復(fù)位穩(wěn)定放大器：The Reset Stabilized A

The reset stabilized amplifier is a form of chopper-stabilized amplifier and is shown in Figure 23. As shown, the amplifier is operated closed-loop with a gain of one.
The connection is useful in eliminating errors due to offset voltage and bias current. The output of this circuit is a pulse whose amplitude is equal to VIN. Operation may be understood by considering the two conditions corresponding to the position of S1. When S 1 is in position 2, the amplifier is connected in the unity gain connection and the voltage at the output will be equal to the sum of the input offset voltage and the drop across R2 due to input bias current. The voltage at the inverting input will be equal to input offset voltage. Capacitor C1 will charge to the sum of input offset voltage and VIN through R1. When C1 is charged, no current flows through the source resistance and R1 so there is no error due to input resistance. S1 is then changed to position 1. The voltage stored on C1 is inserted between the output and inverting input of the amplifier and the output of the amplifier changes by VIN to maintain the amplifier input at the input offset voltage. The output then changes from (VOS + IbiasR2) to (VIN + IbiasR2) as S1 is changed from position 2 to position 1. Amplifier bias current is supplied through R2 from the output of the amplifier or from C2 when S1 is in position 2 and position 1 respectively. R3 serves to reduce the offset at the amplifier output if the amplifier must have maximum linear range or if it is desired to DC couple the amplifier.
An additional advantage of this connection is that input resistance approaches infinity as the capacitor C1 approaches full charge, eliminating errors due to loading of the source resistance. The time spent in position 2 should be long with respect to the charging time of C1 for maximum accuracy.
The amplifier used must be compensated for unity gain operation and it may be necessary to overcompensate because of the phase shift across R2 due to C1 and the amplifier input capacity. Since this connection is usually used at very low switching speeds, slew rate is not normally a practical consideration and overcompensation does not reduce accuracy.

復(fù)位穩(wěn)定放大器是斬波穩(wěn)定放大器的一種，其電路如圖23所示。在圖中的狀態(tài)為，放大器工作于閉環(huán)增益為1的情況。
這種電路可有效消除因失調(diào)電壓和偏置電流引起的誤差。該電路的輸出是一個(gè)幅值等于VIN的脈沖。為了理解其工作原理，需要分析一下將開(kāi)關(guān)S1撥向不同的位置時(shí)，電路結(jié)構(gòu)如何改變。先將S1撥到位置2，此時(shí)放大器為單位增益接法——輸出端電壓等于輸入失調(diào)電壓加上R2與輸入偏置電流產(chǎn)生的電壓，反相端的電壓等于輸入失調(diào)電壓，電容C1持續(xù)充電直到其端電壓等于輸入失調(diào)電壓與VIN的代數(shù)和。C1充電完畢時(shí)，流經(jīng)信號(hào)源內(nèi)阻及R1的電流為零，因此信號(hào)源內(nèi)阻不會(huì)造成誤差。接著S1撥到位置1，使C1的電壓加在放大器的輸出端和反相輸入端之間，（由于電容端電壓不能突變，所以）輸出端的電壓將產(chǎn)生大小等于VIN的變化，使放大器輸入端維持輸入失調(diào)電壓，即當(dāng)開(kāi)關(guān)S1從位置2撥到位置1時(shí)，輸出端電壓將從(VOS + IbiasR2)變?yōu)?VIN + IbiasR2)。S1處于位置2時(shí)，放大器的偏置電流是由輸出端經(jīng)電阻R2提供的，當(dāng)S1轉(zhuǎn)向位置1時(shí)則由C2提供。R3的作用是降低輸出失調(diào)電壓，如果要求放大器具有最大的線性范圍，或者采用直流耦合，就應(yīng)該使用該電容。
上面這個(gè)電路的另一個(gè)優(yōu)點(diǎn)在于，當(dāng)電容C1充電完畢后，輸入電阻將趨于無(wú)窮大，因此可以消除因信號(hào)源內(nèi)阻產(chǎn)生的誤差。開(kāi)關(guān)接通位置2的持續(xù)時(shí)間要足夠長(zhǎng)，以保證電容C1完成充電，這樣才能獲得最佳的精度。
此電路必須使用單位增益穩(wěn)定的OP，因?yàn)镽2—C1產(chǎn)生的相移，所以還可能需要一定的過(guò)補(bǔ)償。鑒于本電路常用于切換頻率很低的場(chǎng)合，轉(zhuǎn)換速率（SR）一般不是關(guān)鍵指標(biāo)，因此過(guò)補(bǔ)償不會(huì)降低精度。