參數(shù)資料
型號: LOG104AIDR
廠商: Texas Instruments, Inc.
英文描述: LOGARITHMIC AND LOG RATIO AMPLIFIER
中文描述: 對數(shù)和對數(shù)比率放大器
文件頁數(shù): 8/13頁
文件大?。?/td> 273K
代理商: LOG104AIDR
www.ti.com
LOG104
SBOS243B
8
A
2
A
1
I
1
Q
1
Q
2
+
V
OUT
= (0.5V) LOG
I
1
I
2
I
2
I
1
I
2
+
R
2
V
OUT
V
L
R
1
V
BE1
V
BE2
also
V
V
R
R
R
R
V
R
R
n V
I
I
OUT
L
OUT
T
=
+
=
+
1
2
1
1
2
1
1
2
log
V
V
I
I
OUT
=
0 5
1
2
.
log
Using the base-emitter voltage relationship of matched
bipolar transistors, the LOG104 establishes a logarith-
mic function of input current ratios. Beginning with the
base-emitter voltage defined as:
V
V
I
I
where V
kT
q
BE
T
C
S
T
=
=
ln
:
k = Boltzman
s constant = 1.381
10
23
T = Absolute temperature in degrees Kelvin
q = Electron charge = 1.602
10
19
Coulombs
I
C
= Collector current
I
S
= Reverse saturation current
From the circuit in Figure 11, we see that:
V
V
V
L
BE
BE
=
1
2
Substituting (1) into (2) yields:
V
V
I
I
V
I
I
L
T
S
T
S
=
1
1
1
2
2
2
ln
ln
If the transistors are matched and isothermal and
V
TI
= V
T2
, then (3) becomes:
V
V
I
I
I
I
V
V
I
I
and
ce
x
x
V
n V
I
I
L
T
S
S
L
T
L
T
=
=
=
=
1
1
2
1
2
10
1
2
2. log
ln
ln
ln
sin
ln
log
where n = 2.3
INSIDE THE LOG104
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(9)
(10)
(11)
FIGURE 11. Simplified Model of a Log Amplifier.
(8)
or
FIGURE 13. Transfer Function with Varying I
2
and I
1
.
(5)
3.0
2.5
3.5
2.0
1.5
1.0
0.5
0.0
0.5
3.0
3.5
2.0
2.5
1.0
1.5
1nA
10nA
100nA
1
μ
A
10
μ
A
100
μ
A
1mA
10mA
100pA
V
O
(
I
2
=100pA
I
2
=1nA
I
2
= 10nA
I
=100nA
I
2
=1
μ
A
I
2
μ
A
I
2
=1mA
I
2
=100
μ
A
I
1
V
OUT
= (0.5V)
LOG (I
1
/I
2
)
DEFINITION OF TERMS
TRANSFER FUNCTION
The ideal transfer function is:
V
OUT
= 0.5V
logI
1
/I
2
See Figure 12 for the graphical representation of the transfer
over valid operating range for the LOG104.
ACCURACY
Accuracy considerations for a log ratio amplifier are some-
what more complicated than for other amplifiers. This is
because 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.
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