NPN biasing

Chapter NPN biasing

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • modeling the behavior of bipolar transistors, so that their operation is easier to understand.A dual-diode NPN transistor model is shown in Fig. 22-3. The base is formed by theconnection of the two diode anodes. The emitter is one of the cathodes, and the collec-tor is the other.402 The bipolar transistor22-3At A, simple NPN circuitusing dual-diodemodeling. At B, theactual transistor circuit.The normal method of biasing an NPN transistor is to have the emitter negative andthe collector positive. This is shown by the connection of the battery in Fig. 22-3. Typi-cal voltages for this battery (although it might be, and often is, a dc power supply) rangefrom 3 V to about 50 V. Most often, 6 V, 9 V, or 12 V supplies are used.The base is labeled “control” in the figure. This is because the flow of currentthrough the transistor depends critically on the base bias voltage, EB, relative to theemitter-collector bias voltage, EC.Zero biasSuppose that the base isn’t connected to anything, or is at the same potential as theemitter. This is zero base bias, sometimes simply called zero bias. How much currentwill flow through the transistor? What will the milliammeter (mA) show?The answer is that there will be no current. The meter will register zero.Recall the discussion of diode behavior from the previous chapter. No current flowsthrough a P-N junction unless the forward bias is at least equal to the forward breakover