Frequency Response of Amplifiers. Concept of frequency response, Human ear response to audio frequencies, significance of Octaves and Decades. The decibel unit. Square wave testing of amplifiers, Miller's theorem. Effect of coupling, bypass, junction and stray capacitances on frequency response for BJT and FET amplifiers. Concept of dominant pole.
Frequency response requirements differ depending on the application. In high fidelity audio, an amplifier requires a frequency response of at least 20–20,000 Hz, with a tolerance as tight as ±0.1 dB in the mid-range frequencies around 1000 Hz, however, in telephony, a frequency response of 400–4,000 Hz, with a tolerance of ±1 dB is sufficient for intelligibility of speech
Frequency response curves are often used to indicate the accuracy of electronic components or systems.When a system or component reproduces all desired input signals with no emphasis or attenuation of a particular frequency band, the system or component is said to be "flat", or to have a flat frequency response curve.
Frequency response of a low pass filter with 6 dB per octave or 20 dB per decade
The frequency response is typically characterized by the magnitude of the system's response, measured in decibels (dB), and the phase, measured in radians, versus frequency. The frequency response of a system can be measured by applying a test signal, for example:
1. - Applying an impulse to the system and measuring its response
2. - Sweeping a constant-amplitude pure tone through the bandwidth of interest and measuring the output level and phase shift relative to the input
3.- Applying a signal with a wide frequency spectrum (for example digitally-generated maximum length sequence noise, or analog filtered white noise equivalent, like pink noise), and calculating the impulse response by deconvolution of this input signal and the output signal of the system
These typical response measurements can be plotted in two ways: by plotting the magnitude and phase measurements to obtain a Bode plot or by plotting the imaginary part of the frequency response against the real part of the frequency response to obtain a Nyquist plot
Once a frequency response has been measured (e.g., as an impulse response), providing the system is linear and time-invariant, its characteristic can be approximated with arbitrary accuracy by a digital filter. Similarly, if a system is demonstrated to have a poor frequency response, a digital or analog filter can be applied to the signals prior to their reproduction to compensate for these deficiencies.
Frequency response measurements can be used directly to quantify system performance and design control systems. However, frequency response analysis is not suggested if the system has slow dynamics.
Frequency Response of Amplifiers.
The essential purpose of an amplifier is to accept an input signal and provide an enhanced copy of that signal as an output. . In addition to being classified by function, amplifiers are classified by frequency response. The frequency response of an amplifier refers to the band of frequencies or frequency range that the amplifier was designed to amplify. However there is a fundamental relationship between signal frequency and gain such that a given gain cannot be maintained over an arbitrarily large frequency range. Physically it takes time for electric charge in a device to redistribute itself in response to a control signal, and so the response of a device to a control signal inevitably becomes jumbled for very fast signal changes. This is an ultimate limit to circuit response; degradation of the response may begin at lower signal frequencies because of delays associated with other circuit components. Circuit components can introduce degradation of the frequency response of a circuit at low frequencies as well as high, as will be seen.
In general the frequency response of an electronic circuit.
Low Frecuency High Frecuency
There is a 'mid-band' range of operation for which the gain is substantially independent of frequency, bounded by 'high' and 'low' frequency ranges in which the gain is degraded. An amplifier is 'wide-band' if the ratio of a frequency measuring the onset of the high-frequency degradation to a corresponding frequency for low frequency degradation is relatively large. A basic audio amplifier, for example, has a substantially 'flat' response extending from about 100 Hz to roughly 10KHz. 'Narrow-band' amplifiers, used for more specialized purposes, approximate selective amplification at a single frequency.
To simplify consideration of the frequency response of wide-band amplifiers analysis generally is separated into three frequency ranges. The argument used to justify this separation is that those circuit components associated with low-frequency degradation have by definition lost significant influence on the response in mid-band, and supposing a monotonic behavior have no influence on the high-frequency response. The converse argument removes the influence at low frequencies of those components affecting the high frequency response. And, of course, in mid-band by definition neither of these sets of components influences the response significantly. The mid-band range is the one assumed in previous work, for example by neglecting the influence of coupling and bypass capacitors.
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