Static current amplification

Chapter Static current amplification

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • If you look at Fig. 22-4 closely, you’ll see that there are some bias values at which atransistor won’t give current amplification. If the E-B junction is not conducting, or ifthe transistor is in saturation, the curve is horizontal. A small change (to the left andright) of the base voltage, EB, in these portions of the curve, will cause little or noup-and-down variation of IC.But if the transistor is biased near the middle of the straight-line part of the curvein Fig. 22-4, the transistor will work as a current amplifier.Static current amplificationCurrent amplification is often called beta by engineers. It can range from a factor of justa few times up to hundreds of times.One method of expressing the beta of a transistor is as the static forward currenttransfer ratio, abbreviated HFE. Mathematically, this isHFEIC /IBThus, if a base current, IB, of 1 mA results in a collector current, IC, of 35 mA, HFE35/1 35. If IB 0.5 mA yields IC 35 mA, then HFE 35/0.5 70.This definition represents the greatest current amplification possible with a giventransistor.Static current amplification40522-5At A, simple PNPcircuit using dual-diodemodeling. At B, theactual transistor circuit.