低噪聲放大器最大限度地提高接收機靈敏度

　　在每個射頻接收系統(tǒng)，如那些在基站，發(fā)現(xiàn)手機、GPS、無線局域網(wǎng)接口，和其他應(yīng)用中，低噪聲放大器（LNA）可用于具有較高的內(nèi)部噪聲水平更換放大器提高信噪比。這些系統(tǒng)中的噪聲信號來自外部源（外部）和內(nèi)部電路組件源（內(nèi)在），表現(xiàn)出設(shè)計者可以計算和補償?shù)奶匦?。外部噪聲源可以被過濾，以提供更清潔的信號給放大器，通過精心設(shè)計和選擇放大器，信號噪聲比（SNR）可以最大化。

　　噪聲系數(shù)（NF）是描述信號鏈中放大器提供的附加噪聲量的設(shè)計參數(shù)。噪聲會降低模擬信號或數(shù)字通信系統(tǒng)的比特和幀錯誤率。在雷達系統(tǒng)中，接收機噪聲會限制雷達的有效范圍。理想情況下，設(shè)計者應(yīng)該優(yōu)化總體信噪比，以提供最佳的系統(tǒng)性能。噪聲系數(shù)定義為噪聲因子F，表示分貝［ NF = 10×日志（F）］，與F定義為（Si／Ni）/（/無），比信噪比在輸入信噪比在輸出。

　　All amplifiers have some amount of thermal noise and other types of noise that they add to the signal they are trying to amplify. And since most RF signals are relatively weak when they reach the receiving antenna， the amplification process should introduce as little noise as possible to deliver the cleanest signal to the rest of the system. In many applications， multiple gain stages are cascaded to greatly improve the signal strength. The Friss equation shows that the noise figure， F1， of the first receiver stage dominates the added noise， and subsequent stages have progressively lower impact on the total noise.