Abstract
After an introduction to amplifiers in communications and an exposition of the literature specifically relevant to high linearity power amplifiers, this study investigates more thoroughly various aspects of envelope feedback as applied to Bipolar Tuned Power Amplifiers at HF and VHP. It is discovered that under the correct conditions a new mode of linear operation exists where gain compression, AM-PM conversion and input impedance are simultaneously linearised, and in this region DC-RF power efficiency is also improved. Spectral measurements are presented from an envelope feedback amplifier constructed to operate over this region. A computerised system is described for measuring accurately the gain and phase shift of the test amplifier against variation of collector supply, quiescent bias current and RF drive power. The results from these measurements are presented as 3-dimensional projections and as contour plots. Subsequently the stored data is used to re-construct two-tone spectra, which is then analysed to show contributions to the spectrum from the gain compression and AM-PM conversion mechanisms separately. Conclusions are drawn with respect to effects of bias on these two mechanisms. A mechanism has been discovered which gives a symetric spectra without requiring AM-PM conversion at the fundamental frequencies. An attempt is made to model the amplifier with a non-linear circuit transient analysis program (SPICE). Good correlation is obtained for some parameters and these results are also plotted in 3-D and in contour.