Structure and use of PNP transistor:

The figure opposite is a schematic presentation of a PNP transistor related to two voltage sources. To construct the transistor conduct appreciable current (on the order of 1 mA) from E to C, V_EB have to be above a minimum value occasionally considered to like the cut-in voltage. The cut-in voltage is generally about 600 mV for silicon BJTs at room temperature but can be different depending upon the type of transistor and its biasing. This applied voltage results the upper P-N junction to 'turn-on' permitting a flow of holes from the emitter into the base. In active mode, the electric field lay between the emitter and the collector (caused by V_CE) causes the majority of these holes to cross the lower P-N junction into the collector to make the collector current I_C. The remainder of the holes again combines with electrons, the majority carriers in the base, forming a current through the base connection to make the base current, I_B. As displayed in the diagram, the emitter current, I_E, is the total transistor current that is the sum of the other terminal currents (that is., I_B=I_B+I_C).

In the figure, the arrows presenting current point in the direction of conventional current - the flow of holes is in similar direction of the arrows since holes carry positive electric charge. The ratio of the collector current to the base current is termed as the DC current gain, in active mode. This gain is generally 100 or more, but robust circuit designs do not depend upon the exact value. The value of this gain for DC signals is considered to as h_FE, and the value of this gain for AC signals is considered to as hfe. Though, when there is no specific frequency range of interest, the symbol β is employed.

It should as well be noted that the emitter current is related to V_EB exponentially. At room temperature, increment in V_EB by approximately 60 mV increases the emitter current by a factor of 10. Since the base current is approximately proportional to the collector and emitter currents, they vary in similar way.

Posted Date: 1/10/2013 6:52:06 AM | Location : United States

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