Chap. 5 The Operational Amplifier

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1 Chap. 5 The Operational Amplifier
2018/11/15 Chap. 5 The Operational Amplifier Contents 5.1 Operational Amplifier Terminals 5.2 Terminal Voltages and Currents 5.3 The Inverting-Amplifier Circuit 5.4 The Summing-Amplifier Circuit 5.5 The Noninverting-Amplifier Circuit 5.6 The Difference-Amplifier Circuit 5.7 A More Realistic Model for the OP Amplifier Objectives 能指明和描述運算放大器的五個端子，並應用電壓和 電流的限制予以簡化，導出一理想運算放大器。 2.能分析包含理想放大器的簡單電路，並分辨下列運算 放大器電路：反相放大器、和值放大器、非反相放大器 與差值放大器。 3.了解比較實際的運算放大器模型，並能利用它來分析包 含這些放大器的簡單電路。

2 5.1 Operational Amplifier Terminals
運算放大器的五個重要端子為： ◆ 反相輸入端(Inverting Input) ◆ 非反相輸入端(Noninverting Input) ◆ 輸出端(Output) ◆ 正電源(V ＋ ) ◆ 負電源(V － ) 2

3 5.2 Terminal Voltages and Currents
The voltage transfer characteristic of an op amp. 運算放大器的電壓轉移曲線：當它在線性區運作時，輸出電壓為輸入電壓之差再乘以A 倍，此倍數稱為增益(gain)。 線性區外則輸出電壓是飽和的。 3

4 Input Voltage Constraint For Ideal Op Amp
2018/11/15 Input Voltage Constraint For Ideal Op Amp 理想運算放大器的輸入電壓限制條件： 在線性區時，因增益無限大，所以vp = vn，亦稱為虛短路(virtual short)。 Input Current Constraint For Ideal Op Amp 理想運算放大器的輸入電流限制條件： 因等效輸入電阻無限大，所以ip = in = 0。 Symbol without the power supply terminals. From KCL, 4 4

5 EX 5.1 Analyzing an Op Amp Circuit
2018/11/15 EX 5.1 Analyzing an Op Amp Circuit a) Calculate vo if va = 1V and vb = 0V. b) Repeat (a) for va = 1V and vb = 2V. c) If va = 1.5V, specify the range of vb that avoids amplifier saturation. Ideal Negative feedback  linear region No negative feedback  saturation Then analyze and check! a) Because vo lies between ± VCC, the op amp is in its linear region. b) c) 5 5

6 5.3 The Inverting-Amplifier Circuit
Ideal Note: 6

7 Open-Loop Operation of an Inverting Amp
Open-Loop Gain of the Op Amp Ideal Note: The op amp can operate open loop in the linear mode only if Also note that : 7

8 5.4 The Summing-Amplifier Circuit
Ideal @inverting input terminal: & 8

9 5.5 The Noninverting-Amplifier Circuit
Ideal & & 9

10 5.6 The Difference-Amplifier Circuit
Ideal @inverting input terminal: If 簡化的差值放大器 輸入電壓與輸出電壓關係 10

11 The Difference Amplifier-Another Perspective
重新定義輸入電壓： 差模(differential mode) 和共模(common mode) 電壓輸入。 vo in terms of vcm & vdm If 只放大輸入電壓的差模部分， 而消除共模部分。 11

12 Measuring Difference-Amplifier Performance
當電阻不匹配時會造成共模增益不為零，因此以共模拒斥比(common mode rejection ratio, CMRR) 來表示與理想放大器的接近程度。 CMRR 值愈大代表愈接近理想差值放大器 Let OR 12

13 5.7 A More Realistic Model for the Op Amp
較實際的模型： 輸入電阻Ri 為有限值； 開迴路增益A為有限值； 輸出電阻Ro不為零。 µA741 op amp的典型值： Ri = 2 M  A = 105 Ro = 75  13

14 Analysis of an Inverting-Amplifier Circuit Using the More Realistic Op Amp Model
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15 Analysis of a Noninverting-Amplifier Circuit Using the More Realistic Op Amp Model
where What if Ro0, A, and Ri ? 15