全波整流器和平均值濾波器：The Full-Wave Rectifier and Averaging Filter

The circuit shown in Figure 25 is the heart of an average reading, rms calibrated AC voltmeter. As shown, it is a rectifier and averaging filter. Deletion of C2 removes the averaging function and provides a precision full-wave rectifier, and deletion of C1 provides an absolute value generator.
Circuit operation may be understood by following the signal path for negative and then for positive inputs. For negative signals, the output of amplifier A1 is clamped to +0.7V by D1 and disconnected from the summing point of A2 by D2. A2 then functions as a simple unity-gain inverter with input resistor, R1, and feedback resistor, R2, giving a positive going output.
For positive inputs, A1 operates as a normal amplifier connected to the A2 summing point through resistor, R5. Amplifier A1 then acts as a simple unity-gain inverter with input resistor, R3, and feedback resistor, R5. A1 gain accuracy is not affected by D2 since it is inside the feedback loop. Positive current enters the A2 summing point through resistor, R1, and negative current is drawn from the A2 summing point through resistor, R5. Since the voltages across R1 and R5 are equal and opposite, and R5 is one-half the value of R1, the net input current at the A2 summing point is equal to and opposite from the current through R1 and amplifier A2 operates as a summing inverter with unity gain, again giving a positive output.
The circuit becomes an averaging filter when C2 is connected across R2. Operation of A2 then is similar to the Simple Low Pass Filter previously described. The time constant R2C2 should be chosen to be much larger than the maximum period of the input voltage which is to be averaged.
Capacitor C1 may be deleted if the circuit is to be used as an absolute value generator. When this is done, the circuit output will be the positive absolute value of the input voltage.
The amplifiers chosen must be compensated for unity-gain operation and R6 and R7 must be chosen to minimize output errors due to input offset current.

圖25是一個(gè)平均值輸出，有效值刻度的交流電壓表頭放大電路。它由一個(gè)整流器和一個(gè)平均值濾波器構(gòu)成。如果去掉C2，電路就不再具有平均值濾波功能，只是一個(gè)精密全波整流器，如果去掉C1，電路就變成求絕對值的電路。
為了理解電路原理，下面將從信號路徑入手，先分析輸入電壓小于零的情況，再分析輸入電壓大于零的情況。對于小于零的輸入電壓信號，放大器A1的輸出被二極管D1箝位至+0.7V，二極管D2將其與放大器A2的求和節(jié)點(diǎn)（反相端）隔離開。此時(shí)A2相當(dāng)于一個(gè)簡單的單位增益反相器，輸入電阻是R1，反饋電阻是R2，輸出電壓大于零。
輸入大于零時(shí)，A1起放大作用，輸出經(jīng)電阻R5連到A2的求和節(jié)點(diǎn)。放大器A1接成一個(gè)簡單的單位增益反相器，輸入電阻是R3，反饋電阻是R5（此處反饋電阻應(yīng)該為R4，疑原文筆誤——譯者）。由于D2位于反饋環(huán)路之內(nèi)，所以它不會影響A1的增益精度（譯者認(rèn)為有誤，見注釋1）。正向電流經(jīng)R1流入A2的求和節(jié)點(diǎn)，反向電流經(jīng)電阻R5流出A2的求和節(jié)點(diǎn)。由于R1和R5上的電壓大小相等方向相反，而且R5的阻值是R1的1/2，所以A2的求和節(jié)點(diǎn)處的電流與R1上的電流大小相等流向相反，（譯者認(rèn)為有誤，見注釋2）此時(shí)A2構(gòu)成一個(gè)單位增益反相加法器，輸出仍然為正值。
在R2上跨接電容C2，構(gòu)成了一個(gè)平均值濾波器。此時(shí)A2的作用與前文所述的“簡單低通濾波器”相似。R2C2的時(shí)間常數(shù)必須仔細(xì)選取，使其遠(yuǎn)遠(yuǎn)大于輸入電壓的最長平均周期。
如果去掉電容C1，本電路就會變成一個(gè)計(jì)算絕對值的電路，即電路輸出將等于輸入電壓的絕對值。