Name: op-amp gain, feedback, and saturation
Uploaded: 2025-05-22T09:06:14.000Z
Duration: 1 min 17 s
Channel: JoVE

オペアンプの等価回路モデルについて考えてみましょう。出力セクションは、出力抵抗と直列に接続された電圧制御ソースで構成されています。
入力セクションでは、v1とv2は、それぞれ反転端子と非反転端子からグランドへの電圧を示します。
オペアンプの出力電圧は、入力電圧の差とオープンループ・ゲインの積に等しくなります。
電圧差が小さい場合、オペアンプは直線的に動作します。電圧が電源電圧を超えると、オペアンプは飽和します。
印加電圧と負帰還の組み合わせにより、オペアンプはその線形範囲内で動作することが保証されます。
最新のアンプはゲインと入力抵抗が大きいため、理想的なオペアンプとして近似できます。
理想的なオペアンプは、無限の開ループ利得、無限の入力抵抗、およびゼロ出力抵抗を備えています。入力端子電流と入力オフセット電圧は、理想的なオペアンプではどちらもゼロです。
さらに、帯域幅が無限であるため、任意の周波数の信号を増幅でき、スルーレートは無限であり、時間の経過とともに出力電圧を急速に変化させる能力を示しています。

オペアンプの特性

一般にオペアンプとして知られるオペアンプは、特別に設計された電子回路部品です。その目的は、他の回路要素と連携して、定義された信号処理操作を実行することです。Figure 1 に示すような、オペアンプの等価回路モデルを考えてみましょう。出力セクションは、出力抵抗 Ro と並列の電圧制御電源を備えます。

<img alt="Figure1" src="/files/ftp_upload/15077/15077_PT_Figure_1.svg" style="text-align: center; height: 166px;">
Figure 1: 非理想的なオペアンプの等価回路

Figure 1 は、入力抵抗 Ri が入力端子で観察されるテブナン等価抵抗に等しいことを示しています。同様に、出力抵抗 Ro は、出力で認識できるテブナン等価抵抗に対応します。差動入力電圧 (vd) は、グランドに対する非反転端子の電圧とグランドに対する反転端子の電圧の差として計算されます。

オペアンプはこれら 2 つの入力間の差を識別し、それをゲイン A で増幅し、出力に現れる電圧を生成します。ここで、A は開ループ電圧ゲインを表します。この名前は、出力から入力への外部フィードバックがない場合のオペアンプのゲインを意味するためです。

フィードバックは、オペアンプ回路を研究する際に理解すべき不可欠な概念です。負のフィードバックは、出力をオペアンプの反転端子にフィードバックすることで実現できます。出力から入力へのフィードバック パスが存在する場合、結果として得られる出力電圧と入力電圧の比は閉ループ ゲインとして知られています。

オペアンプには実用上の制限があります。このような制限の 1 つは、出力電圧の大きさが |V_CC|を超えることができないことです。基本的に、電源電圧は出力電圧を決定し、制限します。差動入力電圧に応じて、オペアンプは正の飽和、線形領域、または負の飽和の 3 つのモードのいずれかで機能します。 vd を線形範囲を超えて増加させようとすると、オペアンプは飽和に達し、 v_o = V_CC または v_o = -V_CC になります。

オペアンプの等価回路モデルについて考えてみましょう。出力セクションは、出力と直列に接続された電圧制御ソースで構成されています...

Education

JoVE Core

Electrical Engineering

電気工学

Unit 1

ユニット1

Operational Amplifiers

演算増幅器（オペアンプ）

KEY TERMS AND CONCEPTS

主要用語と概念

Characteristics of OpAmp

op-amp gain, feedback, and saturation

An operational amplifier, or op-amp, is an electronic circuit component used with other circuit parts to carry out a signal-processing task. It is often described with an equivalent circuit model. In that model, the output section includes a voltage-controlled source in parallel with the output resistance, R o.

The model also shows the input resistance, R i, which is the Thevenin equivalent resistance seen at the input terminals. The differential input voltage, v d, is the difference between the noninverting terminal voltage and the inverting terminal voltage, both measured relative to ground. The op-amp compares these two input voltages and amplifies the difference by the gain A to produce an output voltage.

A is called the open-loop voltage gain because it describes the op-amp without feedback from the output back to the input. Feedback is important in op-amp circuits. Negative feedback is created by connecting the output to the inverting terminal. When that feedback path exists, the output-to-input ratio is the closed-loop gain.

Op-amps also have practical limits. Their output voltage cannot exceed the magnitude of the supply voltage, |VCC|. The power supply sets this limit. Depending on the differential input voltage, the op-amp can operate in positive saturation, the linear region, or negative saturation. If v d is pushed beyond the linear range, the output saturates at either v o = VCC or v o = -VCC.

