Example 3: MIMO BLA of a class-C amplifier

To demonstrate how the MIMO BLA of a circuit is estimated, consider the class-C amplifier shown in Figure E3.1.

Figure E3.1 Class-C amplifier used here.

The class-C amplifier is excited by different-phase RPM $R_{1}$ that excites $41$ frequencies in a band of $40\mathrm{MHz}$ around $1\mathrm{GHz}$ with a Root Mean Square (RMS) value of $0.2\mathrm{V}$. The transistor in the amplifier is placed in a common-source configuration, so it will be modelled by a two-port. There’s only one reference multisine in the circuit so a second multisine current source $R_2$ is added at the output (shown in blue in the figure). The tickler multisines are shifted $1\mathrm{Hz}$ away from the frequency grid of the main multisine and are given an RMS current of $40\mathrm{\mu A}$. The resulting $B_{2}$ wave, obtained with HB is shown in Figure E3.2.

Figure E3.2 Spectrum of the $B_{2}$ wave.

In this figure, the frequency bins of the main multisines are shown in black, while its distortion is shown in red. In between the frequency bins of the main multisines, the response to the tickler is visible. The frequency bins of the tickler are indicated with green, while all remaining bins are grey. The amplitude of the tickler was chosen such that the tickler’s response was clearly visible above the numerical noise floor.

The obtained BLAs and their $3\sigma$ uncertainty interval are shown in Figure E3.3.

Figure E3.3 Estimated MIMO BLAs for the transistor in the amplifier.

The dashed lines in Figure E3.3 are the small-signal S-parameters. The largest differences are observed on $S_{21}$ and $S_{22}$, which is to be expected for a class-C biased transistor.