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Time to read 6 min
In electronics, frequency response describes how a system’s output changes as the frequency of the input signal varies. For an Op Amp, it shows the behavior across different signal frequencies in terms of gain (amplification) and phase (timing shift between input and output).
A simple analogy is how a speaker handles bass and treble. At very low frequencies (20-80Hz), it might reproduce deep bass clearly. At very high frequencies (above 10-15kHz), the output may lose strength or clarity.
In earlier discussions, we covered Op Amp negative feedback and derived gain expressions for configurations such as inverting and non-inverting amplifiers. Those derivations assumed an ideal Op Amp with an infinite open-loop gain (AOL) .
In reality, an Op Amp’s open-loop gain is not constant. It decreases as the input signal frequency increases. One effective concept to quantify this change is via Bode Plots, which will be introduced immediately.
Open-Loop Gain Roll-Off in a Bode Plot
This roll-off in gain is typically shown on a Bode plot, where the open-loop gain starts very high at low frequencies, then drops at a predictable slope (often −20 dB/decade) beyond a certain corner frequency. Typically, at low frequencies, an Op Amp’s open-loop gain stays at its maximum value, once the signal reaches the cut-off frequency, the gain is -3dB lower (roughly 71% of the maximum) and then begins to drop at a steady rate of about −20 dB per decade. This downward slope continues until the unity-gain point, where the gain reaches 0 dB. After this unity gain frequency, the Op Amp is not longer capable of amplifications.
This Bode plot gives a clearer view of how open-loop gain changes with frequency. At very low frequencies, such as 10 Hz, the gain can be as high as 120 dB (a factor of one million). By the time the frequency reaches 1 kHz, the gain has dropped to about 80 dB (a factor of ten thousand). At 100 kHz, it falls further to 40 dB (a factor of one hundred). This steady decline illustrates the −20 dB/decade slope and highlights why high-frequency signals experience much less amplification in real op amps compared to low-frequency signals.
Open-Loop Gain – Very high at low frequencies, drops with frequency.
Cut-off Frequency – Point where gain drops 3dB (or 71% of mauxmimum gain).
Unity-Gain Bandwidth – Frequency where gain = 1 (0 dB).
Gain-Bandwidth Product – Gain × bandwidth, roughly constant.
Slew Rate – Limits speed of voltage change.
Phase Margin – Stability measure at unity gain.
When evaluating an Op Amp’s frequency performance, datasheets provide a few key specifications that directly describe its frequency response.
Using the LM358 and TL072 as examples:
Gain-Bandwidth Product (GBW) – Indicates the product of closed-loop gain and bandwidth.
LM358: 0.7 MHz – suitable for low-frequency, general-purpose applications.
TL072: 5.25 MHz – better for audio and higher-speed signal processing.
Slew Rate (SR) – The maximum rate of change of output voltage.
LM358: 0.3 V/µs – limits high-frequency, large-amplitude performance.
TL072: 20 V/µs – handles faster voltage transitions with less distortion.
Another important parameter when considering frequency response is the Slew Rate, which is the maximum speed at which an Op Amp’s output can change.
In the test shown above, a 10 kHz square wave input shows that the LM358 (0.3 V/µs) cannot reproduce sharp edges, resulting in rounded transitions, while the TL072 (20 V/µs) closely follows the input shape.
A higher slew rate allows the op amp to handle faster signals with less distortion.
In industry, frequency response is often measured using specialized instruments such as network analyzers or Bode plotters. These devices can automatically sweep a wide frequency range, record gain and phase data at each point, and generate precise plots for analysis. They also allow engineers to apply controlled signal levels, compensate for measurement errors, and store results for comparison across different devices. Such tools are standard in professional labs where accuracy, repeatability, and efficiency are essential for design verification and troubleshooting.
From an educational perspective, we may not have access to this high-end equipment, but we can still measure frequency response using basic tools such as a function generator and an oscilloscope. By applying a fixed-amplitude sine wave to the amplifier's input and starting from a low frequency, we can gradually increase the frequency until the output amplitude drops noticeably. This manual sweep method gives a clear, hands-on way to visualize how gain changes with frequency.
Usually, 10–20 measurement points are taken across the target frequency range to capture the overall shape of the response. The data can then be plotted to form a frequency response curve. By default, many simple plots use a linear frequency scale , which is fine for a narrow range.
However, for a wider frequency span, a logarithmic frequency scale is always preferred, as it spreads out low-frequency points and compresses high-frequency points, making the gain drop and key features like the −3 dB point much easier to identify. In plotting tools like Excel or similar, set the frequency scale to Logarithm.
Frequency response is a critical factor in determining how an op amp will perform in practical designs. In audio circuits, it influences tonal accuracy, clarity, and the ability to reproduce subtle details. In sensor signal conditioning, it governs how quickly and faithfully the system responds to changes, which is especially important for dynamic or high-speed measurements. In active filter design (a topic we will explore in upcoming sections and thoroughly explained in our kit on Analog Signals and Filter Design) it directly sets the passband, stopband, and the sharpness of the transition between them. A well-chosen op amp with sufficient bandwidth and slew rate ensures that the circuit operates over the desired frequency range without introducing distortion, excessive phase shift, or loss of signal integrity, ultimately improving both performance and reliability.
Op amp frequency response describes how gain and phase change with signal frequency, defining usable bandwidth.
Open-loop gain roll-off and GBW set limits on closed-loop performance, while slew rate affects large-signal, high-speed accuracy.
Op Amp's datasheet parameters like GBW and slew rate can help predict frequency behavior and guide Op Amp selection.
Simple lab tools like a function generator and oscilloscope can effectively measure frequency response for educational purposes.
Internal capacitances and compensation networks create a low-pass effect, causing the open-loop gain to roll off beyond a certain frequency.
For a single-pole op amp, they are the same. GBW is the product of closed-loop gain and bandwidth, which remains roughly constant for a given device.
To some extent—using proper feedback networks, reducing capacitive loading, and choosing suitable closed-loop gains can help, but the op amp’s inherent limits remain.
We will dig more into this in future articles on this topic.
It provides better visibility across wide frequency ranges, spreading low-frequency data points and compressing high-frequency ones for easier interpretation.
Daniel Cao
Daniel Cao is the founder of EIM Technology, where he creates hands-on, beginner-friendly electronics education kits that blend practical hardware with clear, structured learning. With a background in engineering and a passion for teaching, he focuses on making complex concepts accessible to learners from all disciplines.
