Physics 116A NotesFall 2004David E. PellettDraft v.0.9 beta• Notes Copyright 2004 David E. Pellett unless stated otherwise.• References:– Text for course:Fundamentals of Electrical Engineering, second edition, by LeonardS. Bobrow, published by Oxford University Press (1996)– Others as noted1Physics 116A, 12/9/04Last Class Quick Wrap-Up• BJT amplifier large signal response quick overview:– load lines and power dissipation– Push-pull output stage to improve output power efficiency• Negative feedback voltage amplifier improvements: Prob. 10.35• Some realistic op-amp details– Why negative feedback can cause oscillation– Dominant pole compensation and the 741– Gain-bandwidth product– Maximum slew rate– Input currents and offset voltage2BJT CE Large Signal Performance• The maximum output voltage swing is set by BJT cutoff and saturation• Start with the BJT curves of ICvs. VCEfor various values of IB, locateQ point• Draw straight line through Q point with slope dIC/dVCEfor midband ACsignals (AC Load Line) to determine useful range• For AC, vc= −RCicso AC load line slope = ic/vc= −1/RCin this case.Output voltage swing follows AC load line.3CE Amplifier: DC and AC Load Lines0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0-0.50.00.51.01.52.02.53.03.5IC vs VCE for 2N2222A npn BJT (SPICE simulation) IC (mA)VCE (V)IB = 8 µAIB = 0 µAIB = 4 µAIB = 12 µAIB = 16 µAQ PointAC Load LineDC Load LineVCCRC + REVCC• Max. symmetrical voltage swing when Q point centered on AC load line• At Q, no input, BJT power dissipation p ≈ VCEIC= 4 V × 2 mA = 8 mW• If the Q point is centered, the average power dissipated by the BJT ismax. with no AC input – actually less when producing a signal. (Seesec. 9.4 in text for details)4Push-Pull Emitter Follower• Base bias chain keeps both BJTs just at cutoff (or slightly “on”) at Qpoint – ≈ No BJT power dissipated if no input signal.• AC input causes one or the other BJT to provide the output.• Maximum average BJT power now 0.1VCEQiC(sat)– much more efficientuse of B JTs and power – useful for driving low impedance loads at highpower5Negative Feedback Example: Voltage AmplifierReview Negative Feedback Advantages• Improved input impedance• Improved output impedance• Improved linearity• Improved frequency response• However, gain reduced, must avoid oscillation6• See solution of assigned Prob. 10.35 for proof of relations. (I did AFingeneral before. May do RiFon board if time permits.)• Note RiFis increased (improved) and RoFis decreased (also improved).• Example: If A = 200000, Rin= 2 MΩ, Ro= 75 Ω and B = 1/20,then A = 20, RiF= 20 GΩ and RoF= 7.5 mΩ.7Voltage Amplifier: AFdependence on AB = 1/20, AF= A/(1 + AB) = [A−1+ B]−1:A AF200000 19.998100000 19.99610000 19.961000 19.6500 19.2200 18.2100 16.750 14.3• AFmax= 20. If A >> AFmax, AFis insensitive to A. AFis down ≈3 dB frommaximum when A = 50.• Reduces distortion due to A nonlinearity, allows for variations in amplifiergain from device t o device. (What Black wanted back in the 1920’s forhis telephone long-distance line amplifiers)• Suppose A is 200000 at low frequency (say 1 Hz) but falling with fre-quency like 1/f at high frequencies due to a built-in low-pass filter withfc= 5 Hz. With feedback, the -3 dB bandwidth would be improved,since AFremains high until A has fallen many orders of magnitude.8Op Amp: LM741 SchematicDiagramc! 2000 National Semiconductor Corp.• Note C1. It connects a high-gain CE amplifier output to its input, multi-plying C1by (1+|gain|). This sets HF fc= 5 Hz a nd leads to a ≈ constantHF gain-bandwidth product of 1 Mhz.• This dominates the HF response. Phase shift less than 90 degrees at1 MHz (BW for AV F= 1). Can’t oscillate with any choice of resistivefeedback network B. Also limits usefulness at high frequencies.9741 SpecificationsInput Bias Current 80 nAInput Offset Current 20 nAInput Offset Voltage 1 mVMax. Slew Rate 0.5 V/µsOpen Loop Gain 200000Gain-BW Product 1 MHzInput Resistance 2 MΩOutput Resistance 75 ΩCMRR typ. 80-100 dB• Output protected against short-circuits• Input offset voltage can be bala nced out w ith external pot• See Sec. 10.2 for details10Bode Plot for µA702A Op-Amp• No large C1: Could make amplifier with BW of several MHz.• Considerable gain left when phase shift equals 180 degrees at 12.5 MHz.• Not fool-proof: A unity gain voltage follower would
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