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參數(shù)資料
| 型號(hào)： | LTI969CMS |
| 英文描述： | Voltage-Feedback Operational Amplifier |
| 中文描述： | 電壓反饋運(yùn)算放大器 |
| 文件頁(yè)數(shù)： | 14/20頁(yè) |
| 文件大?。?/td> | 285K |
| 代理商： | LTI969CMS |
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14
LT1969
APPLICATIOU
wasted in the termination resistor. Second, the signal is
halved so the gain of the amplifer must be doubled to have
the same overall gain to the load. The increase in gain
increases noise and decreases bandwidth (which can also
increase distortion). Third, the output swing of the ampli-
fier is doubled which can limit the power it can deliver to
the load for a given power supply voltage.
An alternate method of back-termination is shown in
Figure 7. Positive feedback increases the effective back-
termination resistance so R
BT
can be reduced by a factor
of n. To analyze this circuit, first ground the input. As R
BT
=
R
L
/n, and assuming R
P2
>>R
L
we require that:
V
a
= V
o
(1 – 1/n) to increase the effective value of
R
BT
by n.
V
p
= V
o
(1 – 1/n)/(1 + R
F
/R
G
)
V
o
= V
p
(1 + R
P2
/R
P1
)
W
U
U
Figure 7. Back-Termination Using Positive Feedback
+
–
1969 F07
R
F
R
BT
R
P2
R
P1
R
G
V
i
V
a
V
P
V
o
R
L
R
F
R
G
1 +
(
R
L
n
(
=
V
o
V
i
= 1 –
–
1
n
FOR R
BT
=
)
R
F
R
G
1 +
(
)
R
P1
R
P1
+ R
P2
R
P1
R
P2
+ R
P1
R
P2
/(R
P2
+ R
P1
)
1 + 1/n
)
Eliminating Vp, we get the following:
(1 + R
P2
/R
P1
) = (1 + R
F
/R
G
)/(1 – 1/n)
For example, reducing R
BT
by a factor of n = 4, and with an
amplifer gain of (1 + R
F
/R
G
) = 10 requires that R
P2
/R
P1
=12.3.
Note that the overall gain is increased:
R
n
R
i
F
(
1 1
1
/
/
/
V
V
R
R
R
R
[
R
(
R
o
P
P
P
G
P
P
P
=
+
(
)
)
+
(
)
[
]
+
)
]
2
2
1
1
2
1
/
/
A simpler method of using positive feedback to reduce the
back-termination is shown in Figure 8. In this case, the
drivers are driven differentially and provide complemen-
tary outputs. Grounding the inputs, we see there is invert-
ing gain of –R
F
/R
P
from –V
o
to V
a
V
a
= V
o
(R
F
/R
P
)
Figure 8. Back-Termination Using Differential Positive Feedback
–
+
R
BT
R
F
R
G
R
P
R
P
R
G
R
L
R
L
–V
i
V
a
–V
a
V
i
–V
o
V
o
–
+
R
BT
1969 F08
R
F
R
L
n
=
V
o
V
i
n =
1 –
(
2
FOR R
BT
=
R
F
R
P
R
F
R
P
)
+
R
F
R
G
1 +
1 –
R
F
R
P
1
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