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INTRODUCTION TO ELECTRONICS EHB 222E Bipolar Junction Transistors II (BJT) Asst. Prof. Onur Ferhanoğlu Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 1 BJTs – Exercise 1 Determine node voltages & branch currents (assume β = 100) Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 2 BJTs – Exercise 2 Determine node voltages & branch currents (assume β = 30) pnp transistor! Assume ACTIVE: • Base current is small -> VB ~ 0V • VE = VB + 0.7 = 0.7V • IE = (5-0.7)/1kΩ = 4.3 mA • IC max ~5V/10 k Ω = 0.5 mA IE >> IC -> NOT ACTIVE! Assume SAT Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 3 BJTs – Exercise 2 continued Determine node voltages & branch currents (assume β = 30) pnp transistor! Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 4 BJTs – Exercise 3 Determine node voltages & branch currents (assume β = 100) Simplify base circuit using Thevenin`s theorem Cancel voltage sources to find Req (Thevenin) Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 5 BJTs – Exercise 3 continued.. Determine node voltages & branch currents (assume β = 100) Loop equation Assume active Isolate IE from 1st equation VC > VB ACTIVE verified Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 6 BJTs – Exercise 4 Determine node voltages & branch currents (assume β = 100) 1st stage same as previous question Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 7 BJTs – Exercise 4 continued.. Determine node voltages & branch currents (assume β = 100) 1st stage same as previous question Assume identical solution • Since part of collector current will flow to Q2, VC1 will be different • If IC1 >> IB2 (Same as previous) Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 8 BJTs – Exercise 4 continued.. Determine node voltages & branch currents (assume β = 100) Q2 -> pnp • Emitter is connected to + 15V -> VEB2 forward biased Since IC2 ends up at ground VCE is possibly reverse biased -> Assume Q2 ACTIVE Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS Q2 ACTIVE as assumed 9 BJTs – Exercise 5 Determine node voltages & branch currents (assume β = 100) Q1 & Q2 cannot be simultaneously conducting 10V / ~ 0 resistance -> infinite current along supplies Assume Q2 ON Q1 OFF • Current will flow from ground through Q2 • VB2 is negative as VEB is forward biased • But, current should not move from negative voltage to +5 V, through 10kΩ Assume Q1 ON Q2 OFF Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 10 BJTs – Exercise 5 Determine node voltages & branch currents (assume β = 100) Q1 ACTIVE or SAT? VCB > 0 -> reverse biased -> ACTIVE Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 11 BJT in amplifier design Simple amplifier circuit with BJT`s Amplification (highest slope region) Segment Y-Z: Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 12 Biasing the BJT Simple amplifier circuit with BJT`s DC voltage & current at Q Signal to be amplified is superimposed on the bias voltage Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 13 Biasing the BJT – graphical illustration Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 14 Small-signal voltage gain Amplifier output = Amplifier gain x Amplifier input Amplifier gain: Av is maximum when VCE = VCE SAT Not preferred for the sake of signal symmetry Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 15 Exercise 6 IC = Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 16 Exercise 6 continued.. Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 17 Exercise 6 continued.. Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 18 Exercise 6 continued.. Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 19 Graphical Analysis • For each value of vBE, the circuit will be operating at the point of intersection of the ic & vCE graph and the straight line (load line) Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 20 Small signal operation • Base-emitter junction is forward biased • Reverse biased at collector base function is established using the source VCC • vbe, which is superimposed on VBE, is to be amplied • First consider DC bias -> vbe = 0 Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 21 Small signal operation • Base-emitter junction is forward biased • Reverse biased at collector base function is established using the source VCC • vbe, which is superimposed on VBE, is to be amplied • When vbe is applied for small vbe (<10mV) small-signal approximation transconductance Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 22 Small signal operation - transconductance Small signal operation is restricted to A linear segment of the curve Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 23 Base current & resistance • Determine the resistance seen by vbe Small-signal resistance between base and emitter, looking into the base Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 24 Emitter current & resistance Small-signal resistance between base and emitter, looking into the emitter Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 25 Voltage gain • The transistor senses the input voltage vbe and causes a proportional current gmvbe • Output voltage becomes: Substitute for gm Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 26 Small-signal transistor models: Hybrid-π • BJT is represented as a voltage controlled current source, which includes input resistance looking into the base (rπ) KCL at emitter yields: Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 27 Small-signal transistor models: Hybrid-π variation • BJT is represented as a current controlled current source, which includes input resistance looking into the base (rπ) KCL at emitter yields: Yields to correct expression for ie Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 28 Small-signal transistor models: T-model • Alternative models may be more convenient for other situations Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 29 Small-signal transistor models: T-model variation • Alternative models may be more convenient for other situations Collector current: Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 30 Exercise 7 Determine voltage gain (vo/vi), assume β = 100 STEP 1) Determine DC operating point Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 31 Exercise 7 continued.. Determine voltage gain (vo/vi), assume β = 100 STEP 2) Determine small-signal parameters Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 32 Exercise 7 continued.. Determine voltage gain (vo/vi), assume β = 100 STEP 3) Eliminate DC sources Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 33 Exercise 7 continued.. Determine voltage gain (vo/vi), assume β = 100 STEP 4) Replace with small-signal equivalent model Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 34 Exercise 7 – solving directly on the circuit graph Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 35 Accounting for the Early-effect Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS 36