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Nonideal OP AMP Characteristics Nonideal AC Characteristics. 學生 : 陳俊傑 執導教授 : 梁治國. Outline. Frequency Response Slew Rate Noise Frequency Compensation. Frequency Response (1/3). 開迴路的頻域響應. 理想 OP Amp ： 非理想 OP Amp ：. AoL (dB). 100. 80. -20 dB/decade. 60.
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Nonideal OP AMP CharacteristicsNonideal AC Characteristics 學生: 陳俊傑 執導教授: 梁治國
Outline • Frequency Response • Slew Rate • Noise • Frequency Compensation
Frequency Response (1/3) 開迴路的頻域響應 • 理想OP Amp： 非理想OP Amp： AoL (dB) 100 80 -20 dB/decade 60 40 20 f 1 10 100 1 K 10 K 100 K 1 M AoL 的頻域響應
Frequency Response (2/3) 造成STC的低通特性 C Vo Vid 提供極低 的輸出阻抗 反相電壓放大 輸出緩衝電路 差動放大器 OP Amp內部結構圖
Frequency Response (3/3) 整體線路的轉換函數： ：代表直流狀況，其增益最大 ：此時的 為半功率點或稱為-3 dB時的頻率 ：代表當頻率增加時，其放大率下降 ：OP Amp 的增益為1
Slew Rate • Slew Rate：OP Amp輸出電壓的最大改變率。 • 轉動率產生的原因： • OP Amp內部電容的大小 • 內部電流限制
Noise (1/2) • 溫度變化所產生的雜訊 • 具有固定雜訊頻譜的寬頻帶白雜訊(Broadband white noise)，主要由輸入級的電晶體所引起。 • 在頻譜中的雜訊下降是與頻增加，通常此雜訊再超過10 Hz時便不重要了。 • 具有能量頻譜爆炸雜訊 (Burst noise)，通常在200~300 Hz以上，有時會在某些積體電路出現。
Noise (2/2) • 雜訊解決方法： • 將電路裡的雜訊增益減至最低。 • 雜訊在放大器電路裡，在反相輸入端和接地或非反相輸入端和接地電阻是要相同的。 • 避免大電壓在電阻上
Frequency CompensationPrinciples of Frequency Compensation Cutoff frequency ：10 KHz Voltage gain：0.707 Phase shift：45° (A) 1. Voltage gain remains fairly constant (near unity). 2. The dB-per-decade drop in output voltage is very slight. 3. The phase shift is near 0° (<6°). (B) 1. Voltage gain decreases by a factor of 10 for each decade increase in frequency. 2. The dB-per-decade drop continues at about -20 dB per decade. 3. The phase shift is near 90° (>84°). (C) 1. Voltage gain decreases by a factor of 100 for each decade increase in frequency. 2. The dB-per-decade drop continues at -40 dB per decade. 3. The phase shift is near 180°.
Frequency CompensationPrinciples of Frequency Compensation The frequency /phase response of a simple RC circuit.
Frequency CompensationPrinciples of Frequency Compensation Good news：頻寬增大 Bad news：相位角超過180° 解決辦法：加入一個閉迴路響應 在開迴路響應上。 開迴路+閉迴路 (補償) An uncompensated frequency response provides both “good” and “bad” news. 開迴路
Frequency CompensationPrinciples of Frequency Compensation To ensure against oscillation, the intersection of the close-loop gain and the open-loop gain curves must occur with a net slop of less than 40 dB per decade.
Frequency CompensationInternal Frequency Compensation Adding a compensation capacitor increases stability but reduces the bandwidth of an amplifier
Frequency CompensationExternal Frequency Compensation • Use LM301A op amp • It is an externally compensate, general-purpose op amp. • In large-signal frequency response obtained by using 3-picofarad capacitor instead of a 30-picofarad. • The curve for 3 picofarads does not extend to unity gain (0 dB). So we must design the circuit to have substantial (40 dB) gain to insure stability.
Frequency CompensationFeed-Forward Frequency Compensation The use of feedforward compensation produces all of the following effects: 1. Increased small-signal bandwidth 2. Increased full-power bandwidth 3. Improved slew rate
Input Resistance Actual input impedance directly input:
- + Output Resistance Accurate estimate of the closed-loop output impedance
