实验八 放大器参数测量实验

1. 实验八 放大器参数测量实验

2. 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

3. Op Amp Circuit Symbol and Terminals V + positive power supply non-inverting input + – output inverting input 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)

4. I+ V+ + + I- Vout = A(V+ - V-) V- - - Op.Amp. Between ideal and the real

5. 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)

6. Vout = A(V+ - V-) Then Vout/A = V+ - V- Let A infinity then, V+ - V- 0 Then V+=V- We name this express ‘virtual short’. Rin = infinity Then I+=I-=0 We call this express ‘virtual cut off’ Summary of op-amp behavior

7. 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

8. Meeting with Vos (also Vio) In the ideal world 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?

9. Vos: define and model • 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.

10. Finding it! ——simple method • measure the output with your digital multi-meter configured as a DC voltmeter with the highest sensitivity available • Vout =Vos (1+R2/R1), Vos = Vout / (1+R2/R1) Note:Where we suppose without other imperfections

11. Finding it——Improve method • When we take the Ibias into account, which would be instructed after.

12. Where Vos come from? Input stage with input offset voltage

13. Ibias and Ios • Define： Vos = Ip – In Vbias = (Ip+In)/2

14. Op amp bias current (IB) 741 IB = 500 nA or less CMOS input op amp  IB = 0.000001 nA range

15. Finding it！ Step1, find Ip Step2, find In • 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

16. Balance resistor Rb Rb just correct Ibias, but Ios. Because, generally, Ios < Ibias, For 741

17. Trim them comment: Not all Op-Amp are trimmed in this configuration.

18. The worse thing ——They all drift with Temperature

19. Output Current limit

20. PSS and PSRR • Power Supply Sensitivity, PSS (uV/V) • The change in output voltage due to change in Supply Voltage. PSS=(Vout1-Vout2)/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)

21. Inside the 741

22. AC imperfections • Slew Rate • Gain Bandwidth Product, GBP

23. Slew rate Slew-rate (SR) = d(Vout)/dt |max = Volts/sec (V/ms) Vin Vout = Vin SR = 2.5V/ms

24. Finding it

25. Frequency response of open-loop op-amp

26. - - Vout = A(V+ - V-) Vout = A(V+ - V-) C + + + + - - Frequency response of op-amps Model of an op-amp with frequency response Model of an ideal op-amp We assume low-pass filter behavior

27. Gain Bandwidth Product GBP Mhz The product of the closed-loop voltage gain and the -3dB frequency at which it is measured.

28. Constant Gain-Bandwidth product ft = |A| fb 1MHz = (10 000) 100Hz 1MHz = (1000) 1KHz 1MHz = (10) 100KHz Tradeoff between gain and bandwidth