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 power 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 impedance, 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 bandwidth 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 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 minus;3 dB bandwidth. Bandwidths (otherwise called 'frequency responses') for other response tolerances are sometimes quoted (minus;1 dB, minus;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 design, the amps frequency response should extend considerably beyond this (one or more octaves either side) and might have minus;3 dB points lt; 10 and gt; 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 potential output power would otherwise be wasted 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 harmonics.
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 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 commonly between 35–55% 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 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 and 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 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 some part of the amplifier becomes saturated and cannot produce any more output; this is called clipping, and results in distortion.
In most amplifiers a reduction in gain takes place before hard clipping occurs; the result is a compression effect, which (if the amplifier 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
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低功耗和高增益的宽带放大器
增益：一个放大器的增益是输出和输入功率或者幅度的比值，通常以分贝测量。当以分贝测量时，其与功率成对数关系：G(dB)=10log(Pout/(Pin))。射频放大器通常根据可获得的最大功率增益来规定，而音频放大器和仪表放大器的电压增益将更多地被指定，因为放大器的输入阻抗通常会远高于源阻抗，并且负载阻抗会高于放大器的输出阻抗。例如：一个增益为20dB的音频放大器的电压增益为10，但输入和输出阻抗相同的情况下，功率增益仅为100。
带宽：放大器的带宽是放大器给出的“令人满意的性能”的频率范围。对于不同的应用，“令人满意的性能”的定义可能不同。然而，一个普遍公认的度量标准是输出频率曲线上的半功率点（即功率下降一半峰值的频率）。因此带宽可以被定义为低端和高端功率点之间的差异。这也同样是被称为-3dB带宽的原因。其他响应容限的带宽也被称为“频率响应”，有时会被引用-1dB,-6dB或“正负1dB”，大概是人们通常可以检测到的声级差。
高质量全频音频放大器的增益在20Hz至20kHz（正常人类听力范围）之间基本保持平坦。在超高保真放大器设计中，放大器的频率响应应该远远超过这个范围（任一侧的一个或者多个八度音阶），并可能具有-3dB点的lt;10和gt;65kHz。专业巡回放大器通常具有输入和/或输出滤波功能，以便将频率响应限制在20Hz-20kHz以上；否则放大太多的放大器的潜在输出功率将被浪费在次声和超声频率上，AM无线电干扰的危险性会增加。现代开关放大器需要在输出端进行陡峭的低通滤波，以消除高频开关噪声和谐波。
效率：效率是衡量电源适用于放大器输出的程度。A类放大器效率非常低，在10-20%的范围内，对于输出的直接耦合，最大效率为25%。输出的感应耦合可以将其效率提高刀最大50%。
B类放大器具有非常高的效率，但由于高失真水平，对于音频工作而言不切实际（请参考：分频失真）。在实际设计中，折中的效果是AB类设计。现代AB类放大器的效率通常在35%-55%，理论最大值在78.5%。
市场上可买到的D类开关放大器据说效率高达90%。C-F类放大器通常被认为是非常高效的放大器。
效率更高的放大器运行温度更低，即使在几千瓦的设计中也不需要任何冷却风扇。其原因是效率损失产生的热量是电力转换过程中损失的能量的副产品。在更高效的放大器中，能量损失更少，因此热量更少。
在RF功率放大器中，例如蜂窝基站和广播发射机，专业设计技术被用于提高效率。Doherty设计使用第二个晶体管，可以在窄带宽内将效率从典型的15%提高到30-35%。通过根据信号包络调整放大器的电源电压，包络跟踪设计可以实现高达60%的效率。
线性：一个理想的放大器将是一个完全线性的器件，但真正的放大器只有在限定范围内才是线性的。
当放大器的信号驱动增加时，输出也会增加，直到达到放大器的某个部分变得饱和并且不能产生更多输出的点，这称为削波，并且会导致失真。
在大多数放大器中，在产生明显的削波现象之前发生增益下降，结果是压缩效应。如果放大器是音频放大器，听起来不太令人舒服。
对于这些放大器，1dB压缩点被定义为增益比小信号增益小1dB的输入功率（或输出功率）。有时候，这种非线性特性是为了减少超载下的听觉不愉快而设计的。
非线性问题最常用负反馈解决。线性化是一个新兴领域，为了避免非线性的不良影响，有许多技术，如前反馈，后失真，EER，LINC，CALLUM，笛卡尔反馈等。
噪声：这是衡量放大过程中引入多少噪声的一种方法。噪声是电子设备和部件的不可取但不可避免的产物，噪声也是由有意的制造经济和设计时间造成的。
电路的噪声性能指标是噪声系数或者噪声因素。噪声系数是输出信噪比和输入信号的热噪声之间的比较。
输出动态范围：输出动态范围是最小和最大有用输出电平之间的范围，通常以dB为单位。最低的有用水平受到输出噪声的限制，而最大的有用水平受到失真的限制。这两个比率被称为放大器动态范围。更确切的说，如果S=最大允许信号功率和N=噪声功率，则动态范围DR为DR=（S N）/N。在很多开关模式放大器中，动态范围受最小输出步长的限制。
摆率：摆率是输出的最大变化率，通常以伏特每秒或微秒表示。许多放大器最终受到转换速率的限制（比如，驱动电流的阻抗必须克服电路某些点的电容效应），这有时会将全功率带宽限制在远低于放大器小信号频率响应的频率。
上升时间：放大器的上升时间tr是由阶跃输入驱动时输出从最终电平的10%变为90%所需的时间。对于高斯响应系统（或者简单的RC滚降），上升时间近似为：tr*BW=0.35，其中tr是以秒为单位的上升时间，BW是以Hz为单位的带宽。
稳定时间和振铃：输出稳定在最终值的一定百分比（例如0.1%）内所需的时间称为稳定时间，通常用于示波器垂直放大器和高精度测量系统。
振铃是指在放大器的最终值之上和之下循环的输出变化，并导致达到稳定输出的延迟。振铃是由欠阻尼电路引起的过冲的结果。
过载：响应阶跃输入，过载是 输出超过其最终稳态值的量。
稳定性：在所有带反馈的放大器中，稳定性都是一个问题，无论是故意添加还是无意中添加反馈。在多个放大阶段应用时尤其如此。
稳定性是射频和微波放大器的一个主要问题。放大器稳定性的程度可以通过所谓的稳定性因子来量化 。有几种不同的稳定性因素，如斯特恩稳定性系数和林维稳定性系数，它们指定了放大器在其两端口参数方面的绝对稳定性必须满足的条件。
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