差分放大器：The Difference Amplifier

差分放大器：The Difference Amplifier

The difference amplifier is the complement of the summing amplifier and allows the subtraction of two voltages or, as a special case, the cancellation of a signal common to the two inputs. This circuit is shown in Figure 5 and is useful as a computational amplifier, in making a differential to single-ended conversion or in rejecting a common mode signal.
Circuit bandwidth may be calculated in the same manner as for the inverting amplifier, but input impedance is somewhat more complicated. Input impedance for the two inputs is not necessarily equal; inverting input impedance is the same as for the inverting amplifier of Figure 1 and the non-inverting input impedance is the sum of R3 and R4. Gain for either input is the ratio of R1 to R2 for the special case of a differential input single-ended output where R1 = R3 and R2 = R4. The general expression for gain is given in the figure. Compensation should be chosen on the basis of amplifier bandwidth. Care must be exercised in applying this circuit since input impedances are not equal for minimum bias current error.

差分放大器是对求和电路的发展，这种电路可以减小甚至去处两个输入信号中的共模成分。电路见图5，这种电路在运算电路中相当有用，比如差分对单端的转换，抑制共模信号。
电路带宽的计算方式和反相电路相同。输入阻抗的计算方式相对复杂。两个输入端的输入阻抗没有必要相等；反相放大部分的输入阻抗和反相放大器的计算方法相同。同相放大部分的输入阻抗为R3、R4之和。在R1=R3，R2=R4，差分输入，单端输出的特殊情况下，两个输入的增益都是R2/R1。一般情况下增益的计算方法见图中说明。补偿可以按照放大器的具体带宽进行。在应用中应该注意一点：两端的输入阻抗是不相等的，要注意输入偏置电流的引起的误差。