Q. With a circuit diagram explain how a transistor in common-base configuration amplify signals.
The common-base terminology is derived from the fact that the base is common to the input and output sides of the configuration. Also the base is the terminal closest to ,or at, ground potential. The input characteristics for a transistor in common base configuration can be obtained by plotting a graph with VBE as x-axis and IE as the y-coordinate at various levels of output voltage VCB. Also the output characteristics can be observed by plotting VCB along x-axis and IC along y-axis for various levels of IE.
The output or collector set of characteristics has three basic regions of interests : the active, cut-off and saturation regions. The active region is the region normally employed for linear (undistorted) amplifiers. In the active region the base-emitter junction is forward biased, while the collector-base junction is reverse-biased.
The signal to be amplified Vs is applied to the input leads, ie. between the emitter and the base. The electrons leaving the base take two paths. A small percentage combines with the holes in the base region and constitute the base current, while the majority of these diffuse towards the collector junction. The collector junction is reverse biased and these electrons can easily cross the junction and form the collector current. This collector current is allowed to pass through a high resistor connected in series with the collector. The voltage developed across the load resistor is the amplified output voltage.
The amount of collector current depend upon the recombination at the base, the smaller the recombination the larger the collector current. A small base current controls large collector current. If the base-emitter voltage is increased slightly, more electrons are injected into the base region, which will be more than the base can use (recombination). This results in a larger collector current. In short, a smallchange in the base voltage cause very large change in the collector current. This is current amplification. The large collector current passing through the load resistor produces high voltage across which will be several times greater than the small input voltage applied to the input.