Frequency Response

Frequency response is the measure of any system's output spectrum in response to an input signal. In the audible range it is usually referred to in connection with electronic amplifiers, microphones and loudspeakers. Radio spectrum frequency response can refer to measurements of coaxial cables, category cables, video switchers and wireless communications devices. Subsonic frequency response measurements can include earthquakes and electroencephalography (brain waves).

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

                                                                                        mid-band

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.

Fuente de origen: wps.prenhall.com/...7/0,9426,1508470-,00.html 

                           http://en.wikipedia.org/wiki/Frequency_response

Datos personales: Lenny. Z Perez M.

Asignatura: EES

---

Connect to the next generation of MSN Messenger  Get it now!

Amplifier Low-Frequency Response

By Abe Michelen

From Electronics I Laboratory Manual

A common-emitter amplifier¹ in general will include three coupling capacitors that play an important role in the low-frequency response of the system.  These capacitors are the input and output coupling capacitors² and the emitter biapass capacitor³.

Figure 19.1 shows a typical common-emitter amplifier with a source, V_s (including its source resistance, R_s) and a load, R_L.  The low-frequency response of this amplifier is determined in part by these three capacitors (C_i, C_o and C_E).  The result is essentially a combination of three high-pass filter networks that allow signals having frequencies greater than the cutoff frequency of the dominant network to pass through while attenuating all others.  This frequency is the so-called 3-dB point.  At this frequency, as you should know, the voltage is only 70.7% of the mid-frequency gain, or in decibels, it is the frequency at which the gain is 3-dB below the mid-frequency gain.
Each one of the three capacitors makes a contribution to the overall frequency response of the amplifier.  Each one behaves like the capacitor in a high–pass filter.  Therefore, each one will contribute with a cutoff frequency of its own.  The cutoff frequencies for each capacitor are given by the following equations:

fc_i  The cutoff frequency due to the input coupling capacitor, C_i, is given by

(19.1)

fC_o  The critical frequency due to the output coupling capacitor, C_o, is given by

(19.2)

fc_E  Finally, the cutoff frequency due to the emitter bypass capacitor, C_E, is

(19.3)

Note: The above equations are derived for the circuit shown in Fig. 19.1  where instead of one we have two emitter resistors.  This is called swamping re.  This is a method used to minimize the effect of rewithout reducing the voltage gain to its minimum value.  By doing this we swamp out the effect of reon the voltage gain.  Swamping is, in effect, a compromise between having a bypass capacitor across RE and having no bypass capacitor at all.

For the common-emitter amplifier shown in Fig. 19.1 the voltage gain is given by

		(19.4)

where

(19.5)

and I_E is the quiescent dc emitter current.
The voltage gain in decibels is given by the following equation

Gv = 20 log(Av) dB

(19.6)

---

¹ Or basically any other type of amplifier.

² They are important if you want to keep the input and output voltages as pure ac voltages.

³ This is important to produce a high voltage gain.




Source: http://www.globalspec.com/reference/9649/359749/Amplifier-Low-Frequency-Response