实验八 放大器参数测量实验. datasheet datasheet...
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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