What?

In an ideal injection point, current is automatically nulled when voltage is nulled, and vice-versa. In other words, the impedance ratio in the forward and backwards direction is infinity or zero in an ideal injection point. In practice, ideal injection point are located at the inputs and outputs of dependent generators.

When?

If, in a single-loop feedback system, an ideal injection point is found that results in the desired \(H_\infty\), then the reverse loop gain must vanish. In other words, no ideal injection point exists in a single-loop feedback circuit with non-zero reverse loop gain, that results in the desired \(H_\infty\).

Note that the emphasis is important, as theoretically always an isolated dependent generator can found. Even though \( T \) will equal the one from the 'correct' injection point, the produced \( H_\infty, T_n \) will be different ('undesired').

Remark that no such property holds for \( H_0 \).