Electronic Circuits for Mechatronics ELCT 609 Lecture 5: BJT Voltage Amplifiers Assistant Professor Office: C3.315 E-mail: eman.azab@guc.edu.eg 1
BJT Modes of Operation Electrical Equations of BJT 2
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 Forward Reverse Q 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 3
BJT NPN I-V Characteristics I C versus V BE and V CE I C = I s exp V BE V T For Active ONLY Q 4
BJT NPN I-V Characteristics 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 5
BJT Large Signal Model in Active Mode Q 6
BJT Voltage Amplifiers Design and Analysis of BJT Amplifiers 7
Objective We want to implement an analog voltage amplifier using BJT What are the Specifications of an Ideal Voltage Amp.? Infinite Input Resistance: R in (Thevinen at the Input Port) A vo Infinite Voltage Gain: A vo Finite Output Resistance (Short Circuit): R out (Thevinen at the Output Port) v v ' out in R A v v v out sig R A in L v A vo R sig R R R in out L 8
Important Notes BJT Amplifiers deals with DC and AC signals, Thus we need to have different symbols to distinguish between them 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 9
BJT Amplifier Large Signal Analysis Voltage Amplifier using BJT Assume that we have instantaneous input voltage signal v I v BE = v I v O = V CC i C R C For Active Mode ONLY i C = I S exp v I V T Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 10
BJT Amplifier Large Signal Analysis Voltage Amplifier using BJT v O = V CC i C R C 1. Cutoff Mode 0 v I 0.5 v O = V CC 2. Active Mode 0.5 v I v BE,sat v O > v CE,sat v O = V CC I S R C exp v I V T 3. Saturation Mode v I v BE,sat v O = V CE,sat 11
BJT Amplifier Large Signal Analysis 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 set the DC Operating point to 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 12
BJT Small Signal Model Derivation 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. 13
BJT Small Signal Model Derivation 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. 14
BJT Small Signal Model Derivation Note: Early effect can be taken into consideration (if V A is given) r o = V A I C 15
Analysis of BJT Amplifiers Objective: Calculate the Voltage gain, Input Resistance and Output Resistance Solution Steps: 1. Determine the DC operating Point (Deactivate AC signals & All External Capacitors impedances are considered open Circuit) 2. Calculate the small signal model parameters: g m, r π 3. Replace the BJT with its small signal model (DC sources are deactivated& All External Capacitors impedances are considered Short Circuit) 4. Analyze the circuit to calculate the voltage gain, Input and Output Resistances 16
BJT Amplifier Configurations Basic Configurations and their Characteristics 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. 18
Common Emitter Amplifier 1. Calculate the DC Current 2. Calculate g m and r π I C = β 1 + β I I 19
Common Emitter Amplifier 3. Draw the equivalent small signal model (Include r o if given) 20
Common Emitter Amplifier 3. Draw the equivalent small signal model (Include r o if given) 4. Calculate the gain, input and output Resistance 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 Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 21
Common Emitter Amplifier Notes on Common Emitter Configuration: Inverting Amplifier Gain is greater than unity High Input Resistance High Output Resistance 22
Common Base Amplifier Objective: Calculate the voltage gain, Input and Output Resistances Input terminal Emitter Output Terminal Collector 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 ) 24
Common Base Amplifier Notes on Common Base Configuration: Non-Inverting Amplifier Gain is greater than unity Low Input Resistance High Output Resistance 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. 26
Common Collector Amplifier Voltage gain, input and Output Resistance A v = v O v sig = 1 + β (r o R L ) r π + 1 + β (r o R L ) 1 + R sig R B + R sig 27
Common Collector Amplifier Voltage gain, input and Output Resistance R in = R B r π + 1 + β r o R L R out = r o r π + R B R sig 1 + β 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 29
Common Emitter with emitter Resistance Exercise: Find the Voltage gain, input and Output Resistance Figure from Sedra/Smith, Copyright 2010 by Oxford University Press, Inc. 30