米勒效应

nullnull§ 9-8 米勒效應電容* •對低頻響應有影響的“大C”→短路(高頻時)。 •對高頻響應有影響的“C”← 主動裝置各端間所感應的”C” 網路連線間的“C效應” •對反相放大器(Vo與Vi差180°→Av<0負值) 輸出入C↑⇐ 元件輸出入端間”極間電容”及放大器“增益”null*[結論] 任何 反相放大器的“輸入電容” 都會由於 米勒效應電容 而增加 ，而此電容乃隨 放大器的“增益” 及 輸出入端間所存在的 “極 間電容” 而變化。§ 9-8 米勒效應電容null*§ 9-8 米勒效應電容§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory【網路參數】§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory• 求高頻響應曲線轉折點及電壓增益：§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory• 頻率響應圖：§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory【輸入網路】【輸出網路】【輸出入戴維寧等效電路】§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory※決定高截止頻率的主要因素：某C所求最低頻率§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory【hfe（或β）的變動】§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory例題 9.11 對於圖 9.48，若其參數與例題 9.9 相同，也就是 RS=1KΩ，R1=40KΩ，R2=10KΩ，RE=2K Ω ，RC=4K Ω ，RL=2.2K Ω CS=10μF， CC=1μF ， CE=20μF ，β=100，ro=∞ Ω ，VCC=20V外加以下參數§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory（a）決定 fHi 和 fHo。 （b）求fβ 及 fT （c）描繪高頻及低頻響應，利用例題 9.9(a) 及 (b)部分 解：（a）從例 題 9.9：§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory（b）利用（9.55）式§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory（c）看圖 9.53 。 fβ 及 fHo 二者都會促使高截止頻率降低至 fHi 以下。其中 fβ 較接近 fHi，所以它的影響較 fHo 大。總之，頻寬將比單由 fHi 所決 定的要窄。事實上，根據這些網路的參數計算，其高截止頻率相當接 近 600 kHz 。所以，通常最低的高截止頻率就決定了系統可能的最大頻寬。§ 9-9 BJT放大器的高頻響應§ 9-9 BJT放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-10 FET放大器的高頻響應§ 9-10 FET放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-10 FET放大器的高頻響應§ 9-10 FET放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory↓反相放大器，須含“米勒效應C”Zo§ 9-10 FET放大器的高頻響應§ 9-10 FET放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-10 FET放大器的高頻響應§ 9-10 FET放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory【輸入電路】【輸出電路】←截止頻率→PS： 這些求截止頻率的一般步驟可用來分析其他電晶體組態。 米勒電容效應只有在反相組態，且在共基極組態下f2>>fβ才會發生§ 9-10 FET放大器的高頻響應§ 9-10 FET放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory例題 9.12 決定圖 9.56 電路的高截止頻率，利用 例題 9.10 的參數 CG=0.01μF， CC=0.5μF， CS=2μF Rsig=10KΩ ， RG=1MΩ ， RD=4.7K Ω ，RS=1KΩ，RL=2.2K Ω IIDSS=8mA， VP=－4V ， rd=∞ Ω ，VDD=20V外加 Cgd=2pF， Cgs=4pF Cds=0.5pF， Cwi=5pF，Cwo=6pF§ 9-10 FET放大器的高頻響應§ 9-10 FET放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory解：§ 9-10 FET放大器的高頻響應§ 9-10 FET放大器的高頻響應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory由上面的結果可以清楚地看出輸入電容及米勒效應電容是決定高截止頻率 的主要因素。 這也是通常在輸出電路中，由於較小的電容Cds及電阻值所造成的結果。§ 9-11 多級頻率效應§ 9-11 多級頻率效應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory 如果第一級放大器的輸出端接上另外一級放大器的話，則整體的頻率 響應會有很大的改變。