参数资料
型号: OP291GS-REEL7
厂商: Analog Devices Inc
文件页数: 9/24页
文件大小: 0K
描述: IC OPAMP GP R-R 3MHZ DUAL 8SOIC
标准包装: 1,000
放大器类型: 通用
电路数: 2
输出类型: 满摆幅
转换速率: 0.5 V/µs
增益带宽积: 3MHz
电流 - 输入偏压: 30nA
电压 - 输入偏移: 80µV
电流 - 电源: 260µA
电流 - 输出 / 通道: 16mA
电压 - 电源,单路/双路(±): 2.7 V ~ 12 V,±1.35 V ~ 6 V
工作温度: -40°C ~ 125°C
安装类型: 表面贴装
封装/外壳: 8-SOIC(0.154",3.90mm 宽)
供应商设备封装: 8-SO
包装: 带卷 (TR)
OP191/OP291/OP491
Rev. E | Page 17 of 24
THEORY OF OPERATION
The OP191/OP291/OP491 are single-supply, micropower
amplifiers featuring rail-to-rail inputs and outputs. To achieve
wide input and output ranges, these amplifiers employ unique
input and output stages. In Figure 61 , the input stage comprises
two differential pairs, a PNP pair and an NPN pair. These two
stages do not work in parallel. Instead, only one stage is on for
any given input signal level. The PNP stage (Transistor Q1 and
Transistor Q2) is required to ensure that the amplifier remains
in the linear region when the input voltage approaches and
reaches the negative rail. On the other hand, the NPN stage
(Transistor Q5 and Transistor Q6) is needed for input voltages
up to and including the positive rail.
For the majority of the input common-mode range, the PNP
stage is active, as is shown in Figure 12. Notice that the bias
current switches direction at approximately 1.2 V to 1.3 V
below the positive rail. At voltages below this, the bias current
flows out of the OP291, indicating a PNP input stage. Above
this voltage, however, the bias current enters the device,
revealing the NPN stage. The actual mechanism within the
amplifier for switching between the input stages comprises
Transistor Q3, Transistor Q4, and Transistor Q7. As the input
common-mode voltage increases, the emitters of Q1 and Q2
follow that voltage plus a diode drop. Eventually, the emitters
of Q1 and Q2 are high enough to turn on Q3, which diverts the
8 μA of tail current away from the PNP input stage, turning it
off. Instead, the current is mirrored through Q4 and Q7 to
activate the NPN input stage.
Notice that the input stage includes 5 kΩ series resistors and
differential diodes, a common practice in bipolar amplifiers to
protect the input transistors from large differential voltages.
These diodes turn on whenever the differential voltage exceeds
approximately 0.6 V. In this condition, current flows between
the input pins, limited only by the two 5 kΩ resistors. This
characteristic is important in circuits where the amplifier may
be operated open-loop, such as a comparator. Evaluate each
circuit carefully to make sure that the increase in current does
not affect the performance.
The output stage in OP191 devices uses a PNP and an NPN
transistor, as do most output stages; however, Q32 and Q33, the
output transistors, are actually connected with their collectors
to the output pin to achieve the rail-to-rail output swing. As the
output voltage approaches either the positive or negative rail,
these transistors begin to saturate. Thus, the final limit on
output voltage is the saturation voltage of these transistors,
which is about 50 mV. The output stage does have inherent gain
arising from the collectors and any external load impedance.
Because of this, the open-loop gain of the amplifier is
dependent on the load resistance.
Q1
8A
5k
Q3
5k
–IN
Q5 Q6
Q11
Q10
Q8
Q7
Q4
Q13
Q15
Q14
Q12
Q9
Q16
Q17
Q18
Q19
Q20
Q21
Q24
Q23
Q22
Q27
Q26
Q30
Q31
Q28
Q25
Q29
Q32
VOUT
Q33
10pF
+IN
Q2
0
02
94
-06
5
Figure 61. Simplified Schematic
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