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參數(shù)資料
| 型號： | LT1228IS8#PBF |
| 廠商： | Linear Technology |
| 文件頁數(shù)： | 22/22頁 |
| 文件大小： | 0K |
| 描述： | IC CRNT FEEDBK AMP 100MHZ 8SOIC |
| 標準包裝： | 100 |
| 放大器類型： | 電流反饋 |
| 電路數(shù)： | 1 |
| 轉換速率： | 3500 V/µs |
| -3db帶寬： | 100MHz |
| 電流 - 輸入偏壓： | 10µA |
| 電壓 - 輸入偏移： | 3000µV |
| 電流 - 電源： | 6mA |
| 電流 - 輸出 / 通道： | 125mA |
| 電壓 - 電源，單路/雙路(±)： | 4 V ~ 36 V，±2 V ~ 18 V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 8-SOIC（0.154"，3.90mm 寬） |
| 供應商設備封裝： | 8-SO |
| 包裝： | 管件 |
LT1228
9
1228fd
APPLICATIONS INFORMATION
The LT1228 contains two amplifiers, a transconductance
amplifier(voltage-to-current)andacurrentfeedbackampli-
fier(voltage-to-voltage).Thegainofthetransconductance
amplifier is proportional to the current that is externally
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
TheLT1228transconductanceamplifierhasahighimped-
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 (gm) is set
by the current that flows into Pin 5, ISET. The voltage at
Pin5 is two forward biased diode drops above the nega-
tive 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.Thetemperaturecoefficientofthisvoltageisabout
–4mV/°C (–3300ppm/°C) and the temperature coefficient
oftheloggingcharacteristicis3300ppm/°C.Itisimportant
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 (gm) is given as
gm = 10 ISET, with gm in (A/V) and ISET in (A).This rela-
tionship holds over many decades of set current (see the
characteristic curves). The transconductance is inversely
proportionaltoabsolutetemperature(–3300ppm/°C).The
input stage of the transconductance amplifier has been
designed to operate with much larger signals than is pos-
sible 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
curveSmall-SignalTransconductancevsDCInputVoltage).
Resistance Controlled Gain
If the set current is to be set or varied with a resistor or
potentiometeritispossibletousethenegativetemperature
coefficient at Pin 5 (with respect to Pin 4) to compensate
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 gm with a 2.5V Reference
LT1228 TA04
LT1004-2.5
V–
gm
5
4
R
ISET
R
Vbe
2.5V
2Eg
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
2.5Vreferenceresultsinzerogainchangewithtemperature:
Sincegm =
q
kT
×
ISET
3.87
= 10 ISET
and Vbe = Eg –
akT
q
where a
= In
cTn
Ic
≈ 19.4 at 27°C c = 0.001, n = 3, Ic = 100A
(
)
Eg is about 1.25V so the 2.5V reference is 2Eg. Solving
the loop for the set current gives:
ISET =
2Eg –2 Eg –
akT
q
R
or ISET =
2akT
Rq
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