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CH11
Specialized Devices
指導老師 : 梁治國教授
學生 : 李玉璽
11.1 Programmable OP Amps






DC supply current
Open-loop voltage gain
Input bias current
Slew rate
Unity gain frequency
Input noise voltage
11.2 Instrumentation Amplifiers
 Offset voltages and drifts are
minimized
 Gain is stabilzed
 Nonlinearity is very low
 Input impedance is very high
 Output impedance is very low
 Common-mode rejection is very high
The basic instrumentation amplifier is essentially
a subtraction circuit preceded by two buffer
amplifiers
Basic instrumentation amplifier used for
numerical analysis
Analysis(1)
 The current through RG can computed
with Ohm’s Law :
iG 
vG 0.1V

 100A
RG 1k
 The resulting voltage drop across the
feedback resistors :
vR1  vR 2  iG R1
 100 A  10k  1V
Analysis(2)
 The voltage on the output of A1
(Kirchhoff’s Voltage Law )
v1 '  v1  vR1
 2 V  1 V
 1 V
 Similarly , the voltage at the
output of A2
 The output of A3
v2 '  v2  vR 2
 2.1 V  1 V
 3.1 V
vO  v'2 v'1
 3.1 V  1 V
 2.1 V
Analysis(3)
 Some basic algebraic
manipulations to
determine an
important equation
for voltage gain
v'1  v1  vR1
v'2  v2  vR2
v2  v1
iG 
RG
vR1  iG R 1

v2  v1 R1

vR2  iG R 2

v2  v1 R2

vO  v'1  v'2
RG
RG
Analysis(4)
 Substituting and simplifying gives us the
following results
vO  v2  vR2   v1  vR1 



v2  v1 R2  
v2  v1 R1 
 v2 
  v1 

RG
RG

 



R1  R2 v2  v1 
 v2  v1  

R
G


Analysis(5)
 Since resistors R1 and R2 are equal , we
can replace the expression R1 + R2 with the
expression 2R .
 2R 

vO  v2  v1 1 
 RG 
Analysis(6)
 Voltage gain is equal to the output voltage
of an amplifier divided by its input voltage,
and the input voltage to our present circuit
v2 – v1 ; therefore, we can now obtain our
final gain equation
2R
Av 
1
RG
11.1
Analysis(7)
 This shows us that the gain of the
instrumentation amplifier is determined by the
value of the external resistor RG .In the case of
the circuit in Figure 11.2 , the voltage gain is
computed as
2R
Av 
1
RG
20 K
1
1 K
 21

11.3 Logarithmic Amplifiers
 Whether the amplifiers are constructed from
discrete components or purchased in an
integrated form, the basic operation remains
the same.
 Of the log amp , the output voltage is
proportional to the logarithm of the input
voltage.
 二極體型(基本型)
 電晶體型
 差動型
二極體型(基本型)
 設計觀念
二極體在偏壓時,其電壓-電流特性方程式約
可表成
iD  I S  e
vD
VT
換言之
 iD
vD  VT  ln 
 IS



電路分析
 缺點



η值並非固定，與 iD 大小有關
熱電壓 VT 是溫度的函數，隨著溫度而變
逆向飽和電流 I S 對溫度變化甚為敏感，每上升5℃就會
倍增因此，此類對數放大器易受到溫度與電流 iD 等因素
影響
電晶體型
 BJT在主動模式下，B-E接
面視為二極體，因此電流
為
iC  I S  e
v BE
VT
 利用電壓電流轉換，將 VI 轉變成
VI
R
，並送入C極
 因虛接地，可知 vC  0 ，故讓B極接地，則可因
vBC  0 而讓BJT確實處於主動模式中，並且 vBC  vBE
 不過對溫度的敏感等缺點依然存在
11.4 Antilogarithmic Amplifiers
 電晶體型
 差動對型
電晶體型
iC  I S  e
v BE
VT
vO  iC R  RI S  e
 RI S  e
vI
VT
 必須強調的是，為了讓
npn操作在主動模式，故
輸入信號必須限制於VI<0
v BE
VT
vO  R F I ESe
38.9v1
附件一 MC1776
附件二
附件(三)MC3476
Thank you for your attention