Power Amplifier Design For CDMA

Feedforward is a correction scheme that takes place on the output of an amplifier. There are two paths for an RF signal to take from the input port of the PA, Error line-up and Main line-up. The input signal to the Error line-up is delayed, (an amount equivalent to the delay through the Main line-up), and then compared with a sample of the output of the Main line-up . If there is no gain or phase distortion then the comparison output will be zero. If any phase or gain distortion occurs then there will be an error RF signal from the comparison circuitry.Doherty amplifier design utilizes traditional methods but requires the management of additional constraints. These constraints are device specific and may not be independent. There are three Doherty specific design task : Maximize the peaking amplifier "off-state" output impedance. Sythesize the Doherty load transformation circuit for optimal carrier amplifier current source capability. Adjust peaking amplifier bias voltage and output match for the best linearity/gain tradeoff. Develop an input and output match. Match carrier and peaking amplifier outputs for best linear transfer function. LDMOS amplifiers require a shunt-L output impedance high, minimize the number of output shunt matching elements. The Doherty configuration efficiency at the transition voltage should be double the equivalent parallel amplifier. Good efficiency requires devices with low Rds(on).Efficiency at the transition voltage improves by reducing Vgp, but trades off linearity and/or gain.Match carrier and peaking amplifier for nominal class B performance. Measure peaking amplifier "off-state" impedance and determine length if phasing line.Select alpha factor. Determine current source capability. Develop load transformation circuit .Adjust matching circuits, phasing line length, and insertion phase to produce best combination of linearity, gain,bandwidth and efficiency.