Lecture (09) Bipolar Junction Transistor 3

Transcription

1 Lecture (09) Bipolar Junction Transistor 3 By: Dr. Ahmed ElShafee ١ I THE BJT AS AN AMPLIFIER Amplification is the process of linearly increasing the amplitude of an electrical signal and is one of the major properties of a transistor. a BJT exhibits current gain (β) When a BJT is biased in the active (or linear) region, ٢

2 DC quantities VBE VCE IC IB IE VB VC AC quantities Vbe Vce Vb Vc r e ٣ An ac voltage, Vs, is superimposed on the dc bias voltage VBB by capacitive coupling as shown. The dc bias voltage VCC is connected to the collector through the collector resistor, RC. ٤

3 The ac input voltage produces an ac base current, which results in a much larger ac collector current. The ac collector current produces an ac voltage across RC, thus producing an amplified, but inverted, internal ac emitter resistance r e is designated in and appears in series with RB. The ac base voltage is The ac collector voltage, Vc, equals the ac voltage drop across RC Since the ac collector voltage is ٥ the ac voltage gain, Av, of the transistor remember that Vb ٦

4 ٧ Example 06 ٨

5 Example 06 ٩ THE BJT AS A SWITCH The second major application area is switching applications. When used as an electronic switch, a BJT is normally operated alternately in cutoff and saturation. Many digital circuits use the BJT as a switch. ١٠

6 ١١ the transistor is in the saturation region because the base emitter junction and the base-collector junction are forward-biased and the base current is made large enough to cause the collector current to reach its saturation value small voltage drop across the transistor of up to a few tenths of a volt normally occurs, which is the saturation voltage, VCE(sat). Conditions in Cutoff Conditions in Saturation ١٢

7 Example 06 ١٣ Example 06 ١٤

8 Example 06 ١٥ Example 06 ١٦

9 A Simple Application of a Transistor Switch ١٧ Example 07 ١٨

10 Example 07 ١٩ Photo transistor In a phototransistor the base current is produced when light strikes the photosensitive semiconductor base region. The collector base pn junction is exposed to incident light through a lens opening in the transistor package. When light strikes the collector base pn junction, a base current, Iλ, is produced that is directly proportional to the light intensity A phototransistor can be either a two lead or a three lead device ٢٠

11 ٢١ Phototransistors are not sensitive to all light but only to light within a certain range of wavelengths. They are most sensitive to particular wavelengths in the red and infrared part of the spectrum, as shown by the peak of the infrared spectral response curve in Figure ٢٢

12 Applications Relay circuits driven by a phototransistor. ٢٣ Optocouplers An optocoupler uses an LED optically coupled to a photodiode or a phototransistor in a single package. Two basic types are LED to photodiode and LED tophototransistor, as shown in Figure ٢٤

13 Transistor Categories Manufacturers generally classify bipolar junction transistors into three broad categories 1. General Purpose/Small Signal Transistors Generalpurpose/small signal transistors are generally used for low or medium power amplifiers or switching circuits. The packages are either plastic or metal cases. dual in-line (DIP) and the small-outline (SO) ٢٥ ٢٦

14 2. Power Transistors Power transistors are used to handle large currents (typically more than 1 A) and/or large voltages. For example, the final audio stage in a stereo system uses a power transistor amplifier to drive the speakers. The metal tab or the metal case is common to the collector and is thermally connected to a heat sink for heat dissipation ٢٧ ٢٨

15 3. RF Transistors RF transistors are designed to operate at extremely high frequencies and are commonly used for various purposes in communications systems and other high frequency applications. ٢٩ Troubleshooting Troubleshooting a Biased Transistor ٣٠

16 out circuit test ٣١ ٣٢

17 In circuit test ٣٣ Thanks,.. See you next week (ISA), ٣٤ I