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The 2-General Feedback Theorem (2-GFT) derives from the General Network Theorem with two injections and is used to analyze feedback systems. A transfer function is decomposed into the ideal transfer function, loop gain and null loop gain or direct forward transmission. In addition, the various components of the loop gain and null loop gain are calculated.

Analytically, this decomposition can be calculated easily using null injection techniques. Furthermore, typically a lot of the lower-level transfer functions can be ignored. This allows to quickly assemble an analytical model that is useful for design. This is explained in Middlebrook's course. This model can checked numerically against the simulation results.

A few of the equivalent 2-GFT representations are:

\begin{align} H &= \Hinf \frac{T}{1+T} + \frac{\Ho}{1+T} \\ &= \Hinf \frac{1+\frac{1}{\Tn}}{1+\frac{1}{T}} \\ &= \Hinf D \Dn \\ &= \Hinf D + \Ho \Do \end{align}

The second-level transfer functions can then be interpreted as:

\( \Hinf \) |
'ideal, desired' transfer function |

\( T \) |
principal loop gain |

\( \Tn \) |
null loop gain |

\( \Ho \) |
direct forward transmission through feedback path plus common-mode gain of the forward path |

$D$ and $\Dn$ are the discrepancy factor and null discrepancy factor, respectively. They express the changes to the closed-loop gain ($H$) compared with the ideal transfer function ($\Hinf$) due the non-idealities in the system, including limited dc gain and bandwidth of the loop gain, reverse transmission through the loop and direct forward transmission.