用非補償運放改進性能

制造商提供單增益穩(wěn)定放大器，期望搶占廣闊的市場，并使學習應用設備變得很容易。但這些銷售商犧牲了潛在但重要的交流性能。了解什么時候考慮非補償運放和能夠提供給你什么東西。

　　設計者經常想使基于傳感器的系統(tǒng)中的振幅誤差最小。這個目的經常導致放大器閉環(huán)增益的特定增益誤差超過傳感器頻率范圍。工程師通常根據其–3dB頻率指定放大器帶寬，但就增益精度點來看，在這個頻率幾乎出現30%的增益誤差。所謂“有效帶寬”與放大器頻率響應和應用所需的增益精度有關。定義有效帶寬為增益誤差小于或等于指定誤差的帶寬。

　　有效帶寬

　　傳感器具有相對低頻響應，頻率越低，由有限開環(huán)增益產生的增益誤差越小。用少于單極、閉環(huán)、頻率響應模式放大器的指定值，計算有效帶寬維持誤差。由特定的通常如在數據手冊上的增益帶寬指標，計算有效帶寬是可行的。放大器閉環(huán)帶寬等于增益帶寬除以增益，對非反轉放大器完全正確，對反轉放大器近似正確。

　　下一個考慮什么是定義最大幅度誤差的基礎。在所有系統(tǒng)中，ADC輸入信號路徑終端的模擬部分，通過擴充，ADC的分辨率定義了誤差的影響因素。這篇文章使用ADC分辨率的½ LSB誤差作為最大誤差。隨著ADC分辨率的增加，最大誤差減小。表1顯示了8到18位ADC分辨率的½ LSB誤差。

　　為容易評估增益誤差在頻率函數中單極模式的效果，計算格式化單極函數。這個計算放置極點在1Hz，表示閉環(huán)增益降為–3dB，用理想閉環(huán)單增益或為0dB。使用這個單極模式，計算增益誤差頻率少于或等于給定誤差。然后可以根據放大器閉環(huán)增益的–3dB帶寬計算有效帶寬。謹記–3dB點為幾乎30%增益誤差，且誤差指標越小的帶寬越窄。

(1)

(2)

(3)

　　表2顯示數據表計算的一部分，將增益隨頻率下降的趨勢形象化。計算公式1~3分別隨列中頻率值變化（圖1）。

　　下一步是找到特定頻率，在這個頻率增益誤差等于指定位分辨率ADC的½ LSB。公式4為假定ADC分辨率的情況下，計算½LSB誤差（表1）。

(4)

　　為計算增益誤差等于½LSB誤差的頻率，用公式1重新整理替代公式4，得到公式5。使用公式4和公式5計算出表3的值。

(5)

　　表3中，縱向表頭“頻率誤差”的給定頻率為增益誤差等于ADC分辨率的½LSB。在更低頻率，增益誤差小于½LSB。帶寬對特定分辨率有效。例如，驅動10位ADC的放大器有效帶寬為–3dB頻率的0.03126，14位ADC的有效帶寬為–3dB頻率的0.007813。如果放大器為閉環(huán)，100kHz為–3dB頻率，有效帶寬分別為31.3和7.81kHz。公式1到公式5，放大器增益帶寬除以其閉環(huán)增益，充分證明了放大器的有效帶寬小于理論使用手冊的–3dB頻率。

　　考慮怎樣在信號路徑中使用放大器，能夠部分減少有效帶寬的迅速降低。放大器通常從傳感器到ADC輸入使用許多增益縮放信號。在許多情況下，增益大過10。使用放大器增益消除單增益穩(wěn)定性的需求，并減少大量放大器使用的內部補償。非補償放大器的優(yōu)點是同樣功耗下增加了可用帶寬和轉換速率。

　　以上為部分翻譯，英文全文：

　　Decompensating amplifiers improve performance

　　Manufacturers offering unity-gain-stable amplifiers hope to address a wide market and minimize the effort of learning to use the devices. Yet these vendors sacrifice a significant portion of the potential ac performance. Learn when to consider decompensated amplifiers and what they can offer you.

　　By Walter Bacharowski, National Semiconductor -- EDN, 12/3/2007

　　Designers often want to minimize amplitude error in sensor-based systems. This goal often leads to specifying the gain error of an amplifier’s closed-loop gain over the frequency range of the sensor. Engineers commonly specify the bandwidth of an amplifier in terms of its –3-dB frequency, but, from a gain-accuracy point of view, almost a 30% gain error occurs at this frequency. The term “effective bandwidth” connects the frequency response of the amplifier and the gain accuracy that the application requires. You define the effective bandwidth as the bandwidth for which the gain error is less than or equal to a specified error.

　　Effective bandwidth

　　Sensors have a relatively low frequency response, and, at lower frequencies, the gain error due to finite open-loop gain is small. You can calculate the effective bandwidth to maintain an error at less than a specified value from the single-pole, closed-loop, frequency-response model of the amplifier. It would be useful to calculate the effective bandwidth from specifications such as gain bandwidth that are commonly available in a data sheet. The relationship of an amplifier’s closed-loop bandwidth being equal to the gain bandwidth divided by the gain is true for noninverting amplifiers and approximately true for inverting amplifiers.

　　The next consideration is what basis to use in defining the maximum amplitude error. In almost all systems, the analog portion of the signal path ends at the input of an ADC, and, by extension, the resolution of the ADC defines the error of interest. This article uses an error of ½ LSB of the ADC’s resolution as the maximum error. As the resolution of the ADC increases, the maximum error decreases. Table 1 shows the ½-LSB error for ADC resolutions of 8 to 18 bits.

　　To easily evaluate the effect of the single-pole model on the gain error as a function of frequency, you calculate a normalized single-pole function. This calculation places the pole at 1 Hz, which represents the –3-dB loss in closed-loop gain, with an ideal closed-loop gain of one, or 0 dB. Using this single-pole model, you calculate the frequency for a gain error less than or equal to the specified error. You can then calculate the effective bandwidth in terms of the –3-dB bandwidth of the closed-loop gain of the amplifier you are evaluating. Keep in mind that the –3-dB point is almost a 30% gain error and that the bandwidth is smaller with a lower error specification.