【正文】 頁(yè) 共 15 頁(yè) Distributed amplifiers Main article: Distributed Amplifier These use transmission lines to temporally split the signal and amplify each portion separately in order to achieve higher bandwidth than can be obtained from a single amplifying device. The outputs of each stage are bined in the output transmission line. This type of amplifier was monly used on oscilloscopes as the final vertical amplifier. The transmission lines were often housed inside the display tube glass envelope. Switched mode amplifiers These nonlinear amplifiers have much higher efficiencies than linear amps, and are used where the power saving justifies the extra plexity. Negative resistance devices Negative resistances can be used as amplifiers, such as the tunnel diode amplifier. Microwave amplifiers Travelling wave tube (TWT) amplifiers Main article: Traveling wave tube Used for high power amplification at low microwave frequencies. They typically can amplify across a broad spectrum of frequencies。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。 the result is a pression effect, which (if the amplifier 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(jì)說(shuō)明書(shū) 第 4 頁(yè) 共 15 頁(yè) amplifier39。s output. Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25% for direct coupling of the output. Inductive coupling of the output can raise their efficiency to a maximum of 50%. Class B amplifiers have a very high efficiency but are impractical for audio work because of high levels of distortion (See: Crossover distortion). In practical design, the result of a tradeoff is the class AB design. Modern Class AB amplifiers are monly between 35–55% efficient with a theoretical maximum of %. Commercially available Class D switching amplifiers have reported efficiencies as high as 90%. Amplifiers of Class CF are usually known to be very high efficiency amplifiers. More efficient amplifiers run cooler, and often do not need any cooling fans even in multikilowatt designs. The reason for this is that the loss of efficiency produces heat as a byproduct of the ener