RF Monte Carlo Calculation of Power Amplifier Efficiency As Function of Signal Bandwidth

We propose a definition of the efficiency bandwidth for a power amplifier (PA) using a modulated signal as the signal bandwidth at which the amplifier efficiency has dropped to a level of 90% of the maximum efficiency at small bandwidth. We introduce a Monte Carlo method to calculate the efficiency bandwidth for some popular PA architectures. The method assumes a given modulated signal at the load. From this load signal the wave forms at the drains of the transistors are derived and the energy dissipation in the transistors can be calculated. Using idealized transistors with no output capacitance the maximum realizable efficiency bandwidth of an asymmetrical Doherty amplifier is 60%, which is much larger than that of an outphasing amplifier, which is 14%. More realistic transistors that include output capacitances, need a matching circuit with a high Q-value which decreases the efficiency bandwidth. Using output capacitance values typical for LDMOS transistors, the asymmetrical Doherty amplifier shows an efficiency bandwidth of about 400 MHz for a signal centered at 1 GHz. For a signal at 2 GHz the efficiency bandwidth is found to be 520 MHz. Due to the fixed values of the output capacitances the efficiency bandwidth does not scale with frequency.