在高頻輸出電容 Co 會包括下一級的接線電容 CW1、 寄生電容 Cbe、及密勒電容 CMi。並且由於第二級對於低頻截止頻率的影響 ，此頻域中的系統增益將會進一步減低。隨著級數的增加，具有最低高截 止頻率的一級將會決定系統的高截止頻率。而系統的低截止頻率則由具有 最高低截止頻率的一級來決定。所以，很明顯地，有一級設計得比較差將 會影響其他設計較好的串接系統。§ 9-11 多級頻率效應§ 9-11 多級頻率效應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-11 多級頻率效應§ 9-11 多級頻率效應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory 增加相同的放大級所產生的影響，可以用圖 9.61 來說明。每一串接 級的高低截止頻率都相同。每一單級的截止頻率如圖分別為 f1 及 f2。當兩 個相同的放大級串接之後，在高及低頻域中衰減的速度已增加至 －12 dB/ 二倍或 －40 dB/十倍。所以在 f1 及 f2，衰減的分貝數是 －6 dB 而非在定 義頻寬增益的－ 3dB。－3 dB 點由於頻寬的縮減移致 f’1 及 f’2。當串接增 至三級時衰減斜率變為 －18 dB/二倍或 －60 dB/十倍。§ 9-11 多級頻率效應§ 9-11 多級頻率效應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory假設各級相同，頻帯與級數（n）的函數關係，推導如下：於低頻域中由於每一級都相同，Av1low＝Av2low＝等等。所以或將此結果設定為 1/(2)1/2（－3 dB 位準）可得§ 9-11 多級頻率效應§ 9-11 多級頻率效應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory或所以§ 9-11 多級頻率效應§ 9-11 多級頻率效應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory則相似的步驟在高頻域中可得，（低頻）（高頻）§ 9-11 多級頻率效應§ 9-11 多級頻率效應*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory 對於 RC耦合電晶體放大器，如果 f2＝fβ 或兩者很靠近足以影響上 3 dB 頻率，則決定 f’2 時級數必須加倍，這是由於因式 1/（1＋jf/fx）數 量增加之故。 如果中頻增益並不隨著級數而變的話，則級數增加並不一定會使頻 寬降低。例如，如果單級放大器產生的增益是 100 且頻寬是 10000 Hz， 則所得的增益頻寬積是 102×104＝106。若是一個兩級的系統，則相同的 增益可由每一級增益為 10 得到（由於 10×10＝100）。每一級的頻寬， 由於所需的增益僅為十分之一，在增益頻寬積為定值的前提下，每一單 級的頻寬可以增加 10 倍，也就是 100000 Hz。當然，設計本身必須能容 許較大的頻寬並維持較小的增益。§ 9-12 方波測試§ 9-12 方波測試*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-12 方波測試§ 9-12 方波測試*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory方波的傅立葉級數展開式如下：• 頻率為基本項的奇數倍而大小隨著更 高的諧波而降低。 • 產生方波需要無限多項。§ 9-12 方波測試§ 9-12 方波測試*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-12 方波測試§ 9-12 方波測試*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory 由於第九諧波的大小還大於基本項的百分之十 【1/9（100％）＝ 11.1％】，所以方波的傅立葉展開最主要的是由基本項至第九諧波。 若某一頻率的方波經放大之後得到一個相當漂亮的方波輸出，則可以 合理的假設，所加頻率直到第九諧波被完整的放大而看不出有所失真 。例如，如果要測一音響的頻寬是否為 20 kHz（可聽範圍從 20 Hz 至 20kHz），則輸入訊號[頻率至少要 20 kHz/9＝2.22kHz。 如果放大器對於輸入方波的響應與輸入信號相同而不失真，則此 放大器的頻率響應（或頻寬）對此輸入頻率是足夠的。§ 9-12 方波測試§ 9-12 方波測試*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-12 方波測試§ 9-12 方波測試*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory• 高截止頻率（或頻寬）：• 低截止頻率：上升時間的定義是波形由峰值的 10％ 上升至峰值的 90％ 所需的 時間，如圖 9.65。藉由測量圖 9.65 輸出波形的傾斜度並代入下式而得§ 9-12 方波測試§ 9-12 方波測試*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory§ 9-12 方波測試§ 9-12 方波測試*Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.Boylestad and Nashelsky Electronic Devices and Circuit Theory解：