Electronic Circuits ELCT604 (Spring 2018) Lecture 2 BJT Amplifiers

Transcription

1 Electronic Circuits ELCT604 (Spring 2018) Lecture 2 BJT Amplifiers Assistant Professor Office: C eman.azab@guc.edu.eg 1

2 Analog Voltage Amplifiers Circuit Design and Configurations 2

3 Objective Implementing a voltage amplifier Circuit on the transistor level What are the Specifications for an Ideal Voltage Amp.? Infinite Input Resistance: R in Infinite Voltage Gain: A vo Finite Output Resistance (Short Circuit): R out A vo v v ' out in A v v v out sig R R A in L v A vo R sig R R R in out L 3

4 Signals in Amplifier Circuits Information signals that we want to amplify must be AC signal However, we will have three types of signals in the circuit. Thus; we need to distinguish between them by symbol: Type of Signal Signal Symbol Example DC Signals AC Signals (Small Signals) Instantaneous (Large Signals) Capital Letter and Subscript Small Letter and Subscript Small Letter and Capital Subscript V BE, V CE, I C v be,v ce, i c v BE, v CE, i C 4

5 BJT Modes of Operation Electrical Equations of BJT I-V characteristics 5

6 BJT NPN Modes of Operation Mode BEJ BCJ Equations Condition Cutoff Reverse Reverse I C = I E = I B =0 Active (Forward) Saturation Reverse Active Forward Q Forward Reverse Q Reverse Forward Forward V BE = 0.7 I E = I C + I B I C = β F I B = α F I E β F α F = 1 + β F V BE = 0.7 V BC = 0.5 V CE = 0.2 I E = I C + I B V BC = 0.5 I C = I E + I B I E = β R I B = α R I C β R α R = 1 + β R V BE < 0.7 V BC < 0.5 V BC < 0.5 Or V CE > 0.2 I C < β F I B V BE < 0.7 6

7 BJT Active mode I-V CKHs I C versus V CE The Early effect V BE ic ISexp( ) 1 VT v V CE A r o i v C CE V I A C Q Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 7

8 BJT Large Signal Model in Active Mode Q Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 8

9 BJT Large Signal Analysis Voltage Amplifier using BJT Assume that we have instantaneous input voltage signal v I The instantaneous output voltage v O is measured from collector referring to the ground Using KVLs: v I = v BE v O = v CE v O = V CC i C R C Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 9

10 BJT Large Signal Analysis Voltage Amplifier using BJT The transistor mode of operation depends on the value of v I Cutoff Mode 0 v I 0.5 i C = 0 Active Mode v O = V CC 0.5 v I v BE,sat v O > v CE,sat i C = I S exp v BE V T v O = V CC I S R C exp Saturation Mode v I V T v I v BE,sat v O =

11 BJT as an Amplifier We can easily separate DC and AC Signals (Superposition) Under the assumption that the AC signal amplitude is very small, such that the transistor s mode will remain the same for the complete cycle BJT must work in Active Mode to avoid signal distortion DC Sources are used to make sure BJT operates in Active mode Input terminals of the amplifier are Base/Emitter Output terminals of the amplifier are Collector/Emitter Equivalent Circuit for Small Signal Analysis can be derived v BE = V BE + v be i C = I S exp V BE + v be V T 11

12 BJT as an Amplifier Assume BJT is in active mode & v be <<V T i C = I S exp V BE + v be V T i C = I S exp V BE V T exp v be V T i C I C 1 + v be V T g m = i C v be = I C V T i C = g m v be = βi b Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 12

13 BJT Small Signal Model We can place a resistance between Base and Emitter to have a path for the base current i C = g m v be = βi b r π = v be i b = β g m = V T I B Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 13

14 BJT Small Signal Model Note that: Early effect can be taken into consideration r o = V A I C Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 14

15 Analysis of BJT Amplifiers Objective: Calculate the voltage gain, Input and Output Resistances 1. Determine the DC operating Point (Deactivate AC signals) 2. Calculate the small signal model parameters: g m, r π 3. Replace the BJT with its small signal model (DC sources are deactivated) 4. Analyze the circuit to calculate the voltage gain, Input and Output Resistances 15

16 BJT Amplifiers Configurations 16

17 Common Emitter Amplifier Objective: Calculate the voltage gain, Input and Output Resistances Input terminal Base Output Terminal Collector Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 17

18 Common Emitter Amplifier 1. Calculate the DC Current 2. Calculate g m and r π I C = β 1 + β I I 18

19 Common Emitter Amplifier 3. Draw the equivalent small signal model (Include r o if given) 19

20 Common Emitter Amplifier 3. Draw the equivalent small signal model (Include r o if given) 4. Calculate the gain, input and output Resistance Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. A v = v O v sig = g m (r o R C R L ) R B r π R B r π + R sig R in = R B r π R out = r o R C R L 20

21 Common Emitter Amplifier Notes on Common Emitter Configuration: Inverting Amplifier Gain is greater than unity High Input Resistance High Output Resistance 21

22 Common Base Amplifier Objective: Calculate the voltage gain, Input and Output Resistances Input terminal Emitter Output Terminal Collector 22

23 Common Base Amplifier Voltage gain, input and Output Resistance (r o is neglected) A v = v O = g m(r C R L ) v sig R 1 + S (R E r π 1 + β ) R in = R S + (R E R out = R C R L r π 1 + β ) 23

24 Common Base Amplifier Notes on Common Base Configuration: Non-Inverting Amplifier Gain is greater than unity Low Input Resistance High Output Resistance 24

25 Common Collector Amplifier Objective: Calculate the voltage gain, Input and Output Resistances Input terminal Base Output Terminal Emitter Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 25

26 Common Collector Amplifier Voltage gain, input and Output Resistance A v = v O v sig = 1 + β (r o R L ) r π β (r o R L ) 1 + R sig R B + R sig 26

27 Common Collector Amplifier Voltage gain, input and Output Resistance R in = R B r π β r o R L R out = r o r π + R B R sig 1 + β 27

28 Common Collector Amplifier Notes on Common Collector Configuration: Non-Inverting Amplifier Gain is less than unity Emitter Follower (Buffer) High Input Resistance Low Output Resistance 28

29 Common Emitter with R E Exercise: Find the Voltage gain, input and Output Resistance Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 29