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Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader

Lecture 7. Bipolar Junction Transistor (1). Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader. Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader. By: Dr Tarek Abdolkader. OUTLINE. What is a Bipolar junction transistor?

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Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader

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  1. Lecture 7 Bipolar Junction Transistor (1) Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader By: Dr Tarek Abdolkader

  2. OUTLINE • What is a Bipolar junction transistor? • Construction and symbol • Active mode of operation • Cut-off and saturation modes • BJT characteristics • BJT specifications • BJT as an amplifier • BJT as a switch Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  3. What is a Bipolar junction transistor? • A transistor is a semiconductor device used to amplify and switch electronic signals. • The name transistor is a combination of the words transfer and resistor - a transferred resistor • This is because it can amplify electrical signals by transferring a current I from a low to a high-resistance circuit. Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader • Bipolar junction transistor (BJT) is one of the most famous types of transistors. • Bipolar: current is carried by two types of carriers (electrons and holes). • Junction: current passes through pn-junctions • BJT is a three terminal device. Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  4. Can diode be used for amplification? Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Amplification of signals Current increases with vi but vo has the same amplitude as vi (no amplification) Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  5. Construction and symbol • There are two main types of BJT: • 1. npn transistor2. pnp transistor Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  6. Construction and symbol Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader • The arrow on the emitter is in the direction from p to n. Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  7. Active mode of operation In the normal operation of the transistor (when it is used as an amplifier), The Base-Emitter junction (BE) is forward biased and the Base-Collector junction (BC) is reverse biased. Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  8. Active mode of operation Ef Ef BE is forwarded and BC is reversed at equilibrium p n p n p p         Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader n p n n p n Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  9. Active mode of operation • The emitter is made heavily doped to emit large number of carriers into base. • The base is made very thin so the least number of carriers injected from the emitter is lost by recombination before reaching the collector. • The collector collects the minority carriers from the emitter through the base. The collector usually has the lightest doping concentrations of the three regions. • The main source of current collected at the collector is the current sent from the emitter. The collector own current is very small (reverse-biased junction). • The collector current is controlled by the voltage in a different circuit (VBE). Small changes in VBE leads to large changes in collector current (IC). • The main difference between transistor and p-n junction is that the current is collected in a different circuit with different resistance (transfer resistor). Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  10. Active mode of operation Transistor currents: Hole diffusion in the forward-biased BE junction. Electron diffusion in the forward-biased BE junction (it is less than hole diffusion because base is lightly doped while emitter is heavily doped). 2 3 p n p 4 1 5 6 Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Some of the holes injected from the emitter are recombined with some of the electrons diffusing from the base (small portion because the base is thin). Most of the hole current injected from the emitter is collected by the collector. Minority holes are drifted by the reverse-bias field of the BC junction. Minority electrons are drifted by the reverse-bias field of the BC junction. Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  11. Active mode of operation Transistor equations in active mode: • The common-base dc current gain αDC : αDC < 1 • The common-emitter dc current gain βDC: βDC >> 1 • From KCL: IE = IC + IB IC =βDCIB Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader IE = (1 + βDC ) IB VBE = VBC + VCE • Remember that for forward-biased Si pn junction, VBE = 0.7 V • All above equations are valid only in the active mode. Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  12. Active mode of operation Example Determine all currents and voltages in the circuit shown. βDC = 150 Solution Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  13. Cutt-off and Saturation modes Cut-off mode: BE is reverse-biased and BC is also reverse-biased • No current will be in both circuits except the reverse saturation currents (can be neglected). IB = 0, IC = 0, IE = 0     n p n Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader n p n Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  14. Cutt-off and Saturation modes Saturation mode: BE is forward-biased and BC is also forward-biased n p n n p n     Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader • The field in the BC junction is no longer sufficient to collect all the current injected by the emitter. • In this case, increasing the base current will not result in an increase in the collector current. We say that the current is saturated. In this case IC < βDCIB . • Saturation starts before the BC junction is completely forward-biased (VBC 0.5 ~ 0.7 V , VCE(sat) 0 ~ 0.2 V). Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  15. BJT characteristics npn VBC VBC inverse active saturation Cut-off Active region VBE VBE saturation inverse Cut-off active IC =βDCIB Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader saturation pnp As long as BC junction is reverse biased (VCE > VCE(sat) ), the collector is able to withdraw αDCIE (βDCIB) and the current will depend only on IB and not on VCE. Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  16. BJT characteristics Example Sketch an ideal family of collector curves for the circuit shown for IB= 5 A to 25 A in 5 A increments. βDC = 100 Solution Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader 0.3 V Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  17. Operating point and load line Load line analysis: • Determine IBfrom the input circuit. (Use VBE = 0.7 V for forward-biased BE junction). • Draw the load line and determine the operating point. IC = βDCIB Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader VCC= ICRC + VCE • Note that the onset of saturation for all base currents is nearly the same. This value is called VCE(sat) ~ 0 → 0.2 V Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  18. Operating point and load line Example Determine the mode of operation of the transistor of the circuit shown. Assume VCE(sat) =0.2 V, βDC = 50 Solution There are three methods to solve this problem: Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader • Draw the collector characteristics and the load line. Then determine the Q-point. • Assume the transistor is in the active mode (IC =βDCIB), then find VCE. If VCE > VCE(sat), then the transistor is really in active mode. Otherwise, it is saturation. • Assume the transistor is in the saturation mode (VCE = VCE(sat) ), then find IC. If IC < βDCIB, then the transistor is really in saturationmode. Otherwise, it is active. Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  19. BJT data sheet: Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  20. BJT as an amplifier Vin Ib Ic Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader • Amplification is the process of linearly increasing the amplitude of an ac signal. • The transistor in the active mode amplifies the base current linearly (Ic=βIb ) • Large changes in collector current leads to large changes in the collector voltage Vc. • Voltage gain is the ratio Vc/Vin. Vc Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  21. BJT as a switch Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader • Input voltage is zero (low). • Transistor is in cut-off mode. • Base current is zero. • Collector current is zero. • CE junction is open-circuited. • VCE = VCC (high) • Input voltage is +VBB (high). • Transistor is in saturation mode. • Base current is high. • Collector current is IC(sat) (high) . • CE junction is nearly short-circuited. • VCE = VCE(sat) ~ 0.2 V (low) Transistor switch is called an inverter because low input results in high output and vice versa. Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

  22. BJT as a switch Example For the circuit shown, (a) Find VCE for VIN = 0, (b) Find the maximum value of RB to drive the transistor into saturation with VIN= 5 V, βDC = 200. Neglect VCE(sat). Solution Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Dr Tarek Abdolkader Electronic devices (802311) Lecture 7 Dr Tarek Abdolkader

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