实验八 放大器参数测量实验

实验八 放大器参数测量实验实验八 放大器参数测量实验

DatasheetDatasheet

Datasheet Datasheet 是由器件制造商提供的有关器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资料。从普通二极管到高级 CPUCPU ，负责任，负责任的厂商都同样会对其产品提供详细的的厂商都同样会对其产品提供详细的 datasdatasheetheet 。通常可以从互联网上下载到其电子。通常可以从互联网上下载到其电子版本（版本（ .pdf.pdf 文件）。文件）。

比如这里的比如这里的 741741 的的 datasheetdatasheet 来自于德来自于德（克萨斯）州仪器公司的网站（克萨斯）州仪器公司的网站 http://http://www.ti.comwww.ti.com

Op Amp Circuit Symbol and TerminalsOp Amp Circuit Symbol and Terminals

+

–

V +

V –

non-inverting input

inverting input

positive power supply

negative power supply

output

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 reaOp.Amp. Between ideal and the reall

V+

V-

Vout = A(V+ - V-)+-

+

-I-

I+

+

-

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)

Ideal Op-AmpIdeal Op-Amp

Summary of Summary of op-ampop-amp behavior behavior

Vout = A(V+ - V-) Then Vout/A = V+ - V-Let A infinitythen, V+ - V- 0Then V+=V-We name this express We name this express

‘virtual short’.‘virtual short’.

Rin = infinityThen I+=I-=0

We call this express ‘virtual cut off’

DC imperfectionsDC imperfections

Initial Offset Voltage, Vos Initial Offset Voltage Temperature oefficient, T

CVos Input Bias Current, Ib Input Offset Current, Ios Input Offset Current, Temperature Coefficient, TCIos Power Supply Rejection Ratio, PSRR

Meeting with Vos (also Vio)Meeting with Vos (also Vio)

If we tie the input pins together so that Vn= Vp, the output Vo will not be zero ， but to be saturate. Why?

+

− Vo=0

In the ideal world In the real world

+

− Vo—>Vcc or Vee

Vos: define and modelVos: define and model

+

−

Vos

Real Op Amp

Ideal Op Amp

0v

+

−

Vos

Real Op Amp

Ideal Op Amp

Initial Offset Voltage The DC voltge that must be applied between t

he input terminals of the amplifier to force the quiescent dc output voltage to zero.

Finding it! ——simple Finding it! ——simple methodmethod

+

−

Vos

R2R1

Vout

measure the output with your digital multi-meter configured as a DC voltmeter with the highest sensitivity available

Vout =Vos (1+R2/R1), Vout =Vos (1+R2/R1), Vos = Vout / (1+R2/R1)Vos = Vout / (1+R2/R1)

Note:Where we suppose without otNote:Where we suppose without other imperfectionsher imperfections

+

−

1.234

Finding it——Improve Finding it——Improve methodmethod

+

−

R2R1

Rb=R1//R2

When we take the Ibias into account, which wWhen we take the Ibias into account, which would be instructed after.ould be instructed after.

Where Vos come from?

Input stage with input offset voltage

Ibias and IosIbias and Ios

DefineDefine ：：Vos = Ip – InVos = Ip – InVbias = (Ip+In)/2Vbias = (Ip+In)/2

+

−

Ip

In

Ideal Op Amp

Real Op Amp

Op amp bias current (IB)

741 IB = 500 nA or less

CMOS input op amp IB = 0.000001 nA range

Finding itFinding it ！！

+

−

1.234

1M

Ip+

−

1.234

1M

In

Step1, find IpStep1, find Ip Step2, find In 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 InStep3 Calculate Ibias and IosStep3 Calculate Ibias and Ios

Balance resistor RbBalance resistor Rb

+

-

R2R1

+

-

R2R1

Rb=R1//R2

In

Ip

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

Trim themTrim them

+

-

R2R1

Rb=R1//R2

In

Ip1

54

741

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

The worse thingThe worse thing

————They all drift with TemperatureThey all drift with Temperature

Output Current limitOutput Current limit

+

−

4Ώ

+0.1V

-0.1V

PSS and PSRRPSS and PSRR

Power Supply Sensitivity, PSS (uV/V) The change in output voltage due to ch

ange 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)

Inside the 741Inside the 741

AC imperfectionsAC imperfections

Slew RateSlew Rate Gain Bandwidth Product, GBP

Slew rateSlew rateSlew-rate (SR) = d(Vout)/dt |max = Volts/sec (V/s)

VinVout = Vin

SR = 2.5 V/s+

−

Finding itFinding it

+

−

10k

Frequency response of open-looFrequency response of open-loop op-ampp op-amp

Vout = A(V+ - V-)+

-+

-

Model of an ideal op-amp Model of an op-amp with frequency response

Vout = A(V+ - V-)+

-+

-

C

We assume low-pass filter behavior

Frequency response of op-amps

Gain Bandwidth Product GBP Mhz The product of the closed-loop voltage gai

n and the -3dB frequency at which it is measured.

Constant Gain-Bandwidth product Constant Gain-Bandwidth product ft = |A| fb

1MHz = (10 000) 100Hz

1MHz = (1000) 1KHz

1MHz = (10) 100KHz

Tradeoff between gain and bandwidth