NEGATIVE FEEDBACK
As stated previously, the open loop gain of the 741 operational amplifier is about 200,000. This amount of gain is not practical for most applications, so something must be done to reduce this gain to a reasonable level. One of the great advantages of the op amp is the ease with which the gain can be controlled, Figure 57–5. The amount of gain is controlled by a negative-feedback loop. This is accomplished by feeding a portion of the output voltage back to the inverting input. Since the output voltage is always opposite in polarity to the inverting input voltage, the amount of output voltage fed back to the input tends to reduce the input voltage. Negative feedback has two effects on the operation of the amplifier. One effect is that it reduces the gain. The other is that it makes the amplifier more stable.
The gain of the amplifier is controlled by the ratio of resistors R2 and R1. If a noninverting amplifier is used, the gain is found by the formula (R2 + R1)/R1. If resistor R1 is 1K ohms and resistor R2 is 10K ohms, the gain of the amplifier would be 11 (11,000 -:- 1,000 = 11).
If the op amp is connected as an inverting amplifier, however, the input signal will be out of phase with the feedback voltage of the output. This will cause a reduction of the input voltage applied to the amplifier and a reduction in gain. The formula (R2/R1) is used to compute the gain of an inverting amplifier. If resistor R1 is 1K ohms and resistor R2 is 10K ohms, the gain of the inverting amplifier would be 10 (10,000 -:- 1,000 = 10).
There are some practical limits, however. As a general rule, the 741 operational amplifier is not operated above a gain of about 100. If more gain is desired, it is generally obtained by using more than one amplifier, Figure 57–6.
As shown in Figure 57–6, the output of one amplifier is fed into the input of another amplifier. The reason for not operating the 741 at high gain is that at high gains it tends to become unstable. Another general rule for operating the 741 op amp is the total feedback resistance (R1 + R2) is usually kept more than 1,000 ohms and less than 100,000 ohms. These general rules apply to the 741 operational amplifier and may not apply to other operational amplifiers.