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Frequency Response

Author: N.H. Crowhurst

Amplifiers produce another kind of distortion because they do not amplify all frequencies uniformly (by the same amount). Low frequencies are reduced in amplitude by the effect of coupling capacitors. Various stray capacitances and the leakage inductance in the output transformer result in loss of high frequencies. Thus no amplifier amplifies all frequencies absolutely uniformly.

Distortion may be caused by the unequal amplification of low and high frequencies

While this non-uniformity does not result in the introduction of any spurious or unwanted frequencies, it does result in the change of the relative magnitude of different frequencies in a composite audio waveform. The frequencies in the middle of the band will usually be amplified more than the extremely low or extremely high frequencies and this will cause a change in the resultant waveform. Fortunately the difference in amplification over the audio range in modern amplifiers is extremely small.

This form of distortion can readily be measured by taking frequency-response measurements of the amplifier. Audio voltages are fed into the amplifier at different frequencies, from the lowest to the highest, and the output voltage is carefully measured to see how closely it corresponds with the input voltage. If an input of 1 millivolt at 1000 cycles produces an output of 10 volts, then 1 millivolt is applied to the amplifier at all frequencies from 20 cycles up to 20,000 cycles and the output voltage is also measured. This will deviate up or down from 10 volts, according to the frequency response of the amplifier. The measurements are usually converted into db according to the ratio of the actual output voltage to the 10-volt output that should be there.

Taking amplifier frequency response

Calculating response in dB

Last Update: 2010-11-03