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university:courses:electronics:text:chapter-3 [16 Nov 2016 19:47] Doug Mercer correctin text back to original as a result of correcting error in figure labels G1 G2 |
university:courses:electronics:text:chapter-3 [06 Jun 2017 16:49] Doug Mercer [3.2.3 Loop gain] |
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This loop gain discussion emphasizes that indeed, loop gain is a very significant factor in predicting the performance of closed-loop operational amplifier circuits. The open-loop gain required to obtain an adequate amount of loop gain will, of course, depend on the desired closed-loop gain. | This loop gain discussion emphasizes that indeed, loop gain is a very significant factor in predicting the performance of closed-loop operational amplifier circuits. The open-loop gain required to obtain an adequate amount of loop gain will, of course, depend on the desired closed-loop gain. | ||

- | For example, using equation 3-9, an amplifier with A<sub>VOL</sub> = 20,000 will have an A<sub>VOL</sub>β ~ 2000 for a closed-loop gain of 10, but the loop gain will be only 20 for a closed-loop gain of 1000. The first situation implies an amplifier-related gain error the order of ~ 0.05%, while the second would result in about 5% of error. Obviously, the higher the required gain, the greater will be the required open-loop gain to support an A<sub>VOL</sub>β for a given accuracy. | + | For example, using equation 3-9, an amplifier with A<sub>VOL</sub> = 20,000 will have an A<sub>VOL</sub>β ~ 2000 for a closed-loop gain of 10, but the loop gain will be only 20 for a closed-loop gain of 1000. The first situation implies an amplifier-related gain error the order of ~ 0.05%, while the second would result in about 5% of error. Obviously, the higher the required gain, the greater will be the required open-loop gain to support an A<sub>VOL</sub>β for a given accuracy. |

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+ | **ADALM1000 Lab Activity [[university:courses:alm1k:alm-lab-olg|Open Loop Gain]]** | ||

====3.2.4 Frequency Dependence of Loop Gain ==== | ====3.2.4 Frequency Dependence of Loop Gain ==== | ||

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Figure 3.2 illustrates that the high open-loop gain figures typically quoted for op amps can be somewhat misleading. As noted, beyond a few Hz, the open-loop gain falls at 6dB/octave. Consequently, closed-loop gain stability, output impedance, linearity and other parameters dependent upon loop gain are degraded at higher frequencies. One of the reasons for having DC gain as high as 100dB and bandwidth as wide as several MHz, is to obtain adequate loop gain at frequencies even as low as 100Hz. | Figure 3.2 illustrates that the high open-loop gain figures typically quoted for op amps can be somewhat misleading. As noted, beyond a few Hz, the open-loop gain falls at 6dB/octave. Consequently, closed-loop gain stability, output impedance, linearity and other parameters dependent upon loop gain are degraded at higher frequencies. One of the reasons for having DC gain as high as 100dB and bandwidth as wide as several MHz, is to obtain adequate loop gain at frequencies even as low as 100Hz. | ||

- | A direct approach to improving loop gain at high frequencies other than by increasing open-loop gain is to increase the amplifier open-loop bandwidth. figure 3.2 shows this in terms of two simple examples. It should be borne in mind however that op amp gain-bandwidths available today extend to the hundreds of MHz, allowing video and high-speed communications circuits to fully exploit the virtues of feedback. | + | A direct approach to improving loop gain at high frequencies other than by increasing open-loop gain is to increase the amplifier open-loop bandwidth. figure 3.2 shows this in terms of two simple examples. It should be borne in mind however that op amp gain-bandwidths available today extend to the hundreds of MHz, allowing video and high-speed communications circuits to fully exploit the virtues of feedback. |

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+ | **ADALM1000 Lab Activity [[university:courses:alm1k:alm-lab-gbw|Gain Bandwidth Product]]** | ||

====3.2.5 The Bode Plot: Asymptotic And Actual Responses==== | ====3.2.5 The Bode Plot: Asymptotic And Actual Responses==== |

university/courses/electronics/text/chapter-3.txt · Last modified: 06 Jun 2017 16:49 by Doug Mercer