Noise characteristics of current mirror sinks/sources

General expressions of the limiting and excess noise currents generated by a current mirror sink/source are derived. The analysis, restricted to low and audio frequencies, shows the effect of the transistor base resistance and the noise reduction due to the external emitter degeneration. By means of a computer-aided design (CAD) analysis, spectral density frequency plots are computed, and the total noise content in the audio band calculated for the case of a typical 1/f excess noise generator. The noise characteristics of other current source topologies are compared and shown to be quite similar to the basic current mirror. Noise measurements of current mirrors implemented with IC transistor pairs showed a good correlation with the previous analytical and CAD results.