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Transistors 4.pptx
- 1. Assistant Professor Electronics and Communications Engineering Narula Institute of Technology Arnima Das M. Tech (WBUT), B. Tech (WBUT) Transistor Biasing and Stability 4
- 2. Topics Covered in this lecture • h-parameters in CE configuration • Hybrid model • Expression of • voltage gain • current gain • input & output impedance • Transconductance in Emitter follower circuit
- 3. h-parameters 𝒉𝟏𝟏 = 𝑽𝟏 𝑰𝟏 𝑽𝟐=𝟎 = 𝒉𝒊 This is known as the Input impedance with output short circuit 𝒉𝟏𝟐 = 𝑽𝟏 𝑽𝟐 𝑰𝟏=𝟎 = 𝒉𝒓 This is the fraction of output voltage at input with input open circuited or Reverse voltage gain with input open circuited Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 3
- 4. h-parameters 𝒉𝟐𝟏 = 𝑰𝟐 𝑰𝟏 𝑽𝟐=𝟎 = 𝒉𝒇 This is known as the negative of current gain with output short circuited 𝒉𝟐𝟐 = 𝑰𝟐 𝑽𝟐 𝑰𝟏=𝟎 = 𝒉𝒐 This is the output admittance with input open circuited Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 4
- 5. h-parameters in common emitter bias • For a transistor in Common Emitter mode, the two port network input and output will change as – • Ib will be the input current and Vb the input voltage • Ic will be the output current and Vc the output voltage Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 5 • The type of configuration in which the transistor is operated can also be reflected in the subscripts of the h-parameters with c, b, or e – • E.g. in the typical amplifier configuration, i.e. common emitter bias the parameters can be written as – 𝒉𝒊𝒆, 𝒉𝒓𝒆, 𝒉𝒇𝒆, 𝒉𝒐𝒆
- 6. h-parameters in common emitter bias • 𝒉𝟏𝟏 = 𝜹𝑽𝒃 𝜹𝑰𝒃 𝑽𝒄=𝑪 = 𝒉𝒊𝒆 • 𝒉𝟏𝟐 = 𝜹𝑽𝒃 𝜹𝑽𝒄 𝑰𝒃=𝑪 = 𝒉𝒓𝒆 • 𝒉𝟐𝟏 = 𝜹𝑰𝒄 𝜹𝑰𝒃 𝑽𝒄=𝑪 = 𝒉𝒇𝒆 • 𝒉𝟐𝟐 = 𝜹𝑰𝒄 𝜹𝑽𝒄 𝑰𝒃=𝑪 = 𝒉𝒐𝒆 Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 6 The quantities 𝛿𝑉𝑏, 𝛿𝐼𝑏, 𝛿𝑉 𝑐, 𝛿𝐼𝑐 represent the incremental base and collector voltage and current
- 7. Conversion of h-parameters Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 7
- 8. Advantages of h-parameters • At audio frequency the h-parameters are real numbers, so, reactive elements are absent in two- port devices • h-parameters can be readily measured • They offer simple mathematical expressions for various performance quantities, such as voltage gain, current gain, input & output impedance, Transconductance, etc. • They are convenient to use in circuit analysis and design • A set of h-parameters of the transistor is usually specified by the transistor manufacturers Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 8
- 9. Hybrid model of transistor • The four h-parameters can be used to construct a mathematical model known as the hybrid model or h-parameter equivalent circuit • Here, the input impedance, output admittance, reverse voltage gain and forward current gain all are represented by equivalent linear components Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 9
- 10. Hybrid model of transistor • The hybrid model can be shown in various bias configuration for a transistor • The figure here shows the Common Emitter configuration for the transistor where – • Input terminals are Base (B) and Emitter (E) • Output terminals are Collector (C) and Emitter (E) Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 10
- 11. Emitter follower circuit • In the emitter follower or Common collector amplifier using n-p-n transistor, when the base voltage increases, the emitter current also increases. • So, the voltage drop across the load resistance increases, making emitter more positive • Here, the emitter voltage follows the base voltage • So, it is called Emitter Follower circuit Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 11
- 12. Expression of Gain • Current gain in the emitter follower amplifier can be given as – 𝐴𝐼 = − 𝐼𝑒 𝐼𝑏 = − ℎ𝑓𝑐 1+ℎ𝑜𝑐𝑅𝑒 As, ℎ𝑓𝑐 = − 1 + ℎ𝑓𝑒 & ℎ𝑜𝑐 = ℎ𝑜𝑒 So, 𝐴𝐼 = 1+ℎ𝑓𝑒 1+ℎ𝑜𝑒𝑅𝑒 • Voltage gain in the emitter follower amplifier can be given as – 𝐴𝑉 = 𝑉𝑒 𝑉𝑏 = 𝐴𝐼𝑅𝑒 𝑅𝑏 Assuming, ℎ𝑓𝑒 ≫ 1 & ℎ𝑜𝑒 ≈ 0 So, 𝐴𝑉 = ℎ𝑓𝑒𝑅𝑒 ℎ𝑖𝑒+ℎ𝑓𝑒𝑅𝑒 Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 12
- 13. Expression of impedance • Input impedance in the emitter follower amplifier can be given as – 𝑅𝑖 = 𝑉𝑏 𝐼𝑏 = ℎ𝑖𝑐 + 𝐴𝐼ℎ𝑟𝑐𝑅𝑒 As, ℎ𝑖𝑐 = ℎ𝑖𝑒 & ℎ𝑟𝑐 = 1 So, 𝑅𝑖 = ℎ𝑖𝑒 + 𝐴𝐼𝑅𝑒 • Output impedance in the emitter follower amplifier can be given as – 𝑅𝑜 = 𝑅𝑏 + ℎ𝑖𝑐 𝑅𝑏ℎ𝑜𝑐 + ℎ𝑖𝑐ℎ𝑜𝑐 − ℎ𝑓𝑐ℎ𝑟𝑐 = 𝑅𝑏 + ℎ𝑖𝑒 1 + 𝑅𝑏ℎ𝑜𝑒 + ℎ𝑖𝑒ℎ𝑜𝑒 + ℎ𝑓𝑒 Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 13
- 14. Transconductance • The rate of change of output current with respect to input voltage when output voltage is constant is known as transconductance of a two-port network • For the emitter follower circuit this can be defined as - 𝑔𝑚 = 𝛿𝐼𝑒 𝛿𝑉𝑏 𝑉𝑒=𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 • Therefore, 𝑔𝑚 = ℎ𝑓𝑐 ℎ𝑖𝑐𝛿𝐼𝑏 𝑉𝑒=𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = ℎ𝑓𝑒 ℎ𝑖𝑒 𝑉𝑒=𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 14
- 15. Transconductance • The circuit here shows simplification of the h-parameter model using transconductance. • Here the voltage and current source is replaced by a single source of value 𝑔𝑚𝑉𝐵𝐸 • This is known as the simplified low- frequency hybrid-pi model for BJT Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 15
- 16. Further study • https://www.youtube.com/watch?v=RnClfkGvk_c • https://www.youtube.com/watch?v=wT8fZJIl3Nk • https://www.youtube.com/watch?v=4rcopzJWayQ Arnima Das - Dept. of Electronics and Communications Engineering - Narula Institute of Technology 16