Common Collector Configuration

Common Collector Configuration:

Look at the figure, input is applied to the base terminal and output is obtained from the emitter. Whereas the collector is a common terminal to both the input and output signal. This configuration is also called the emitter follower. The common collector has high input impedance and low output impedance. Unity voltage gain and high current gain.

• Input voltage is base-collector voltage
• input current is IB
• The output voltage is the emitter voltage
• output current is IE

Biasing:

In figure 1 (a) shows a transistor in a common collector configuration. A biased transistor is shown in figure 1 (b). As a beginner, you might confuse with the polarities. How to apply proper biasing?

Follow these steps to bias a common collector transistor configuration

• Look at figure 1(a). The direction of currents are indicated by arrows
• Apply biasing voltage such that base-emitter junction becomes forward biased and base-collector junction reverse biased
• The first battery is connected in between the base and collector junction. Connect the positive terminal of the battery to the base of the NPN transistor
• The second battery is connected in between the collector and emitter junction. Connect positive terminal to the collector
• The collector resistance must be zero

Figure: Common Collector Configuration - Input and output characteristics and biasing 

Input Characteristics:

The graph plotted between base current IB and the base-emitter voltage VBC at constant collector-emitter voltage VCE is called input characteristics. Base current IB  is taken along the y-axis (dependent variable) and base-emitter voltage VBC along the x-axis. 

Figure 1(c) shows input characteristics of common collector configuration. CC has different input characteristics from the common emitter and common base configurations.  

Look at the schematic diagram below, 

we can write 

VCE = VBC + VBE

You know that VBE is the small forward voltage drop of 0.7 V. VCE = VCC and VBC = VBB 

VBB = VCE - 0.7… equation 1

From equation 1, it is concluded that VBC is determined by the voltage VCE. 

From the graph in figure 1(c), we can observe that as IB is dependent on VBC.

Output Characteristics:

It is the graph plotted between IE (output current) and (VCE) collector to emitter voltage at fixed base current. IE is plotted along the y-axis and VCE is plotted along the x-axis.

The output characteristic curve is almost similar to the CE characteristic curve. The reason is obvious. IC is approximately equal to IE.  In CE configuration, the characteristic curve is plotted in between IC and VCE. While here the curve is plotted in between IE and VCE. 

Current Amplification:

As you know the current amplification factor is defined by the ratio of output current to the input current. Here input current is base current IB and output current is emitter current IE. Hence the current amplification factor is 

β* = IE / IB

Since IB is very small, so the value of β is quite high. 

β* = ( IC + IB) / IB

β* = β  + 1

Input Resistance:

In this configuration, the input resistance is the ratio of base to collector voltage VBC to the base current IB.

Ri = VBC / IB

Since IB is very small, Ri is quite higher than the CE configuration.

Output Resistance:

In this configuration, output resistance is the ratio of output voltage VCE to the output current IE.

RO = VCE / IE

Voltage Gain:

As you know voltage gain is the ratio of output voltage to the input voltage. Here the input is applied at the base. Input voltage is vbc, output terminal is the emitter. The output voltage is vec.

AV = vec / vbc

AV = ieRL / ibRi

AV = (1+β )ibRL / ibRi

AV = (1+β )RL / Ri

Power Gain:

As I discussed, the power gain is the ratio of output power to the input power. 

Instantaneous input power Pi = i2bRi

Instantaneous output power Po  = i2eRL = (1+β)2ib2RL

AP = (1+β)2ib2RL / i2bRi

AP = (1+β)2RL / Ri

Common Emitter (CE) Configuration

Look at the figure 1(a),  input is applied to the base terminal and output is obtained from collector. Whereas emitter is a common terminal to both the input and output signal. This is common emitter configuration. The common emitter configuration has medium input impedance and medium output impedance. Both the current and voltage gains are high. 

To understand the complete behaviour of a transistor, we need to analyse the input and output characteristics. For this analysis, the transistor must be in active region.

