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参数资料
| 型号： | LOG100 |
| 英文描述： | Precision LOGARITHMIC AND LOG RATIO AMPLIFIER |
| 中文描述： | 精密对数和对数比率放大器 |
| 文件页数： | 5/9页 |
| 文件大小： | 123K |
| 代理商： | LOG100 |
LOG100
5
THEORY OF OPERATION
The base-emitter voltage of a bipolar transistor is
V
BE = VT
n
where: V
T =
(1)
K = Boltzman’s constant = 1.381 x 10–23
T = Absolute temperature in degrees Kelvin
q = Electron charge = 1.602 x 10–19 Coulombs
I
C = Collector current
I
S = Reverse saturation current
From the circuit in Figure 1, we see that
V
OUT' = VBE1 – VBE2
(2)
Substituting (1) into (2) yields
V
OUT' = VT1
n
– V
T2
n
(3)
If the transistors are matched and isothermal and V
T1 = VT2,
then (3) becomes:
V
OUT' = VT [
n
–
n
]
(4)
V
OUT' = VT
n
and since
(5)
n x = 2.3 log
10 x
(6)
V
OUT' = n VT log
(7)
where n = 2.3
(8)
also
V
OUT = VOUT'
(9)
=
n V
T log
(10)
or
V
OUT = K log
(11)
FIGURE 1. Simplified Model of Log Amplifier.
I
2
I
1
R
1 + R2
R
1
R
1 + R2
R
1
I
2
I
1
FIGURE 2. Transfer Function with Varying K and I
1.
FIGURE 3. Transfer Function with Varying I
2 and I1.
KT
q
A
2
A
1
I
1
Q
1
Q
2
V
OUT = K LOG
I
1
I
2
I
2
I
1
I
2
++
––
R
2
V
OUT
V
OUT
R
1
V
BE1
V
BE2
It should be noted that the temperature dependance associ-
ated with V
T
= KT/q is compensated by making R
1
a
temperature sensitive resistor with the required positive
temperature coefficient.
DEFINITION OF TERMS
TRANSFER FUNCTION
The ideal transfer function is V
OUT = K log
where:
K = the scale factor with units of volts/decade
I
1 = numerator input current
I
2 = denominator input current.
ACCURACY
Accuracy considerations for a log ratio amplifier are some-
what more complicated than for other amplifiers. The reason
is that the transfer function is nonlinear and has two inputs,
each of which can vary over a wide dynamic range. The
accuracy for any combination of inputs is determined from
the total error specification.
I
2
I
1
10
8
6
4
2
0
–2
–4
–6
–8
–10
1A
10A
100A
1mA
100nA
10nA
1nA
V
OUT = K LOG
I
1
I
2
I
2 = 1A
Fixed value of I
2.
V
OUT
(V)
K = 5
K = 3
K = 1
I
1
10
8
6
4
2
0
–2
–4
–6
–8
–10
1A
10A 100A
1mA
100nA
10nA
1nA
V
OUT = K LOG
I
1
I
2
K = 3
Fixed value of K.
V
OUT
(V)
I
2 = 10nA
I
2 = 1A
I
2 = 100A
I
1
l
I
2
I
1
l
I
S
I
C
l
I
S1
I
1
l
I
S2
I
1
l
I
S
I
1
I
S
I
2
l
I
2
I
1
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LOG100JP | 功能描述:特殊用途放大器 Prec Logarithmic and Log Ratio Amp RoHS:否 制造商:Texas Instruments 通道数量:Single 共模抑制比（最小值）: 输入补偿电压: 工作电源电压:3 V to 5.5 V 电源电流:5 mA 最大功率耗散: 最大工作温度:+ 70 C 最小工作温度:- 40 C 安装风格:SMD/SMT 封装 / 箱体:QFN-20 封装:Reel |
| LOG100JP-2 | 制造商:Rochester Electronics LLC 功能描述:LOGARITHMIC AMPLIFIER - Bulk |
| LOG100JP-3 | 制造商:Rochester Electronics LLC 功能描述:LOG & LOG RATIO - Bulk |
| LOG100JP-4 | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
| LOG101 | 制造商:BB 制造商全称:BB 功能描述:Precision LOGARITHMIC AND LOG RATIO AMPLIFIER |
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