We have seen that negative feedback can enhance the performance of amplifier circuits. This is done by sampling a portion of the output signal and summing it out of phase with the input signal. In order to maintain stability, the gain of the amplifier must be less than unity by the time its phase reaches -180$$^{\circ}$$. There are four basic variants of negative feedback: Series-Parallel, Parallel-Series, Parallel-Parallel, and Series-Series. In all cases, gain and distortion are lowered by the sacrifice factor $$S$$, and the bandwidth is increased by $$S$$. Parallel connections reduce impedance by $$S$$, whereas series connections increase the impedance by $$S$$. At the input, parallel connections are current sensing, and series connections are voltage sensing. At the output, parallel connections produce a voltage-source model, and series connections produce a current-source model. The sacrifice factor is the ratio between the openand closed-loop gains. It is a function of frequency, and therefore, the effects of negative feedback lessen at the frequency extremes.