Biasing:

How to design or bias a CE transistor configuration?

Here are a few steps to follow.

• Indicate all the directions of current (consider non transistor)
• Introduce supply voltage. The polarities of the supply should support the directions of IB and IC
• Here we consider electron current (the electron current flow from negative to positive terminal)
• Repeat the same procedure for pnp transistor. For pnp transistor all the polarities are reversed

So, look at the complete circuit shown in figure 1(a) and
1(b)

Figure: Common Emitter Configuration - Input and output characteristics and biasing 

Supply voltages are such that they support the direction of transistor currents. (Means the direction of current from supply is in the same direction as that of transistor currents).  Positive terminal of the supply is connected to the base, makes the base-emitter junction forward biased. While the positive terminal of the supply is connected to collector, makes the base-collector junction reverse biased. 

In common emitter configuration:

• Input voltage is base-emitter voltage
• input current is IB
• Output voltage is collector-emitter voltage
• output current is IC

Input Characteristics:

The graph plotted between base current IB and the base emitter voltage VBE at constant collector emitter voltage VCE is called input characteristics. Base current IB  is taken along y- axis (dependent variable) and base emitter voltage VBE along x-axis. 

From figure 1(c) we can conclude that: 

• The input characteristics curve of CE configuration resembles the diode characteristics curve
• After threshold voltage, the small change in VBE,  IB increases rapidly
• Small dynamic input resistance. It is the ratio of change in VBE to the resulting change in IB , for a given value of collector emitter voltage. 

ri = ∆VBE / ∆IB | VCE = constant

Output Characteristics:

The graph plotted between collector current IC and collector emitter voltage VCE at constant values of base current IB is called output characteristics. Collector current IC is taken along y-axis and collector emitter voltage VCE is taken along x-axis.

From figure 1(d) we can conclude that:

• The value of βdc is simply calculated by the ratio of IC to IB. For βac we take the ratio of small change in IC (∆IC) to the small change in IB (∆IB). It means dynamic output resistance of CE configuration is high  

βac = ∆IC / ∆IB | ∆VCE = 0

• From the graph you can observe, the change in VCE  will cause little change in IC for constant IB. It means dynamic output resistance of CE configuration is high

rO = ∆VCE / ∆IC | IB = constant

• The output characteristics curve has all three regions, active, saturation and cut-off

Current Amplification:

As you know the current amplification factor is defined by the ratio of output current to the input current. Here input current is IB and output current is IC. Hence the current amplification factor is 

β = IC / IB

Since IB is very small, so the value of β is quite high. 

Input Resistance:

Input resistance of CE is the ratio of base to emitter voltage VBE to the base current IB. 

Ri = VBE / IB

IB is negligibly small and VBE is also a small value (VBE =0.7V). So, the input resistance is moderately high.

Output Resistance:

In this configuration, output resistance is the ratio of base to collector voltage VCE to the output current IC.

RO = VCE / IC

Voltage Gain:

Voltage gain is the ratio of output voltage to the input voltage. Input voltage is the small forward base emitter voltage (VBE = 0.7V). Output voltage is the voltage across collector terminal and it is quite high. So, the voltage gain is high for CE configuration.

AV = vce / vbe = ic*Rc

AV = β*ib*RL / ib Ri

Av = β*RL / Ri

Power Gain:

Power gain is the ratio of output power to the input power. 

Instantaneous input power Pi = i2bRi

Instantaneous output power Po  = i2cRL

AP = Po/Pi

AP = β2*i2b*RL / i2bRi

AP = β2 *RL / Ri

Learn more about other configurations

Common Collector Configuration
Common Base Configuration

Understand The Biasing Effect On BJT

Modes Of Operations:

Outline:

• Discuss different modes of operation in BJT
• Active mode
• Cut-off mode
• Saturation mode
• Reverse Active mode
• Transistor operation

As I talked over, there are two types of BJT (NPN and PNP). The operation of both types of transistors is the same except for the direction of current or polarities. So, there is no need to explain both types in detail here. If you can understand the operation of the npn transistor, you will understand the operation of PNP as well. So I am going to discuss the operation of npn only.

