The present paper focuses on solutions of the experimental difficulties rising up when a full low frequency noise characterization of bipolar transistors exhibiting high noise levels and low input dynamic input impedances is carried out using a direct characterization technique. First, a typical direct characterization experimental set-up is introduced pointing out the difficulties to be faced when a high noise, low input impedance transistor is addressed. Then, an improved version of the experimental set-up is proposed and successfully applied to AlGaAs/GaAs heterojunction bipolar transistors. (c) 2005 Elsevier Ltd. All rights reserved.