This article discusses the common collector amplifier which is the amplifier topologies. The common collector amplifier is one of the three basic BJT amplifier topologies. In this circuit, the base of the transistor serves as an input, emitter as the output and the collector is grounded that is, common for both emitter and base.
It is also called as an emitter follower. This configuration acts as a buffer. This circuit provides offer low output impedance while taking high input impedance. This configuration is mainly used in digital circuits with logic gates and has many applications. The load resistor in the common collector amplifier being placed in series with the emitter circuit receives both the base current and collector currents.
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Since the emitter of a transistor is the sum of the base and collector currents, since the base and collector currents always add together to form the emitter current, it would be reasonable to assume that this amplifier will have a very large current gain. The common collector amplifier has quite a large current gain, larger than any other transistor amplifier configuration. The characteristics of the common collector amplifier as mentioned below. The common collector characteristics are quite different from the common base and common emitter characteristics. This is because the input voltage Vbc is largely determined by the output voltage Vec.
As Vbc increases with Vec constant Veb decreases hence Ib decreases. Here as Vcc increases Ie also increases. Just as in common emitter output characteristics Ic increases with increasing Ib, so Ie also increases here with the increase in Ib. Hence, for constant Vec, Ie increases with Ib.
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The small signal circuit performance can now be calculated. Total circuit performance is the sum of quiescent and small signal performance. The AC model circuit is shown below. The current gain is defined as the ratio of the load current to the input current. The current gain is dependent only on the BJT characteristics and independent of any other circuit element values. In the below image an ideal class A amplifier is shown. As we can see in the image, there is one active element, a transistor.
The bias of the transistor remains ON all of the time. Due to this never turn off feature, Class A amplifier provides better high frequency and feedback loop stability. Other than these advantages, Class A amplifier is easy to construct with a single-device component and minimum parts count. Despite the advantages and high linearity, certainly, it has many limitations. Due to continuous conducting nature, the class A amplifier introduce high power loss. Also, due to high linearity, Class A amplifier provides distortion and noises.
The power supply and the bias construction need careful component selection to avoid unwanted noise and to minimize the distortion. Because of high power loss in Class A amplifier, it emits heat and requires higher heat sink space. The Class B amplifier is a bit different from the Class A. It is created using two active devices which conduct half of the actual cycle , ie degrees of the cycle. Two devices provide combined current drive for the load. In the above image, an Ideal Class B amplifier configuration has been shown.
It consists two active devices which get biased one by one during the positive and negative half cycle of sinusoidal wave and thus the signal gets pushed or pulled to the amplified level from both positive and negative side and combine the result we get complete cycle across the output. We can see each device input and output signal graph in the below image.
The heat dissipation is minimized in this class providing a low heat sink space.
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But, this class also have limitation. A very profound limitation of this class is the crossover distortion.
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As two devices provides each half of the sinusoidal waves which are combined and joined across the output, there is a mismatch cross over in the region, where two halves are combined. This is because when one device complete the half cycle, the other one needs to provide the same power almost at the same time when other one finish the job. It is difficult to fix this error in class A amplifier as during the active device the other device remains completely inactive.
The error provides a distortion in the output signal. Due to this limitation, it is a major fail for precision audio amplifier application. An alternate approach to overcome the cross-over distortion, is to use the AB amplifier. Same as class B, it has the same configuration with two active devices which conducts during half of the cycles individually but each device biased differently so they do not get completely OFF during the unusable moment crossover moment.
Each device does not leave the conduction immediately after completing the half of the sinusoidal waveform, instead they conduct a small amount of input on another half cycle. Using this biasing technique, the crossover mismatch during the dead zone is dramatically reduced. But in this configuration, efficiency is reduced as the linearity of the devices is compromised. The efficiency remains more than the efficiency of typical Class A amplifier but it is less than the Class B amplifier system.
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Also, the diodes need to be carefully chosen with the exact same rating and need to be placed as close as possible to the output device. In some circuit construction, designers tend to add small value resistor to provide stable quiescent current across the device to minimize the distortion across the output. Class C amplifier is tuned amplifier which works in two different operating modes, tuned or untuned. Class C amplifier uses less than degree conduction angle. During the untuned mode, the tuner section is omitted from the amplifier configuration.
In this operation, Class C amplifier also gives huge distortion across the output. When the circuit is exposed to a tuned load, the circuit clamps the output bias level with the average output voltage equal to the supply voltage. The tuned operation is called as clamper. During this operation, the signal gets its proper shape and the center frequency became less distorted.
The conduction angle is not a factor in such case as the direct input signal is changed with a variable pulse width.
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