Common Base Configuration
The common base terminology is derived from the fact that the base is common to both the input and output sides of the configuration.The base is usually the terminal closest to,or at,ground potential.
The figure below showing the notation and symbol used for npn transistor.
In CB configuration, the base is common to both input (emitter) and output (collector). For normal operation, the E-B junction is forward biased and C-B junction is reverse biased.
In CB configuration, IE is +ve, IC is –ve and IB is –ve. So,
VEB=f1 (VCB,IE) and
With an increasing the reverse collector voltage, the space-charge width at the output junction increases and the effective base width ‘W’ decreases. This phenomenon is known as “Earlyeffect”. Then, there will be less chance for recombination within the base region. With increase of charge gradient within the base region, the current of minority carriers injected across the emitter junction increases.The current amplification factor of CB configuration is given by,
α= ∆IC/ ∆IE
The input or driving point characteristics and the output or collector characteristics for a common-base transistor amplifier is shown below
The common-base amplifier can provide a reasonable level of voltage gain but suffers from low input impedance and a current gain of less than one. However, this circuit is used extensively for high-frequency applications because its terminal characteristics at high frequencies are better than those of a common-emitter configuration using the same
The standard common-base configuration also requires two DC supplies rather than the one needed for the common-emitter and common-collector configurations. There is, however, a method to be introduced in this experiment that permits the proper biasing of a common-base amplifier with only a single supply.
1 What is the biasing rule of transistor?
Ans: Emitter base junction must be forward biased and collector base junction must be reversed biased.
2.What are three basic transistor connection,which mode is most preferred and Why?
Ans: Common-Emitter, Common-Base & Common-Collector connections.
Common emitter mode is most preferred because this modes gives Highest voltage gain.