Potential instability in a transistor current source

A high-frequency bipolar transistor, often used as a switched current source in sampling and in current-steering circuits, is potentially a Colpitts oscillator. This consideration is generally overlooked. The key to instability is the driving point admittance. Provided that the load resistance is lower than that which meets a derived criterion, free oscillators will take place at a frequency of a resonant tank circuit. The solution based on analysis, simply requires that a resistor be added in series with the load in order that the criterion for unconditional stability shall be met.