In electronics, a common-base (also known as grounded-base) amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a current buffer or voltage amplifier. Same as in the case of common-base configuration, the EB junction of the common-emitter configuration can also be considered as a forward biased diode, the current-voltage characteristics is similar to that of a diode: The voltage across the forward biased PN junction is approximately. has little effect on. 3 types of transistor configurations which includes Common Emitter (CE), Common Base (CB) & Common Collector (CC) with Input and Output Characteristics.
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|Published:||3 July 2016|
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The common terminal for both the circuits is the base. Current flow in common base amplifier For the sake of understanding, let us consider NPN transistor in common base configuration.
The npn transistor is formed when a single p-type semiconductor layer is sandwiched between two n-type semiconductor layers. Due to the forward bias voltage VBE, the free electrons majority carriers in the emitter region experience a repulsive force from the negative terminal of the common base configuration of bjt similarly holes majority carriers in the base region experience a repulsive force from the positive terminal of the battery.
As a result, free electrons start flowing from emitter to base similarly holes start flowing from base to emitter. Thus free electrons which are flowing from emitter to base and holes which are flowing from base to emitter conducts electric current.
The actual current is carried by free electrons which are flowing from emitter to base. However, we follow the conventional current direction which is from base to emitter. Thus electric current is produced at the base and emitter region.
The free electrons which are flowing from emitter to base will combine with the holes in the base region similarly the common base configuration of bjt which are flowing from base to emitter will combine with the electrons in the emitter region.
The Common-base Amplifier
From the above figure, it is seen that the width of the base region is very thin. Therefore, only a small percentage of free electrons from emitter region will combine with the holes in the base common base configuration of bjt and the remaining large number of free electrons cross the base common base configuration of bjt and enters into the collector region.
A large number of free electrons which entered into the collector region will experience an attractive force from the positive terminal of the battery. Therefore, the free electrons in the collector region will flow towards the positive terminal of the battery.
Thus, electric current is produced in the collector region.
The electric current produced at the collector region is primarily due to the free electrons from the emitter region similarly the electric current produced at the base region is also primarily due to the free electrons from emitter region.
Therefore, common base configuration of bjt emitter current is greater than the base current and collector current. Phase relationships and offsets for PNP common base amplifier. Predicting Voltage Gain Predicting voltage gain for the common-base amplifier configuration is quite difficult, and involves approximations of transistor behavior that are difficult to measure directly.
Common Base (CB) Configuration or Common Base Amplifier
Unlike the other amplifier configurations, where voltage gain was either set by the ratio of two resistors common-emitteror fixed at an unchangeable value common-collectorthe voltage gain of common base configuration of bjt common-base amplifier depends largely on the amount of DC bias on the input signal.
As it turns out, the internal transistor resistance between emitter and base plays a major role in determining voltage gain, and this resistance changes with common base configuration of bjt levels of current through the emitter.
While this phenomenon is difficult to explain, it is rather easy to demonstrate through the use of computer simulation. As the voltage gain changes from one simulation to another, different output voltage amplitudes will be noted.
Gain calculations cannot be based on waveforms that are distorted. The common base circuit stops behaving like a voltage amplifier and behaves like a current follower, as discussed next.
Current follower[ common base configuration of bjt ] Figure 3: Common base circuit with Norton driver; RC is omitted because an active load is assumed with infinite small-signal output resistance Figure 3 shows the common base amplifier used as a current follower.
A note about Phase Shifts In both the the common base and emitter follower configurations, the input and output signals are both in phase. In common emitter configuration common base configuration of bjt, the input and output signals are phase inverted, a positive input resulting in a negative output and vice versa.
The input port is formed by the emitter and base, the output port is formed by the collector and base. The current gain or current transfer ratio of this CB circuit is defined as the ratio between the emitter current treated as the input and the collector current treated as the output, denoted by: