Question 1:
What is the relationship between the gain control elements and distortion?
Answer 1:
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- The gain control elements in this circuit are silicon diodes, which are nonlinear. As the gain (and hence the output amplitude) is increased, the diodes begin to conduct, gently chopping off the peaks and valleys of the sine waves. This symmetrical clipping begins to produce odd harmonics. As gain is further increased, the output waveform approaches a square wave.
Question 2:
What would happen to the distortion components if you replaced one of the diodes in the diode clamp circuit with a Schottky diode, which has a lower forward drop than a silicon diode?
Answer 2:
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- The asymmetrical clipping of the sine wave will produce even harmonics. This is easily visible both in an LTspice® fast Fourier transform (FFT) plot and the actual circuit, measured with Scopy’s spectrum analyzer.
Question 3:
What is the state of the art for distortion measurement instruments in the audio range?
Answer 3:
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- The state-of-the-art Keysight U8903B has a residual total harmonic distortion (THD) specification of –116 dB. Older analog analyzers such as the HP339 allow observation of distortion components on the order of –100 dB.
Question 4:
If you can’t afford a state of the art benchtop distortion analyzer, are there other options? (Hint: Watch the video in Part 1 of this series!)
Answer 4:
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- For signals in the audio range 20 Hz to 20 kHz, a moderately priced (~$30 USD) USB sound adapter can easily achieve 90 dB THD+ noise. Another $100 will get you to 100 dB or so, deducting 10 dB from claimed specifications to be conservative. Beyond that performance level, consider benchtop instruments with guaranteed specifications.
See the article: https://www.analog.com/en/resources/analog-dialogue/studentzone/studentzone-november-2025.html