Determination of the transition frequency of a bipolar transistor using optoelectronic current sources

The principle is described of a measurement by a method of comparison which allows the transition frequency f_T of a bipolar transistor to be measured. The AC test signal is obtained by optoelectronic injection. The same sine-modulated monochromatic light is shone successively on two photodiodes. The transistor does not need an actual excitation current at the input or short-circuit at the output since a method of comparison is used. The frequency response of the two photoelectric signals approximates a first-order low-pass function. Optoelectronic injection allows the presence of any parasite synchronous induced signals to be detected, leading to more accurate measurements. For a signal-to-induction ratio of 40 dB and a signal-to-noise ratio of 50 dB, the accuracy over the determinations of f_T is about ±5% for both amplitude and phase shift measurement