In calculating the input resistance, output resistance, and overall gain of transistor feedback amplifiers, it is necessary, in general, to account for the loading effect of the feedback circuit on the input and output of the amplifier proper. In considering practical transistor feedback amplifiers, account should be taken of finite gain, finite load, and the loading effects of the feedback circuit at the input and output of the amplifier. Moreover, because the source and load resistances have been included within the modified amplifier, the feedback amplifier is effectively open-circuited at the output and is supplied at the input from an ideal source of zero source impedance. The feedback becomes positive at some frequency, resulting in oscillations at this frequency. A sinusoidal oscillator can be implemented by connecting an amplifier and a frequency selective circuit in cascade in a positive feedback loop.