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实验八 放大器参数测量实验 Datasheet Datasheet 是由器件制造商提供的有关器件 特性参数、用途说明和设计参考的权威资 料。从普通二极管到高级CPU，负责任的厂 商都同样会对其产品提供详细的datasheet。 通常可以从互联网上下载到其电子版本 （.pdf文件）。 比如这里的741的datasheet来自于德（克 萨斯）州仪器公司的网站 http://www.ti.com Op Amp Circuit Symbol and Terminals V+ non-inverting input inverting input positive power supply + – output V – negative power supply The output voltage can range from V – to V + (“rails”) The positive and negative power supply voltages do not have to be equal in magnitude (example: 0V and +3V DC supplies) Op.Amp. Between ideal and the real V+ V- I + + I- + - Vout = A(V+ - V-) + - Ideal Op-Amp Characteristics of an ideal op-amp Rin = infinity Rout = 0 Avo = infinity (Avo is the open-loop gain, sometimes A or Av of the op-amp) Bandwidth = infinity (amplifies all frequencies equally) Summary of op-amp behavior Vout = A(V+ - V-) Then Vout/A = V+ - VLet A infinity then, V+ - V0 Then V+=VWe name this express ‘virtual short’. Rin = infinity Then I+=I-=0 We call this express ‘virtual cut off’ DC imperfections Initial Offset Voltage, Vos Initial Offset Voltage Temperature oefficient, TCVos Input Bias Current, Ib Input Offset Current, Ios Input Offset Current, Temperature Coefficient, TCIos Power Supply Rejection Ratio, PSRR Meeting with Vos (also Vio) + − In the ideal world + Vo=0 − Vo—>Vcc or Vee In the real world If we tie the input pins together so that Vn= Vp, the output Vo will not be zero，but to be saturate. Why? Vos: define and model Real Op Amp Real Op Amp Ideal Op Amp Ideal Op Amp + − Vos + 0v − Vos Initial Offset Voltage The DC voltge that must be applied between the input terminals of the amplifier to force the quiescent dc output voltage to zero. Finding it! ——simple method − + R1 1.234 sensitivity available R2 − + Vos measure the output with your digital multi-meter configured as a DC voltmeter with the highest Vout Vout =Vos (1+R2/R1), Vos = Vout / (1+R2/R1) Note:Where we suppose without other imperfections Finding it——Improve method R1 R2 − Rb=R1//R2 + When we take the Ibias into account, which would be instructed after. Where Vos come from? Input stage with input offset voltage Ibias and Ios Real Op Amp Ideal Op Amp Ip Define： Vos = Ip – In Vbias = (Ip+In)/2 + In − Op amp bias current (IB) 741 IB = 500 nA or less CMOS input op amp IB = 0.000001 nA range Finding it！ In 1M − 1M + − 1.234 + 1.234 Ip Step1, find Ip VP = –RIP, so that V1 = Vos – 1M*Ip, by the superposition principle. Apply power, measure Vo, and calculate Ip = (VOS – V1)/R, with Vos as found in ‘Finding Vos’ By similar reasoning, calculate the In Step3 Calculate Ibias and Ios Step2, find In Balance resistor Rb R2 R1 R2 R1 In + Rb=R1//R2 + Ip Rb just correct Ibias, but Ios. Because, generally, Ios < Ibias, For 741 Trim them R2 R1 In - 741 Rb=R1//R2 + Ip 4 5 1 comment: Not all Op-Amp are trimmed in this configuration. The worse thing ——They all drift with Temperature Output Current limit − + +0.1V 4Ώ -0.1V PSS and PSRR Power Supply Sensitivity, PSS (uV/V) The change in output voltage due to change in Supply Voltage. PSS=(Vout1Vout2)/Vss(1-Vss2) Power Supply Rejection Ratio, PSRR,dB The Power Supply Sensitivity divided by the closed loop gain. When measured in dB it is equal to: 20log10(PSS/Gain) Inside the 741 AC imperfections Slew Rate Gain Bandwidth Product, GBP Slew rate Slew-rate (SR) = d(Vout)/dt |max = Volts/sec (V/ms) − Vin + Vout = Vin SR = 2.5 V/ms Finding it − 10k + Frequency response of openloop op-amp Frequency response of op-amps Model of an ideal op-amp - + - + Model of an op-amp with frequency response Vout = A(V+ - V-) - C + Vout = A(V+ - V-) + We assume low-pass filter behavior Gain Bandwidth Product GBP Mhz The product of the closed-loop voltage gain and the -3dB frequency at which it is measured. Constant Gain-Bandwidth product ft = |A| fb 1MHz = (10 000) 100Hz 1MHz = (1000) 1KHz 1MHz = (10) 100KHz Tradeoff between gain and bandwidth