Op-Amp Gain, Feedback, and Saturation

operational amplifiers

Operational Amplifier Pinout and Signal Use

An operational amplifier, or op-amp, is a flexible circuit building block used in many electronic designs. It can act like a voltage-controlled voltage source, and it can also be used to create a voltage- or current-controlled current source. When resistors and capacitors are connected to its terminals, an op-amp can perform mathematical operations on signals. Op-amps are able to sum signals, amplify a signal, integrate it, or differentiate it. These features make them useful in practical...

design example: vintage mixing console

Op-Amp Preamp for a Vintage Console

A sound engineer designs an op-amp pre-amplifier for a vintage mixing console. The console’s output is too low for modern recording equipment, so the signal must be boosted before it can be used. The goal is to build a preamp that raises the audio level without changing the console’s character. The design must meet three clear requirements. It needs a gain of 10, an input impedance above 10 kilo-ohms so the console is not overloaded, and a frequency range that covers the full audio spectrum...

characteristics of opamp

An operational amplifier, or op-amp, is an electronic circuit component used with other circuit parts to carry out a signal-processing task. It is often described with an equivalent circuit model. In that model, the output section includes a voltage-controlled source in parallel with the output resistance, R o. The model also shows the input resistance, R i, which is the Thevenin equivalent resistance seen at the input terminals. The differential input voltage, v d, is the difference between...

inverting and non-inverting opamps

Op-Amp Gain: Inverting, Noninverting, Buffer

Operational amplifiers can be used as inverting, noninverting, and buffer amplifiers. In these circuits, the output voltage depends on the input voltage and the resistor setup around the op-amp. In an inverting amplifier, the input voltage is sent through a resistor to the inverting terminal. The non-inverting terminal is grounded, and a feedback resistor links the inverting terminal to the output. Using Kirchhoff's Current Law and an ideal op-amp model, the gain can be found. This circuit...

sum and difference opamps

Op-Amp Adders and Subtractors

Operational amplifiers can be wired to add or subtract signals. In these two op-amp setups, the summing amplifier combines several input signals into one output, while the difference amplifier compares two inputs and amplifies their difference. A summing amplifier is also called an adder. Input signals, such as audio signals, enter through input resistors. These resistors send currents through the feedback resistor, which produces an output voltage. Using Kirchhoff's current law and Ohm's Law,...

characteristics of practical op amps

Op Amp Noise Rejection and Buffering

Practical op amps are used to separate useful signals from unwanted noise. A difference amplifier should ideally amplify only the difference between two input signals. In real circuits, though, the output depends on both differential gain and common-mode gain. The common-mode rejection ratio, or CMRR, measures how well an op amp rejects common-mode signals. Common-mode signals are the same on both inputs. A higher CMRR helps an op amp ignore noise and amplify the wanted signal, such as music...

integrator and differentiator

Op-Amp Circuits in Cruise Control

Op-amp circuits are used in cruise control systems to help cars keep a constant speed. In these systems, operational amplifiers, or op-amps, can work as integrators and differentiators. Both circuits use resistors and capacitors, which makes them compact and cost-effective. An integrator circuit produces an output that is proportional to the integral of the input signal. It is made by replacing the feedback resistor in a standard inverting amplifier with a capacitor. The output and input...

cascaded op amps

Op Amp Stages in Series

Operational amplifiers can be connected in series as separate stages. This cascade works because op-amps have very high input resistance and very low output resistance. Those properties help each stage keep its input-output relationship while driving the next stage. In a cascaded op-amp circuit, the output of one stage becomes the input of the next. Each stage adds gain to the signal as it passes through. The overall gain is the product of the gain of each stage. This design can improve...

instrumentation amplifier

ECG Signal Amplifier Design

An ECG signal amplifier helps an electrocardiography machine measure the heart’s electrical activity. ECG electrodes detect tiny voltage changes on the skin that come from depolarization of the heart muscle during each heartbeat. These signals are in the microvolt range, so they can be easily hidden by noise or interference. To solve this problem, ECG machines use an instrumentation amplifier. This amplifier boosts the ECG waveform while reducing common-mode noise, which is interference that...

key terms and concepts

Learn how an op-amp compares input voltages, uses open-loop and closed-loop gain, and reaches positive or negative saturation when output limits are exceeded.

Op-Amp Gain, Feedback, and Saturation | JoVE K12

JoVE Core 電気工学 は、電気回路の基本概念と回路解析テクニックを網羅したアニメーションビデオシリーズです。オペアンプと回路変換および 1 次・ 2 次回路の概念を網羅しています。実験を撮影した実写動画では、設計例や現代社会での応用が含まれており、複雑な概念も分かりやすく学習いただけます。

electrical engineering

Op-Amp Gain, Feedback, and Saturation

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