this diagram shows a low-noise composite amplifier built by adding a low noise JFET pair (Q1 and Q2) as an input
preamplifier for the OPA172. Transistors Q3 and Q4 form a 2-mA current sink that biases each JFET with 1 mA
of drain current. Using 3.9-kΩ drain resistors produces a gain of approximately 10 in the input amplifier, making
the extremely-low, broadband-noise spectral density of the JFET pair, Q1 and Q2, the dominant noise source of
the amplifier. The output impedance of the input differential amplifier is large enough that a FET-input amplifier
such as the OPA172 provides superior noise performance over bipolar-input amplifiers.


AV = (1 + R3 / R4)

The resistances shown are standard 1% resistor values that produce a gain of approximately 100 (99.26) with
68° of phase margin. Gains less than 10 may require additional compensation methods to provide stability.
Select low resistor values to minimize the resistor thermal noise contribution to the total output noise.


电源模块内的跟踪放大器 Tracking Amplifier in Power Modules
商用电源 Merchant Power Supplies
传感器放大器 Transducer Amplifiers
桥式放大器 Bridge Amplifiers
温度测量 Temperature Measurements
应力计放大器 Strain Gauge Amplifiers
精密积分器 Precision Integrators
测试设备 Test Equipment


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