📚 Electronics | 电子学
Electronics is a major topic in the CIE A-Level Physics A2 syllabus. It brings together sensors, potential dividers, operational amplifiers and output devices. You need to understand how transducer signals are conditioned and amplified, and how op-amps can be used as comparators or amplifiers.
电子学是 CIE A-Level 物理 A2 阶段的重要课题。它将传感器、分压器、运算放大器和输出器件结合在一起。你需要理解传感器信号如何被调理和放大，以及运算放大器如何用作比较器或放大器。
1. Sensing Devices | 传感器件
Electronic systems usually begin with a sensing device that converts a physical quantity into a resistance or voltage. Common examples are the LDR, thermistor, strain gauge and piezoelectric transducer.
电子系统通常从传感器件开始，它把物理量转换为电阻或电压。常见的例子有光敏电阻、热敏电阻、应变计和压电换能器。
An LDR has a resistance that falls as light intensity increases. This makes it useful in light-operated switches and automatic lighting circuits.
光敏电阻的电阻随光照强度增大而减小。这使它可用于光控开关和自动照明电路。
An NTC thermistor shows a fall in resistance as temperature rises. A strain gauge changes resistance when stretched or compressed, while a piezoelectric sensor generates a small e.m.f. under stress.
负温度系数热敏电阻的电阻随温度升高而下降。应变计在受到拉伸或压缩时电阻发生变化，而压电传感器在受力时产生小电动势。
2. Potential Divider Circuits | 分压器电路
A potential divider is two resistors in series connected across a supply. The output voltage is taken across one resistor.
分压器是由两个串联电阻跨接在电源上组成。输出电压取自其中一个电阻两端。
Vout = Vin × R₂ / (R₁ + R₂)
If the sensor is placed as R₂, its changing resistance directly changes Vout. This is how a non-electrical input is converted into an electrical signal.
如果把传感器放在 R₂ 的位置，它变化的电阻会直接改变 Vout。这就是非电学输入转换为电信号的方式。
To produce a rising output voltage with rising light intensity, the LDR is usually placed as the top resistor R₁, so the output across the fixed resistor increases when the LDR resistance falls.
为了使输出电压随光照增强而升高，通常把光敏电阻放在 R₁ 的位置，这样当光敏电阻阻值下降时，固定电阻两端的输出电压会上升。
3. The Ideal Operational Amplifier | 理想运算放大器
An operational amplifier, or op-amp, is a high-gain differential amplifier with two inputs and one output. The non-inverting input is labelled V⁺ and the inverting input is labelled V⁻.
运算放大器是一种高增益差分放大器，有两个输入端和一个输出端。同相
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