What is the typical input impedance of an op-amp?
The correct answer is D: Very high. The typical input impedance of an operational amplifier (op-amp) is very high, typically in the megohm range (millions of ohms). This very high input impedance is a fundamental characteristic of op-amps that makes them ideal for voltage sensing applications.
Op-amps are designed with very high input impedance so they draw minimal current from the signal source. This means the op-amp doesn't load down the source circuit, allowing accurate voltage measurements and signal processing. The very high input impedance (often 1 megohm or more) contrasts with the very low output impedance (typically a few ohms) of op-amps. This high input/low output impedance combination makes op-amps excellent buffer amplifiers and voltage followers.
Exam Tip
Op-amp input impedance = Very high. Remember: Op-amps have very high input impedance (megohms) so they don't load the signal source. This contrasts with their very low output impedance.
Memory Aid
"**O**p-amp **I**nput = **M**egohms (think 'OI = M' = Op-amp Input = Megohms, very high)"
Real-World Application
You're using an op-amp to measure a voltage from a high-impedance sensor. The op-amp's very high input impedance (maybe 10 megohms) means it draws virtually no current from the sensor, so it doesn't affect the sensor's output voltage. This allows accurate voltage measurements without loading down the source circuit.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. 100 ohms is very low input impedance. Op-amps have very high input impedance, not low.
Option B: Incorrect. 10,000 ohms (10 kΩ) is still too low. Op-amp input impedance is typically in the megohm range.
Option C: Incorrect. Very low is the output impedance characteristic, not input impedance. Op-amps have very high input impedance.
题目解析
The correct answer is D: Very high. The typical input impedance of an operational amplifier (op-amp) is very high, typically in the megohm range (millions of ohms). This very high input impedance is a fundamental characteristic of op-amps that makes them ideal for voltage sensing applications. Op-amps are designed with very high input impedance so they draw minimal current from the signal source. This means the op-amp doesn't load down the source circuit, allowing accurate voltage measurements and signal processing. The very high input impedance (often 1 megohm or more) contrasts with the very low output impedance (typically a few ohms) of op-amps. This high input/low output impedance combination makes op-amps excellent buffer amplifiers and voltage followers.
考试技巧
Op-amp input impedance = Very high. Remember: Op-amps have very high input impedance (megohms) so they don't load the signal source. This contrasts with their very low output impedance.
记忆口诀
**O**p-amp **I**nput = **M**egohms (think 'OI = M' = Op-amp Input = Megohms, very high)
实际应用示例
You're using an op-amp to measure a voltage from a high-impedance sensor. The op-amp's very high input impedance (maybe 10 megohms) means it draws virtually no current from the sensor, so it doesn't affect the sensor's output voltage. This allows accurate voltage measurements without loading down the source circuit.
错误选项分析
Option A: Incorrect. 100 ohms is very low input impedance. Op-amps have very high input impedance, not low. Option B: Incorrect. 10,000 ohms (10 kΩ) is still too low. Op-amp input impedance is typically in the megohm range. Option C: Incorrect. Very low is the output impedance characteristic, not input impedance. Op-amps have very high input impedance.
知识点
Operational amplifier, Input impedance, Very high impedance, Op-amp characteristics
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.