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