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Chapter 30 Operational Amplifiers Introduction • Characteristics – High input impedance – Low output impedance – High open-loop gain – Two inputs – One output – Usually + and – dc power supplies 2 Introduction • Ideal Characteristics – zin (inverting) ≈ ∞ – zin (non-inverting) ≈ ∞ – zout ≈ 0 – Av ≈ ∞ Inverting Input V+ - Output + Non-Inverting Input V- 3 Introduction • Uses – Comparators – Voltage amplifiers – Oscillators – Active filters – Instrumentation amplifiers Inverting Input V+ - Output + Non-Inverting Input V- 4 Introduction • Single-ended amplifier – One input grounded – Signal at other input • Double-ended amplifier/Differential amplifier Inverting Input V+ - Output + Non-Inverting VInput – Signals at both inputs 5 Differential Amplifier and Common-Mode Signals • Basic differential amplifier – Q1 identical to Q2 – RC1 = RC2 – IC1 = IC2 and emitter currents equal – Also, IC ≈ IE for high β – and VBE ≈ 0.7 V • Similar calculation of Bias 6 Differential Amplifier and Common-Mode Signals ◦ IC1 VCC RC1 RC2 Q1 __ _ IE Q2 ◦ IC2 __ _ - RE –VEE 7 Differential Amplifier and CommonMode Signals • Apply same signal to both Bases Vout = Vout1 – Vout2 ≈ 0 – Eliminates commonmode signals – 60 Hz – Noise ◦ VCC IC1 RC1 Q1 __ _ - RC2 IC2 Q2 IE RE __ _ - ◦ –VEE 8 Differential Amplifier and CommonMode Signals • Apply sinusoids to both bases: – Same amplitude, 180° difference in phase, if Vin1 = –Vin2 Vout = 2Vin ◦ VCC IC1 RC1 Q1 __ _ - RC2 IC2 Q2 IE RE __ _ - ◦ –VEE 9 Differential Amplifier and CommonMode Signals • Common-mode signals – Differential voltage gain vout Avd vd also called open-loop voltage gain 20,000 ≤ Av ≤ 200,000 10 Differential Amplifier and CommonMode Signals • Common-mode signals – Common-mode voltage gain vout Avc vc 11 Differential Amplifier and CommonMode Signals • Common-mode rejection ratio (CMRR) – Equations Avd CMRR Avc [CMRR]db 20 log10 (CMRR) – Values 70db CMRR 90db 12 Differential Amplifier and Common-Mode Signals • Noise – Static in audio signal – Increases as signal is amplified – Common mode signal – Significantly reduced by differential amplifier vnoise vin __ _ - __ _ - + 13 Negative Feedback • Op-amp – Large differential, open-loop voltage gain • Avol ≈ 100,000 – Small input yields saturated output (VCC or VEE) 14 Negative Feedback • Negative feedback – Returns a portion of output signal to the input – Open-loop voltage gain decreased 15 Negative Feedback • Input impedance still high • Output impedance low • Circuit voltage gain, Av – Adjustable – Stable vout Av vin Negative Feedback + vout vin __ _ 16 Inverting Amplifier • Basic circuit 17 Inverting Amplifier • Output 180° out of phase with input • Significant decrease in gain – Gain now called closed-loop voltage gain • Output impedance ≈ 0 • vd ≈ 0 18 Inverting Amplifier • Inverting input at virtual ground, vin(-) ≈ 0 • iin to op-amp ≈ 0 • Input current only dependent on vin and R1 • Avcl only dependent on input resistor and feedback resistor vin iin R1 vout iin RF Avcl vin iin R1 RF Avcl R1 19 Inverting Amplifier • Model vd ≈ 0 Rin ≈ ∞ iin = if zin ≈ R1 RF R1 i=0 + i vin in if + vd Rin + - __ _ - Rout Avolvd internal +__ _ vout(OC) - - 20 Inverting Amplifier • Low output impedance zout zout RF vout (OC ) iout ( SC ) Rout (1 Avcl ) Avol R1 i=0 + i vin in if + vd Rin - __ _ - Rout - Avolvd internal+__ _ - i1 iout(sc) __ _ 21 Non-Inverting Amplifier • Circuit 22 The Non-Inverting Amplifier • • • • Very high input impedance Voltage gain related to the two resistors Very low output impedance Excellent buffer 23 Non-Inverting Amplifier • Differential voltage – vd ≈ 0 • Input current to op-amp –i=0 • Closed-loop voltage gain (Avcl) is a resistor ratio 24 Non-Inverting Amplifier vout Av vin RF iF R1iF Av R1iF Avcl RF 1 R1 25 Non-Inverting Amplifier • Model • Input impedance vd iin Rin Avol zin 1 Rin Avcl vin iin zin + vd Rin - + R1 - __ _ - - vout Rout Avolvd internal __ _ + - + RF if 26 Non-Inverting Amplifier • Model • Output impedance iout ( sc ) i2 i f zout Avcl Avol Rout iin - + R1 - __ _ - i2 + vd Rin - Rout Avolvd internal __ _ + - + RF if __ _ - iout(sc) 27 Non-Inverting Amplifier • • • • • Very high zin Very low zout Good buffer circuit Also called voltage follower (gain = 1) Or adjustable gain > 1 28 Non-Inverting Amplifier • Voltage Follower Buffer Circuit – Gain = 1 – High impedance source drives low impedance load 29 Op-Amp Specifications • LM 741 series – Inexpensive – Widely used – Good general specifications – Characteristic of all op-amp specifications • Provide Minimum, Typical, and Maximum ratings 30 Op-Amp Specifications • Input Offset Voltage, Vio – LM741C, Vio typical is 2 millivolts – Model is voltage source with value, Vio in series with + input 31 Op-Amp Specifications • Input Offset Voltage, Vio – Without feedback this would saturate output with no input – With negative feedback, output due to Vio is closed-loop gain times Vio 32 Op-Amp Specifications • Input Offset Current, Ios • Ios = Difference between bias currents at + and – inputs of op-amp • 741C typical Ios is 20 nanoamps • Multiplying resistor used to measure Ios 33 Op-Amp Specifications • Input Resistance – 741C: minimum = .3 MΩ, typical = 2 MΩ • Open-Loop Voltage gain (Avol) – 741C: Avol = Large Signal Voltage Gain • minimum = 20,000, typical = 200,000 – Closely related to Bandwidth, BW 34 Op-Amp Specifications • Gain-bandwidth product – 741C = 1,000,000 = 106 MHz 35 Op-Amp Specifications • Gain versus frequency curve for op-amp 36 Op-Amp Specifications • Slew rate – Maximum rate of change of output voltage V Slew Rate t • 741C maximum slew rate = 0.5 V/μsec 37 Op-Amp Specifications • Fastest time for output to go from 0 to 10 volts is 20 μsec • Can distort waveforms that have too fast a rise time 38 Op-Amp Specifications • Slew rate required for Sinusoid with frequency f and amplitude A • Maximum amplitude of a sine wave with frequency f for a given slew rate v A sin(t ) dv A cos(t ) dt slew rate 2 fA slew rate f max 2 A slew rate Amax 2 f 39 Op-Amp Specifications • Bias Compensation: use RC = R1||RF 40 Troubleshooting an Op-Amp Circuit • Problems occur when circuit is first built • Most important – Correct connection of dual power supply • Connecting a – supply to a + input (or vice versa) can burn out an op-amp • Single earth ground • Short connecting wires 41