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Impact of self-heating in LF noise measurements with voltage amplifiers

Voltage Amplifiers have been used to characterize the low-frequency noise of Heterojunction Bipolar Transistors (HBTs). They generally feature not only a lower noise floor, but also have less impact on simultaneous (two-port) measurements than Transimpedance Amplifiers, when moderate to high DC current regimes are considered. However, when the Device Under Test (DUT) is characterized under these regimes, common concepts such as unilateralism and frequency-independent small-signal parameters are no longer valid due to the frequency-dependent thermal response of the DUT (self-heating). It will be shown that depending on the conditions under which the measurements are carried out, the experimental data may vary for some orders of magnitude, leading to an incorrect characterization if the effect is disregarded.