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Linear cellular Power Amplifier without any load harmonic termination?

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yolande_yj

yolande_yj

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I am designing linear power amplifier for cell phone, all of our PAs (L/M/HB) need 2nd harmonic termination to get good linearity and PAE. Removing the 2nd harmonic termination degrades performance a lot no matter how we tune it. However I know that removing the harmonic termination while maintaining the performance is possible. Does anyone here knows the trick?
 

Adjusting the Bias Point properly to achieve "Nulling Point for Second Order HD" may give you a flexibility.I don't know which technology you use ( GaN, Silicon, GaAs. etc.) but it's a quite practical technique for PAs.This wouldn't work for every type of PA but if you research of Gm of the FET like transistors, you will see that the second derivative of Gm crosses the null @ certain bias point.
 

Thanks for your reply. Yes, adjusting the bias is a key tuning knob that we are using, but that is not the solution. The problem is the nulling point is lower in power comparing with the one with 2nd harmonic termination. Another problem is without harmonic termination, the ACLR AB hump at backoff power is high and the PAE is low. The performance at rated power is far worse than the original design. BTW, we are using GaAs HBT.
 

Using Push-Pull topology with a planar transformer may be a solution to suppress even mode harmonics.In this case, the PA will work in AB -Class that is preferable method for Cellular Infrastructures
 

Solving this problem is complex, and is hard to make a compromise.
Without the 2nd harmonic circuit (usually a series LC circuit), the PA output is not anymore matched for optimum linearity and efficiency.

An option which may solve this problem is to increase a bit the bandwidth of the output matching network adding another LC network cell, and get an intermediate transforming impedance.
Tune this intermediate impedance for optimum linearity and efficiency.
 

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