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
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
|