A BJT can be used in different ways like signal amplification, digital logic circuits. In signal amplification, it will remain in active mode. While in the digital logic circuit it will work in both saturation and cut-off modes. Now the question is, how do you derive a transistor in different modes? This is what an engineer has to do. It is all about how to bias a transistor. If you understand how to bias a transistor, you can easily design an amplifier or digital logic circuit. 

There are four possible modes of BJT, as shown in the table below. We can get different modes with the help of biasing. Biasing means applying voltage. There are two junctions emitter-base junction (EBJ) and collector-base junction (CBJ). We can bias these two junctions in four different ways and get four different modes of operation.

Modes	Emitter - Base Junction (EBJ)	Collector - Base Junction (CBJ)	Use
Cut off	Reverse biased	Reverse biased	Open switch
Saturation	Forward biased	Forward biased	Closed switch
Forward Active	Forward biased	Reverse biased	Normal amplifier
Reverse Active	Reverse biased	Forward biased	Low gain amplifier

So, the basic principle involves:

• The voltage between two terminals controls the current in the third terminal
• So it is a three-terminal voltage-controlled current source

Active Mode 

To turn on the transistor, EBJ must forward biased. EBJ is not forward biased until 

VBE = Vγ = 0.7 V

BJT is in active mode if and only if it satisfies the condition below

VBE ⩾ 0 

VB - VE ⩾ 0

and 

VCB ⩾ 0 or VBC ≤ 0

VC - VB ⩾ 0

VC > VB > VE

EBJ is forward biased and CBJ is reverse biased. The purpose of the emitter is to emit or inject electrons into the base. The base is lightly doped and very thin, with only a limited number of holes available. The recombination process occurs between electrons from the emitter and holes from the base. But the base has only a few holes, so only a small number of holes recombined with electrons. The collector has collected almost all of these electrons. In this mode change in the collector, the voltage does not affect the collector current. 

Fig 1: collector current Vs collector voltage q 

Saturation Mode:

In saturation mode both the junctions (EBJ and CBJ) are forward biased. Saturation and cut-off modes are used in switching applications. In this mode, the transistor conducts heavily and acts as a closed switch.

VBE > 0

VB - VE > 0

VB > VE

The base should at a higher potential than the emitter. When the base junction is at a higher potential than the base-emitter junction becomes forward biased.

Similarly

VBC > 0

VCE = VCE(sat) = 0.2V

Cut-Off Mode:

In a cut-off mode both the junctions (EBJ and CBJ) are reverse biased. When both junctions are reverse biased, it means there is no flow of electrons. That is 

VBE < 0

VBC< 0

And hence under this condition 

IB = 0 

But there is a small leakage current called collector leakage current (ICEO). 

In this condition, the transistor behaves as an open circuit or open switch. Leakage currents are neglected in most cases. 

Reverse Active Mode:

Just as the cut-off mode is exactly the opposite of saturation mode, similarly reverse active mode is exactly the opposite of active mode. 

So, you can easily evaluate, in reverse active mode EBJ is reversed biased and CBJ is forward biased.

If EBJ is reversed biased then:

   VBE ≤ 0   

VB - VE < 0

Similarly, if CBJ is forward biased, then it satisfies the condition below:

VCB ≤ 0

VC - VB < 0

The quadrant graph of shows the above concepts. 

Fig 2: Quadrant graph

Conclusion:

Now, you are familiar with all four modes. The next step is to learn about biasing. Biasing means the application of proper DC voltage to the different terminals of the transistor. The desired mode of operation is achieved with the help of proper biasing. There are many different biasing methods. I will discuss the methods below.

• Fixed bias or base bias
• Emitter bias
• Collector feedback bias
• Voltage divider bias