Amplifier distortion can be measured by driving the amplifier with a clean sine wave and observing the output with a spectrum analyzer. In this article, a low-distortion Wien bridge oscillator is used as the 1 kHz signal source, while a Digilent Analog Discovery measures the harmonic content produced by a common emitter transistor amplifier.
The goal is practical: show how an amplifier that appears clean on an oscilloscope may still add noise and harmonics visible in the frequency domain. The demonstration compares small-signal operation with the onset of clipping, where the amplifier leaves its linear operating region and produces strong harmonic distortion.
Do not trust the oscilloscope alone.
Key Takeaways
- A spectrum analyzer reveals amplifier distortion that may not be obvious on an oscilloscope.
- Meaningful distortion testing requires a clean sine-wave source; otherwise, the input harmonics can be mistaken for amplifier distortion.
- A common emitter amplifier provides gain and a 180° phase shift, but real amplifiers also add noise and harmonic content.
- Clipping marks the loss of linear operation and produces strong harmonic content visible in the spectrum.
Last update: 05 Jun 2026
Wien bridge oscillator as a low distortion source
The spectrum analyzer is a useful measurement tool for amplifier analysis. However, for meaningful results we must “feed” the amplifier with a pure sinusoid signal. The ideal amplifier will act as a “wire with gain” providing a faithful replica of the original. Real world amplifiers will add noise, harmonics, and even intermodulation with the power supply ripple frequency or a second tone should you chose to perform a two-tone test.
In this article we will discover the performance of the common emitter transistor amplifier as shown in Figure 1 and schematically as Figure 2. The test will be conducted using a 1 kHz tone generated from the Wien bridge oscillator as described in this previous post. This simple oscillator has remarkably spectral purity which is essential for analyzing amplifier performance. In fact, for the single tone that it produces, the Wien bridge has better performance than many commercially available signal generators.
Figure 1: Picture of a common emitter transistor amplifier with a Wien bridge oscillator in the background.
Figure 2: Multisim live schematic of the transistor circuit. Click on the schematic to view the simulation.
Common Emitter amplifier as the Device Under Test
The Common Emitter (CE) transistor amplifier shown in Figures 1 and 2 is a classic design with degenerative feedback via resistor R5. It features fixed bias via resistors R2 and R3 with capacitive input coupling via C1. The output is also capacitively coupled via C2 to the load resistor R6.
From an AC analysis resistors R4 and R6 are in parallel. The resulting amplifier gain is approximated as
Gain \approx \dfrac{10\ k\Omega \ || \ 10 \ k \Omega} {470 \ \Omega}
Results
Two experiments were conducted to evaluate the amplifier. The first is a small signal test. The second is a larger signal test set with the R1 gain set to the onset of clipping.
The results of the first test are shown in Figures 3, 4, and 5:
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Figure 3 presents the time-domain (oscilloscope) representation of the input (orange) and output signal (blue). This demonstrates that the amplifier gain is approximately 10 and has the expected 180° phase shift associated with the common emitter amplifier. No waveshape distortion is present, although channel 1 does exhibit noise.
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Figure 4 presents the spectrum of the input signal. The waveform shows a pure sinusoid with a -22 dBV level and a noise floor at approximately -90 dBV. Please see this earlier introduction to the Wien bridge oscillator article for more information about the signal source.
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Figure 5 presents the amplifier’s output spectrum. The output is relatively clean with 2nd order harmonics at 2, 4, and 6 kHz. On the far left we can also see a small 60 or 120 Hz component. This is likely line hum picked up from the DC power supply.
Figure 3: Oscilloscope representation of the amplifier input (orange) and output (blue).
Figure 4: Spectrum of the amplifier’s input signal showing a pure sinusoid at approximately 1 kHz.
Figure 5: Spectrum of the amplifier’s output signal showing the 1 kHz fundamental and even harmonics at 2, 4, and 6 kHz.
Tech Tip: The perfect amplifier is a unicorn – there is no such thing as a “wire with gain.” Real world amplifiers will add noise to the signal. In audio amplifier this manifests as a hiss sound plus hum at the line frequency. Amplifiers can also add harmonics. As shown in Figures 5 and 7, the simple transistor amplifier added spectral components that were not present in the Figure 3 input signal.
The results of the second test are captured in Figures 6 and 7. For this test the R1 “volume control” was increased until the output displayed soft clipping as shown in Figure 6. Close visual inspection will show that the tops of the blue output waveform have been flattened.
The spectrum for the overdriven amplifier is shown in Figure 7. The amplifier’s output has strong even harmonics at 2, 4, 6, and 8 kHz. It also has strong odd harmonics at 3, 5, 7, and 9 kHz.
Figure 6: Time-domain signal showing the onset of clipping for the blue output signal.
Figure 7: The overdriven amplifier has strong even and odd harmonics.
Tech Tip: Be mindful of your spectrum analyzer attenuation settings. An overdriven spectrum analyzer will display garbage across the spectrum. Attenuator setting should be one of the first things to check if there is doubt about a signal’s integrity.
Conclusion
An amplifier may be characterized by its spectrum. For these tests we inject the best – spectrally pure – sinusoid available. We then used a spectrum analyzer to monitor the amplifier output. The presence of and nature of the amplifier’s distortion is readily apparent.
Please give this method a try. Years ago, it would have been difficult due to limited test equipment capabilities. Today, if you are a student, chances are very high that the classroom oscilloscopes have a built-in spectrum analyzer capability. Look for the Fast Fourier Transform (FFT) function under the math menu.
Best wishes,
APDahlen
About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.