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Last Time: Common Emitter Amplifier Biased by Current Source ESE 372 / Spring 2013 / Lecture 14 Last time: Common Emitter Amplifier biased by current source. Bias Q * But make sure that BJT is in FA, i.e. VCE > 0.2-0.3V Equivalent circuit based on Hybrid-π model, caps are short circuits for signals 1 ESE 372 / Spring 2013 / Lecture 14 Common Emitter Amplifier biased by current source. Voltage gains MAX Finite output resistance puts upper limit on amplifier AV0 gm rO voltage gain 2 ESE 372 / Spring 2013 / Lecture 14 Common Emitter Amplifier biased by current source. Input/output impedances Not high enough - problem Net voltage gain Depends on β – not too good 3 ESE 372 / Spring 2013 / Lecture 14 Common Emitter Amplifier biased by current source. Short circuit current gain Common emitter current gain 4 ESE 372 / Spring 2013 / Lecture 14 Example npn - BJT with β 100 and VA 100V. Bias current IE 1mA, VCC VEE 10V R B 100k, R C 8k, R S 5k, R L 5k 1. Check if BJT is in FA regime I 1mA I E 10μ B β 1 101 VB 10μ 100k 1V VE 1V 0.7V 1.7V 10V. VC 10V 1m8k 2V VCE 2V 1.7V 3.7V 0.3V 2. Calculate small signal parameters IQ 1mA mA 3. Find voltage gain g C 40 m R in R L Vth 25mV V G A V R R V0 R R β 100 in S L out rπ 2.5kΩ V gm 40mA V A g R || r 296 V0 m C O V VA 100V rO Q 100kΩ R in R B || r 2.4k, R out R C || rO 7.4k IC 1mA V V G V 0.32 296 0.4 38 V V 30mV p-p signal corresponds to about 10mV p-p variation of VBE. This produces about 1.2V p-p across load. 5 ESE 372 / Spring 2013 / Lecture 14 Example – output voltage swing 8 V 3.5 V -1.5 To remain in FA VCE > 0.2 V Maximum amplitude of the undistorted sine wave at the output is limited by maximum negative voltage swing, i.e. by 3.5 V. For 1mA bias current and 10 V power supply what could we do to increase the amplitude of undistorted sine wave? What is the maximum value of this amplitude? 6 ESE 372 / Spring 2013 / Lecture 14 CE amp biased by current source and with RE. Case 1 R E 0 R in R B || r r AV0 gm R C || rO Case 2 (neglect rO) R E 0 R in R B || r R E 1 r R C AV0 r R E 1 Negative feedback resistor RE improves input impedance at the expense of gain 7 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier. No need for CE ! Equivalent circuit for AC analysis 8 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier. Redraw equivalent circuit in more convenient form and neglect rO for beginning. 9 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier. Voltage gain Noninverting amplifier with open circuit voltage gain value similar to that of CE amp Short circuit current gain i.e. no current gain ! 10 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier. Input impedance Very small – PROBLEM for voltage amplifier Output impedance 11 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier. Net voltage gain Could we guess this without analysis? Observe current buffer action of CB amp Equivalent circuit of amp with current gain equal to one. 12 ESE 372 / Spring 2013 / Lecture 14 CB amplifier with BJT having finite output small signal resistance. Voltage gain 13 ESE 372 / Spring 2013 / Lecture 14 CB amplifier with BJT having finite output small signal resistance. Input impedance Impedance When transformation max 14 ESE 372 / Spring 2013 / Lecture 14 Common Collector amplifier Again. No need for CE ! Bias current IE will determine gm, rπ and rO Also R and C can be eliminated B C1 Redraw equivalent circuit in more convenient form 15 ESE 372 / Spring 2013 / Lecture 14 Common Collector amplifier + When i.e. no voltage gain ! 16.
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