Difference Between BJT Configurations

Compare Common Emitter, Common Collector and Common Base Configuration

The article is about all three BJT configurations and their characteristics. You can compare input dynamic resistance Ri, output dynamic resistance RO, current amplification factor Ai, voltage gain AV, power gain AP for different transistor configuration.

Each configuration has its characteristics and area of applications. 

Let's get started. 

Characteristics	Common Emitter	Common Collector	Common Base
Common Terminal	Emitter	Collector	Base
Input Current	IB	IB	IE
Input Voltage	VBE	VBC	VEB
Input Terminal	Base	Base	Emitter
Output Current	IC	IE	IC
Output Voltage	VCE	VEC	VCB
Output Terminal	Collector	Emitter	Collector
Input Dynamic Resistance Ri	High 500 - 5KΩ	Highest 150 - 600KΩ	Low 50 - 500Ω
Output Dynamic Resistance RO	Low 50-500KΩ	Low  100 - 1000Ω	Highest 1 - 10 MΩ
Current Gain Ai	Beta is about 99 β = IC/IB	High (Beta is about 99)	Alpha is less than unity
Voltage Gain Av	High	Less than or equal to unity	High
Power Gain AP	Highest	Less than or equal to unity	Medium
Phase Shift	180° out of phase	0°	0°
Applications	Audio applications	Impedance matching network	High-frequency applications

I have written comprehensive articles on these configurations.

• Common Collector or Emitter Follower Configuration
• Common Base Configuration
• Common Emitter Configuration

Common Base CB Configuration

Common Base Configuration

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

Biasing Of CB Configuration:

Figure 1 (a) shows the CB configuration and figure 1 (b) shows a biased NPN transistor. Now let me explain how to bias a CB transistor configuration. 

• To analyse input and output characteristics of CB configuration, the transistor should be in active mode
• For active mode, base-emitter junction is forward biased and base-collector junction is reverse biased
• To forward bias base-emitter junction, connect the positive terminal of supply to base and negative terminal to the emitter.
• To reverse bias the collector-base junction, connect the positive terminal of supply to the collector and the negative terminal to the base

Input Characteristics Of CB Configuration:

The graph plotted between input current that is emitter current IE and input voltage (emitter to base voltage) VEB at constant output voltage (collector to base voltage) VCB. IE is plotted along the y-axis and VEB along the x-axis.

The input characteristics of CB configuration is exactly similar to CE input characteristics. In CE, the input characteristics curve is plotted between IB and VBE. Since the base emitter junction is forward biased, we expect a similar graph to a forward-biased diode. 

Understanding the effect of VCB on input characteristics curve:

IE increases with increasing VEB. This is the property of a forward-biased diode. Nothing is new. Now understand the effect of VCB on IE.

As you know reverse bias enhances minority carriers. VCB is a reverse biasing voltage. It reverse biases the collector-base junction. The higher value of VCB increases the width of the depletion region at the base-collector junction. As a result, the effective width of the base decreases. Due to a decrease in effective base width, the concentration gradient of holes in the base increases. In other words, there are more charge particles (holes) per unit area. The increased concentration of holes in the base region causes the diffusion of electrons from the emitter. This increases emitter current IE.

And hence, an increase in VCB will increase in IE. 

Output Characteristics Of CB Configuration:

It is the graph plotted between output current (collector current) IC and output voltage (collector to base voltage) VCB at constant input current IE. IC is plotted along the y-axis and VCB along the x-axis.

By observations, the output characteristics of CB is quite similar to CE and CC configuration. The difference is, the saturation region is on the left side of the y-axis (VCB is negative). 

In figure 1(d) each curve starts at IC = 0. IC increases as an increase in VCB. Forward biasing collector-base junction (VCB) causes IC to increase exponentially.  

Current Amplification Of Common Base Configuration:

The current amplification of common Base configuration is less than unity. As you know the current amplification factor is defined by the ratio of output current to the input current. Here input current is IE and outputs current is IC. Hence the current amplification factor is 

α = IC / IE

Since IC is approximately equal to IE , but always less than IE. Hence α is less than unity. 

Input Resistance Of Common Base Configuration:

It has low input resistance. In this configuration, the input resistance is the ratio of base to collector voltage VBE to the base current IB. At constant output voltage VCB

Ri = VEB / IB

Since IB is very small, VBE is also small (VBE = 0.7V). Ri is much smaller than CE and CC configuration.

Output Resistance Of Common Base Configuration:

In CB configuration, output resistance is the ratio of output voltage VCB to the output current IC. At constant  input current IE.

RO = VCB / IC

Voltage Gain Of CB Configuration:

As I discussed earlier, voltage gain is the ratio of output voltage to the input voltage. The output resistance of BJT in the common base configuration is very high. The load resistor (has a small resistance) is in parallel output resistance of BJT. All current will flow through RL. 

AV = vcb / veb

AV = icRL / ieRi

AV = αieRL / ieRi

AV = αRL / Ri

Power Gain:

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

Instantaneous input power Pi = i2eRi

Instantaneous output power Po  = i2cRL = α2i2eRL

AP = Po/Pi

AP = α2i2eRL / i2eRi

AP = α2RL / Ri

Other Configurations:

• Common Emitter Configuration
• Common Collector Configuration

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