Negative feedback automatically adjusts the net input signal of the op-amp by feeding a portion of the output signal back to the input and comparing it with the input signal, thereby accurately setting and stabilizing the closed-loop gain.
- Precise Gain: For op-amps with extremely high yet unstable open-loop gain, a feedback resistor network can set a closed-loop gain that is precise, predictable, and minimally dependent on the op-amp’s own parameters (e.g., -Rf/Rin for inverting amplifiers, 1 + Rf/R1 for non-inverting amplifiers).
- Reducing Gain Sensitivity to Op-Amp Parameters: Closed-loop gain mainly depends on the accuracy of external feedback components (resistors) and is insensitive to the op-amp’s open-loop gain, temperature drift, and component discreteness, improving circuit stability and repeatability.
- Suppressing Oscillations: A properly designed negative feedback network can compensate for the op-amp’s phase lag, increase phase margin, prevent the circuit from self-oscillating at high frequencies, and ensure stability.
Negative feedback trades off a portion of gain for a wider frequency response.
The open-loop gain-bandwidth product of an op-amp is usually a constant. After applying negative feedback to reduce the closed-loop gain, the -3dB bandwidth of the circuit expands accordingly. This is crucial for circuits that need to process wideband signals (e.g., audio amplifiers, data acquisition systems).
Different types of negative feedback topologies significantly alter the input and output impedances of the circuit.
- Voltage-Series Negative Feedback: Increases input impedance and decreases output impedance. This makes the circuit approach the characteristics of an ideal voltage source (low output impedance, strong driving capability) and imposes a small load on the previous-stage circuit (high input impedance). It is highly suitable for voltage amplification, buffering (voltage followers), and sensor interfaces (high input impedance reduces signal source loading).
- Voltage-Shunt Negative Feedback: Decreases both input impedance and output impedance. The input impedance is usually determined by the input resistor (e.g., Rin in inverting amplifiers), and the output impedance is low. It is suitable for current-to-voltage conversion (transimpedance amplifiers) and scenarios requiring specific input impedance matching.
- Current-Series Negative Feedback: Increases both input impedance and output impedance. This makes the circuit approach the characteristics of an ideal current source (high output impedance). It is suitable for voltage-to-current conversion (transconductance amplifiers) and constant current sources.
- Current-Shunt Negative Feedback: Decreases input impedance and increases output impedance. It is suitable for current amplification.
This ability to control impedance allows engineers to select the appropriate feedback topology for impedance matching based on signal source characteristics and load requirements, optimizing signal transmission efficiency and reducing load effects.
Negative feedback can automatically detect deviations (distortions) between the output signal and ideal linear amplification, and generate a correction signal to partially offset such deviations.
It significantly reduces harmonic distortion and intermodulation distortion generated inside the op-amp circuit, improving signal fidelity. This is crucial for applications such as high-fidelity audio amplifiers and precision measurement instruments.
Negative feedback also has a certain suppression effect on noise generated inside the op-amp (e.g., input offset voltage, offset current and their drift, thermal noise).
It helps improve the circuit’s signal-to-noise ratio and DC stability, making the output closer to the ideal amplified signal. Although it cannot completely eliminate external noise or drift of the reference voltage, it improves the performance of internal noise sources.
The configuration of a specific feedback network directly determines the basic function of the circuit.
- Voltage-Series: Non-inverting amplifiers, voltage followers (buffers).
- Voltage-Shunt: Inverting amplifiers, transimpedance amplifiers (current-to-voltage converters, e.g., photodiode preamplifiers).
- Current-Series: Transconductance amplifiers (voltage-to-current converters).
Technical Term Accuracy:
- Core feedback topologies (電壓串聯(lián) / 并聯(lián)、電流串聯(lián) / 并聯(lián)) are translated as "voltage-series/shunt, current-series/shunt"—the standard nomenclature in analog circuit engineering, ensuring consistency with IEEE and industry literature.
- Specialized terms like "跨阻放大器" (transimpedance amplifier), "跨導放大器" (transconductance amplifier), and "相位裕度" (phase margin) adhere to international electronics terminology, avoiding ambiguity for professional readers.
Formula & Parameter Clarity:
- Gain formulas (e.g., "-Rf/Rin, 1 + Rf/R1") and bandwidth indicators ("-3dB bandwidth") are retained in their original symbolic form, as they are universal in technical communication and require no translation.
Logical Cohesion:
- Chinese-style itemization (e.g., "1、", "應用：") is converted to English hierarchical formatting (e.g., "## 1.", "### Applications") for readability. Long explanatory sentences are split into concise clauses while preserving the original logical flow (e.g., linking "feedback resistor network" to its role in "setting closed-loop gain").
Function-Application Alignment:
- The relationship between feedback types and their use cases (e.g., "voltage-series → voltage followers") is explicitly maintained, ensuring readers can directly map topological characteristics to practical applications—critical for engineering reference materials.
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