You need to cherry pick supplies for an SSPA. Does turning the knob spike the voltage? Does it feel like it spent a day at the beach? If you push the knob from side to side without turning it does the voltage shift? If so, put it back on the rack or give it to your worst enemy. Also, you need to try out a power supply before you use it. Just a slight tweak of the knob when you are biasing up the amplifier and you killed it find a five volt, 2 watt supply. Never use a fifty-volt 200W supply for your gate voltage. Selecting a power supplyĬhances are you will use lab power supplies to turn on your first SSPA.
Also, they have no control over how you decouple the supplies and other things you could screw up so belt-and-suspenders may mean better safe than sorry (see next sections). You should follow manufacturer’s guidelines, but realize that they might be over-specifying the solution because they are too lazy to investigate what the minimum requirements are. Other networks that may be required include “de-Qed” caps which are an RC network, and often 10,000 pF caps in shunt as well (which can be located an inch or two away and it won't matter).
Often manufacturers require 100 pF ceramic caps right next to the amp, with the wirebonds configured to ensure that the bias signal has to cross the capacitor (not off to the side like the bad idea on this page). This does not usually provide the “filtering action” that is required on the bias lines to ensures stability. Now back to biasing problems… Stability networksĪ MMIC amplifier is limited on how much bypass capacitance it can carry on-board, usually in the neighborhood of 10 pF on each bias connection. Are all of your constituent amplifiers unconditionally stable at all frequencies? When you put a lid on the amplifier do you create a cavity mode that can resonate in the band? Does your amplifier see a really good fifty ohm load at its output (1.2:1, not a crappy 2:1 VSWR match into fifty ohms?) Did you make sure that the amplifiers are phased up properly (we once saw a branchline coupler rotated in artwork, such that two amplifiers were happily pounding out power into the dump resistor and the actual output was 20 dB down…. There are RF problems you should look at too, but 80% of “missing power” in an SSPA can usually be associated with simple DC biasing problems.
If you are using HBTs you might have to rethink some of the free advice. The following material is written with the preconception that you are using FETs or HEMTs as the amplifying device and they are enhancement mode devices with gate and drain connections.
Watch out for ground feedback in the gate circuit Minimize drain bias line resistance, and ground return resistance.
It might be a good idea to reference this page in an SSPA design review. Many of these issues crop up time and time again, especially in new designs, because people tend to forget lessons they learned previously. This web page will review some of the possible problems that can occur in a solid-state amplifier (or any module that contains multiple power amplifiers), many due to biasing.
Click here to go to our main page on power combiningĬlick here to learn about graceful degradation in generalĬlick here to learn about graceful degradation in SPAsĬlick here to learn how to specify dissipations of SSPA isolation resistors