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Input impedance of Op amp_____.

2MΩ

75Ω

100000Ω

Correct Answer

⚙️ TE • Technical Concept & PrincipleElectricalElectronics

Option A

2MΩ

Quick Summary:

The correct answer is Option A (2MΩ). The practical operational amplifier (Op-Amp) such as the IC 741 has a very high input impedance, typically around 2MΩ, which minimizes loading effects on preceding circuit stages. In an ideal Op-Amp, the input impedance is considered to be infinite.

⚙️TETechnical SolutionConcept & Principle

💡 Explanation

🔢 Key Formulas

$R_{in} = \infty\ \Omega$ — Ideal Op-Amp input resistance

$R_{in} \approx 2\text{ M}\Omega$ — Typical practical Op-Amp (IC 741) input resistance

$I_{in} = \frac{V_{in}}{R_{in}} \approx 0\text{ A}$ — Input current drawn by the Op-Amp

⚙️ Working Principle

The input stage of a standard Op-Amp consists of a differential amplifier built using field-effect transistors (FETs) or bipolar junction transistors (BJTs). Because very little current flows into the differential inputs, the circuit presents an extremely high resistance to any input signal source.

📌 Key Points

▸
High input impedance prevents the loading effect on the signal source.
▸
Ideal Op-Amps draw zero input current ( $I_1 = I_2 = 0$ ).
▸
Practical BJT-based Op-Amps (like IC 741) have an input resistance of approximately $2\text{ M}\Omega$ .
▸
FET-input Op-Amps (like TL081) offer even higher input impedance ( $10^{12}\ \Omega$ ).

✅ Advantages

▸
Prevents loading effects on previous stages
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Ensures high voltage transfer efficiency from low-power sensors

❌ Disadvantages / Limitations

▸
High impedance node is more susceptible to pick up stray noise
▸
Small input bias currents still flow in practical bipolar devices

🛠️ Applications / Uses

▸
Voltage followers / Buffer amplifiers
▸
Instrumentation amplifiers
▸
High-impedance sensor interfaces

📄 Additional Information

▸
Option B (75Ω) represents a typical output impedance value ( $R_{out}$ ) for a practical IC 741 Op-Amp.
▸
Option C (100000Ω or 100kΩ) is a typical input resistance for specific amplifier circuits, but not the standard IC 741 rating.
▸
Option D (0Ω) is the ideal value for output impedance ( $R_{out}$ ), not input impedance.

📊 Diagram / Illustration

✅

A is correct — A practical IC 741 operational amplifier typically exhibits an input impedance ( $R_{in}$ ) of approximately 2MΩ.

Core Concepts Used

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Ideal vs Practical Op-Amp Parameters Input Impedance Loading Effect

💡 EXAM TIP

Always remember the basic IC 741 ideal vs practical parameters: $R_{in}$ is $\infty$ (ideal) / $2\text{ M}\Omega$ (practical), and $R_{out}$ is $0\ \Omega$ (ideal) / $75\ \Omega$ (practical).

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