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TEC 284

Introduction to Transistors. TEC 284. What are Transistors?. Semiconductor devices that consist of three leads The form the building blocks of the majority of electronic circuitry today

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TEC 284

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  1. Introduction to Transistors TEC 284

  2. What are Transistors? • Semiconductor devices that consist of three leads • The form the building blocks of the majority of electronic circuitry today • They are essentially miniature electronic switches, which maintain logic state i.e. they have two operating positions – on and off • Transistors can be connected in circuits to operate as switches or amplifiers

  3. Examples of Transistors

  4. Transistors • Most transistors used in electronic circuits are • Bipolar Junction Transistors (BJTs), • Junction Field Effect Transistors (JFETs) • Metal Oxide Silicon Field Effect Transistors (MOSFETs) • BJTs are also referred to as Bipolar Transistors • JFETs and MOSFETs function in a similar fashion

  5. BJTs (Bipolar Junction Transistors) • These typically have 3 leads • Base • Emitter • Collector • They can be thought of as two diodes connected back to back • Instead of two separate P regions, only one very thin region is used

  6. Transistors - NPN • When talking about a transistor as two diodes, we refer to the diodes as the base-emitter diode and the base-collector diode • The following diagram shows the symbol for an NPN transistor

  7. Transistors - PNP • It is also possible to make transistors with a PNP configuration • Both NPN and PNP transistors can be made from silicon or germanium

  8. BJT NPN Current Flow • If a voltage source is connected to an NPN transistor, current will flow in the direction shown • The current IB flowing through the base-emitter diode is called the base current

  9. Question • Find the base current in the circuit below given that there is a 0.7V drop across the base-emitter diode • Ans: • IB = (Vs – o.7) / 1 kΩ • IB = (3– o.7) / 1 kΩ = 2.3 mA

  10. Will current flow in the circuit shown?

  11. Answer • No! The base-collector diode is reverse-biased

  12. Basic Operation of a Transistor NPN • When power is connected to the base and collector, currents flowing the circuit demonstrate a characteristic of a transistor known as transistor action • If base current flows in a transistor, collector current will also flow • Base current causes the collector current to flow

  13. PNP Transistors • These work in the same way as NPN bipolar transistors • The only difference is that the diodes are oriented in the reverse direction of the NPN transistor and the polarity of the voltages are different

  14. NPN Transistor – One Power Source • The circuit above uses only one power source which is applied to the base and collector • This is a typical configuration for a Bipolar NPN transistor circuit

  15. NPN Transistor • Base current f lows through the power source, resistor RB and the transistor • The base current flows into the base terminal of the transistor • The base current then flows out of the emitter terminal of the transistor • The base current does not flow through the collector terminal of the transistor

  16. NPN Transistor current flow • The collector current flows through • Resistor RC, transistor and power • source • The collector current does not flow unless the base current is flowing

  17. Current Gain β for a transistor • For transistors, the ratio of the collector current to the base current is constant • The ratio of the two currents is called the current gain of the transistor, β • Typical values of β range from 10 to 300 • The collector current is always larger than the base current • β is referred to in manufacturer’s spec sheets as hFE

  18. Current Gain β for a transistor • β = IC / IB β – Current gain of a transistor IC – Collector current of a transistor IB – Base current of a transistor • From the equation we can see that IC = β IB • If more base current flows, then more collector current flows

  19. Current Gain β for a transistor • β is a different number from one transistor part number to another • Transistors with the same part number have β values within a narrow range of each other • Frequently performed calculations in transistor work involve determining the values of collector or base current for a known value of β

  20. Question • If a transistor has a collector current of 500 mA and a current gain β of 100, what is the value of the base current • β = IC / IB

  21. Answer • β = IC / IB • IB = IC / β • IB = 500 mA / 100 = 5 mA

  22. Questions • Calculate the following values: • IC = 2 A, β = 20. Find IB • IB = 1 mA, β = 100. Find IC • IB = 10 µA, β= 250. Find IC • IB = 0.1 mA, IC = 7.5 mA. Find β

  23. Answers • IC = 2 A, β = 20. Find IB • IB = 1 mA, β = 100. Find IC • IB = 10 µA, β= 250. Find IC • IB = 0.1 mA, IC = 7.5 mA. Find β 1. 0.1 A 2. 100 mA 3. 2500 µA or 2.5 mA 4. 75

  24. Transistor Calculation • Find the value of VC • Steps • Determine IC • Determine the voltage drop across RC (VR) • Subtract VR from the supply voltage RC RB

  25. Answer • IB = 10 / 100 k = 0.1 mA • IC = β x IB • IC = 0.1 mA x 50 = 5 mA • VR = 5 mA x 1 k Ω= 5V • VC = 10 – 5 = 5 V RC RB

  26. Transistor Calculation • Using the same circuit find VC when β = 75

  27. Answer • IB = 10 / 100 k = 0.1 mA • IC = β x IB • IC = 0.1 mA x 75 = 7.5 mA • VR = 7.5 mA x 1 k Ω= 7.5V • VC = 10 – 7.5 = 2.5 V

  28. Transistor Calculation • As seen from the previous circuits, you can set VC to any value by choosing a transistor with an appropriate β or by choosing the correct value of RB • When the base current is sufficient enough to produce a collector voltage of 0 V and the maximum collector current possible, this condition is called saturation

  29. Saturation • IB = 10 / 100 k = 0.1 mA • IC = β x IB • IC = 0.1 mA x 75 = 7.5 mA • VR = 7.5 mA x 1 k Ω= 7.5V • VC = 10 – 7.5 = 2.5 V

  30. Saturation • IB = 10 / 100 k = 0.1 mA • IC = β x IB • IC = 0.1 mA x 100 = 10 mA • VR = 10mA x 1 k Ω= 10V • VC = 10 – 10 = 0 V • Since voltage at the collector VC is 0 V, this point is called saturation

  31. Saturation • Saturation can be compared to a closed mechanical switch • Just as a switch is turned “ON”, the transistor is said to be “turned ON” or just “ON”

  32. Open Switch • When the circuit is broken (i.e. no base current is flowing), this can be compared to an “open switch” • The voltage at VC is 10 V • The transistor is said to be “turned off” or just “OFF”

  33. Junction Field Effect Transistor • The JFET like the BJT is used for switching and amplification applications • JFET is preferred when a high input impedance circuit is needed • BJT has a relatively low input impedance as compared to the JFET • JFET has three terminals • Gate (Same as the Base in BJT) • Drain (Same as the Collector in BJT) • Source (Same as the Emitter in BJT)

  34. JFET Construction • JFET consists of one type of semiconductor material with a channel made of the opposite type of semiconductor material running through it • If the channel is N material, it is an N-channel JFET • If the channel is P material, it is a P-channel JFET • Voltage on the gate controls the current flow through the drain and source (just as the voltage on the base controls the collector current in the BJT)

  35. JFET Construction

  36. JFET Operation • Apply a positive voltage on the drain with respect to the source • This allows current to flow • If the gate is at 0 V, drain current is at its largest value and JFET is “ON” • When negative voltage is applied to the gate, drain current is reduced • As gate voltage gets more negative, the current lessens until cutoff which is when the JFET is “OFF” -

  37. ON vs OFF for BJT and JFET • JFET is ON when 0 V is at the gate • BJT is ON by applying a voltage to the base • JFET is turned OFF by applying a voltage to the gate • BJT is OFF when 0V is at the base • JFET is a “normally ON” device but a BJT is a “normally OFF” device • JFET can be used like a switching device just like a BJT

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