Now the base current is 0, so we will gradually increase it from 0 to 5 μA, after that 10 μA, 15 μA like this, so from this we will call active region. And now the value of I_B = 20 μA it means a lot of current is flowing, so obviously the transistor will also have a lot of current flowing through it, due to which we call saturation region.

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Current amplification factor gamma ( γ )

The ratio of change in emitter current (Δ I_E ) to the change in base current (Δ I_B ) in know as the current amplification factor γ.           So the current amplification factor γ = Δ I_E / Δ I_B .

Total collector current I_C = α I_E + I leakage, ‘I’ represent I leakage = I_CBO ( I_CBO = Collector base current with emitter open )             So now we can write I_C = α I_E + I_CBO                                                                                                                                                           Expression for collector current 

I_C = α I_E + I_CBO

We know I_E = I_B + I_C

I_C = α (I_B + I_C ) + I_CBO = α I_B + α I_C + I_CBO

I_C – α I_C = α I_B + I_CBO

So now we take common IC  

I_C ( 1 – α ) = α I_B + I_CBO

I_C = α I_B / (1 – α ) + I_CBO / (1 – α )                                                                                                                               

So it is about transistor configuration it is basically 3 type of configuration and 2 characteristics input output, I hope you will understand very well which i explain, If you have any doubt from this then comment me and share to your friends, Thank you.