【正文】 ier is an audio amplifier) sounds much less unpleasant to the ear. For these amplifiers, the 1 dB pression point is defined as the input power (or output power) where the gain is 1 dB less than the small signal gain. Someti mes this nonlinearity is deliberately designed in to reduce the audible unpleasantness of hard clipping under overload. The problem of nonlinearity is most often solved with negative feedback. Linearization is an emergent field, and there are many techniques, such as feedforward, predistortion, postdistortion, EER, LINC, CALLUM, cartesian feedback, etc., in order to avoid the undesired effects of the nonlinearities. Noise This is a measure of how much noise is introduced in the amplification process. Noise is an undesirable but inevitable product of the electronic devices and ponents, also much noise results from intentional economies of manufacture and design time. The metric for noise performance of a circuit is noise figure or noise factor. Noise figure is a parison between the output signal to noise ratio and the thermal noise of the input signal. Output dynamic range Output dynamic range is the range, usually given in dB, between the smallest and largest useful output levels. The lowest useful level is limited by output noise, while the largest is limited most often by distortion. The ratio of these two is quoted as the amplifier dynamic range. More precisely, if S = maximal allowed signal power and N = noise power, the dynamic range DR is DR = (S + N ) /N.[1] In many switched mode amplifiers, dynamic range is limited by the minimum output step size. Slew rate Slew rate is the maximum rate of change of the output, usually quoted in volts per second (or microsecond). Many amplifiers are ultimately slew rate limited (typically by the impedance of a drive current having to overe capacitive effects at some point in the circuit), which sometimes limits the full power bandwidth to frequencies well below the 中北大學(xué) 2021 屆畢業(yè)設(shè)計說明書 第 4 頁 共 15 頁 amplifier39。s smallsignal frequency response. Rise time The rise time, tr, of an amplifier is the time taken for the output to change from 10% to 90% of its final level when driven by a step input. For a Gaussian response system (or a simple RC roll off), the rise time is approximated by: tr * BW = , where tr is rise time in seconds and BW is bandwidth in Hz. Settling time and ringing The time taken for the output to settle to within a certain percentage of the final value (for instance %) is called the settling time, and is usually specified for oscilloscope vertical amplifiers and high accuracy measurement systems. Ringing refers to an output variation that cycles above and below an amplifier39。s final value and leads to a delay in reaching a stable output. Ringing is the result of overshoot caused by an underdamped circuit. Overshoot In response to a step input, the overshoot is the amount the output exceeds its final, steadystate value. Stability Stability is an issue in all amplifiers with feedback, whether that feedback is added intentionally or results unintentionally. It is especially an issue when applied over multiple amplifying stages. Stability is a major concern in RF and microwave amplifiers. The degree of an amplifier39。s stability can be quantified by a socalled stability factor. There are several different stability factors, such as the Stern stability factor and the Linvil stability factor, which specify a condition that must be met for the absolute stability of an amplifier in terms of its twoport parameters. Electronic amplifiers Main article: Electronic amplifier There are many types of electronic amplifiers, monly used in radio and television transmitters and receivers, highfidelity (hifi) stereo equipment, microputers and other electronic digital equipment, and guitar and other instrument amplifiers. Critical ponents 中北大學(xué) 2021 屆畢業(yè)設(shè)計說明書 第 5 頁 共 15 頁 include active devices, such as vacuum tubes or transistors. A brief introduction to the many types of electronic amplifier follows. Power amplifier The term power amplifier is a relative term with respect to the amount of power delivered to the load and/or sourced by the supply circuit. In general a power amplifier is designated as the last amplifier in a transmission chain (the output stage) and is the amplifier stage that typically requires most attention to power efficiency. Efficiency considerations lead to various classes of power amplifier: see power amplifier classes. Vacuum tube (valve) ampl