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參數(shù)資料
| 型號： | LT1228 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | 100MHz Current Feedback Amplifier with DC Gain Control(100MHz ，電流反饋放大器（帶直流增益控制）) |
| 中文描述： | 100MHz的電流反饋放大器的直流增益控制（100MHz時，電流反饋放大器（帶直流增益控制）） |
| 文件頁數(shù)： | 9/20頁 |
| 文件大?。?/td> | 357K |
| 代理商： | LT1228 |
9
LT1228
U
S
A
O
PPLICATI
U
U
The LT1228 contains two amplifiers, a transconductance
amplifier (voltage-to-current) and a current feedback am-
plifier (voltage-to-voltage). The gain of the transconduc-
tance amplifier is proportional to the current that is exter-
nally programmed into pin 5. Both amplifiers are designed
to operate on almost any available supply voltage from 4V
(
±
2V) to 30V (
±
15V). The output of the transconductance
amplifier is connected to the noninverting input of the
current feedback amplifier so that both fit into an eight pin
package.
TRANSCONDUCTANCE AMPLIFIER
The LT1228 transconductance amplifier has a high imped-
ance differential input (pins 2 and 3) and a current source
output (pin 1) with wide output voltage compliance. The
voltage to current gain or transconductance (g
m
) is set by
the current that flows into pin 5, I
SET
. The voltage at pin 5
is two forward biased diode drops above the negative
supply, pin 4. Therefore the voltage at pin 5 (with
respect to V
–
) is about 1.2V and changes with the log of
the set current (120mV/decade), see the characteristic
curves. The temperature coefficient of this voltage is
about –4mV/
°
C (–3300ppm/
°
C) and the temperature co-
efficient of the logging characteristic is 3300ppm/
°
C. It is
important that the current into pin 5 be limited to less than
15mA. THE LT1228 WILL BE DESTROYED IF PIN 5 IS
SHORTED TO GROUND OR TO THE POSITIVE SUPPLY. A
limiting resistor (2k or so) should be used to prevent more
than 15mA from flowing into pin 5.
The small-signal transconductance (g
m
) is equal to ten
times the value of I
SET
(in mA/mV) and this relationship
holds over many decades of set current (see the character-
istic curves). The transconductance is inversely propor-
tional to absolute temperature (–3300ppm/
°
C). The input
stage of the transconductance amplifier has been de-
signed to operate with much larger signals than is possible
with an ordinary diff-amp. The transconductance of the
input stage varies much less than 1% for differential input
signals over a
±
30 mV range (see the characteristic curve
Small-Signal Transconductance vs DC Input Voltage).
Resistance Controlled Gain
If the set current is to be set or varied with a resistor or
potentiometer it is possible to use the negative tempera-
ture coefficient at pin 5 (with respect to pin 4) to compen-
sate for the negative temperature coefficient of the transcon-
ductance. The easiest way is to use an LT1004-2.5, a 2.5V
reference diode, as shown below:
Temperature Compensation of g
m
with a 2.5V Reference
LT1228 TA04
LT1004-2.5
V
–
g
m
5
4
R
I
SET
I
SET
R
V
be
V
be
2.5V
2E
g
The current flowing into pin 5 has a positive temperature
coefficient that cancels the negative coefficient of the
transconductance. The following derivation shows why a
2.5V reference results in zero gain change with tempera-
ture:
Sinceg
q
kT
I
3 87
.
I
and V
E
akT
q
wherea
IncT
Ic
at
C
c
n
Ic
A
m
SET
SET
be
g
n
=
×
=
×
=
=
≈
°
=
=
=
(
)
10
19 4
27
0 001
.
3
100
–
.
,
,
μ
E
g
is about 1.25V so the 2.5V reference is 2E
g
. Solving
the loop for the set current gives:
I
E
E
akT
q
R
or I
akT
Rq
SET
g
g
SET
=
=
2
2
2
–
–
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| LT1228CN8 | 功能描述:IC CURRNT FEEDBCK AMP100MHZ 8DIP RoHS:否 類別:集成電路 (IC) >> Linear - Amplifiers - Instrumentation 系列:- 標(biāo)準(zhǔn)包裝:73 系列:Over-The-Top® 放大器類型:通用 電路數(shù):4 輸出類型:滿擺幅 轉(zhuǎn)換速率:0.07 V/µs 增益帶寬積:200kHz -3db帶寬:- 電流 - 輸入偏壓:1nA 電壓 - 輸入偏移:285µV 電流 - 電源:50µA 電流 - 輸出 / 通道:25mA 電壓 - 電源，單路/雙路(±):2 V ~ 44 V，±1 V ~ 22 V 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:16-WFDFN 裸露焊盤 供應(yīng)商設(shè)備封裝:16-DFN-EP（5x3） 包裝:管件 |
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