Measurement of RF Linear Operating Area of Bipolar Transistors

The methodology for the measurement of the knee voltage and maximum collector current for the linear operation of bipolar transistors is described and demonstrated on the horizontal current bipolar transistor (HCBT). Both limits are determined by measuring loadlines at 1-dB compression point (P1dB), wherein the hard-limiting nonlinearity occurs only in the positive half-wave of the collector current. The boundary defines the knee voltage, maximum collector current, and the transition between the two in which the onset of the Kirk effect takes place. It is found that the maximum collector current boundary saturates at a constant value for higher collector-emitter voltages and that there are the optimum load impedance and bias point for the maximum output power in the linear regime. They are found for HCBT, which achieves the output power of 21.25 dBm (P1dB) at 2.4 GHz, with the maximum collector-emitter voltage set to the open-emitter breakdown voltage of 9 V.