1、中北大学 2010 届毕业设计说明书 第 1 页 共 15 页 A Broadband Amplifier with Huge Gain-bandwidth Product and Low Power Consumption Gain The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels. (When measured in decibels it is logarithmically related to the powe
2、r ratio: G(dB)=10 log(Pout /(Pin). RF amplifiers are often specified in terms of the maximum power gain obtainable, while the voltage gain of audio amplifiers and instrumentation amplifiers will be more often specified (since the amplifiers input impedance will often be much higher than the source i
3、mpedance, and the load impedance higher than the amplifiers output impedance). Example: an audio amplifier with a gain given as 20 dB will have a voltage gain of ten (but a power gain of 100 would only occur in the unlikely event the input and output impedances were identical). Bandwidth The bandwid
4、th of an amplifier is the range of frequencies for which the amplifier gives satisfactory performance. The definition of satisfactory performance may be different for different applications. However, a common and well-accepted metric is the half power points (i.e. frequency where the power goes down
5、 by half its peak value) on the output vs. frequency curve. Therefore bandwidth can be defined as the difference between the lower and upper half power points. This is therefore also known as the 3 dB bandwidth. Bandwidths (otherwise called frequency responses) for other response tolerances are some
6、times quoted (1 dB, 6 dB etc.) or plus or minus 1dB (roughly the sound level difference people usually can detect). The gain of a good quality full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing). In ultra high fidelity amplifier desig
7、n, the amps frequency response should extend considerably beyond this (one or more octaves either side) and might have 3 dB points 65 kHz. Professional touring amplifiers often have input and/or output filtering to sharply limit frequency response beyond 20 Hz-20 kHz； too much of the amplifiers pote
8、ntial output power would otherwise be wasted 中北大学 2010 届毕业设计说明书 第 2 页 共 15 页 on infrasonic and ultrasonic frequencies, and the danger of AM radio interference would increase. Modern switching amplifiers need steep low pass filtering at the output to get rid of high frequency switching noise and harm
9、onics. Efficiency Efficiency is a measure of how much of the power source is usefully applied to the amplifiers output. Class A amplifiers are very inefficient, in the range of 1020% with a max efficiency of 25% for direct coupling of the output. Inductive coupling of the output can raise their effi
10、ciency 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 commonly between 3555%
11、efficient with a theoretical maximum of 78.5%. Commercially available Class D switching amplifiers have reported efficiencies as high as 90%. Amplifiers of Class C-F are usually known to be very high efficiency amplifiers. More efficient amplifiers run cooler, and often do not need any cooling fans
12、even in multi-kilowatt designs. The reason for this is that the loss of efficiency produces heat as a by-product of the energy lost during the conversion of power. In more efficient amplifiers there is less loss of energy so in turn less heat. In RF Power Amplifiers, such as cellular base stations a
13、nd broadcast transmitters, specialist design techniques are used to improve efficiency. Doherty designs, which use a second transistor, can lift efficiency from the typical 15% up to 30-35% in a narrow bandwidth. Envelope Tracking designs are able to achieve efficiencies of up to 60%, by modulating
14、the supply voltage to the amplifier in line with the envelope of the signal. Linearity An ideal amplifier would be a totally linear device, but real amplifiers are only linear within limits. When the signal drive to the amplifier is increased, the output also increases until a point is reached where
15、 some part of the amplifier becomes saturated and cannot produce any more output； this is called clipping, and results in distortion. 中北大学 2010 届毕业设计说明书 第 3 页 共 15 页 In most amplifiers a reduction in gain takes place before hard clipping occurs； the result is a compression effect, which (if the ampl
16、ifier is an audio amplifier) sounds much less unpleasant to the ear. For these amplifiers, the 1 dB compression point is defined as the input power (or output power) where the gain is 1 dB less than the small signal gain. Sometimes this nonlinearity is deliberately designed in to reduce the audible
17、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 avo
18、id the undesired effects of the non-linearities. 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 components, also much noise results from intentional economies of manufacture and design
19、 time. The metric for noise performance of a circuit is noise figure or noise factor. Noise figure is a comparison 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
20、 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
21、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 overcome capacitive effects at some point in the circuit), which sometimes limits the full power bandwidth to frequencies well below the
