Class AB Balanced power amplifier problem

Hi All,

I am working on a L Band Power amplifier. I use Quadrature hybrid dividers and combiners for balanced operation. To obtain better Harmonics rejection at the output I am planning to use modified Quadrature Hybrid which will provide more than 20dB Secong harmonic rejection. My doubt is if I do so , will the reflected second harmonic will harm the high power amplifier. ( I am using 1000W Power amplifier MRF6VP121KH from freescale).

maybe, you can use isolator to protect PA.

palanisamy2k said:

Hi All,

I am working on a L Band Power amplifier. I use Quadrature hybrid dividers and combiners for balanced operation. To obtain better Harmonics rejection at the output I am planning to use modified Quadrature Hybrid which will provide more than 20dB Secong harmonic rejection. My doubt is if I do so , will the reflected second harmonic will harm the high power amplifier. ( I am using 1000W Power amplifier MRF6VP121KH from freescale).

Click to expand...

So you will be combining two 1KW PA's to get ~2KW out ? You have to be careful,
because if you have a device in line with even one PA (output side) it can get burned
quickly.

i.e. 1KW = 60dBm, if you have an Isolator in line that has 1dB of insertion loss that's:
60dBm - 1 = 59dBm --> 59dBm = 794.33 W
so that Isolator just absorbed (1000 - 794 = ) 206Watts.

Also I know you have to be careful with what type of resistor you use on the
Isolated port. The Port might be "Isolated", but it will get power sinked to it.
and at 2000W Pin that load resistor is going to get hot !

In my previous weather radar employer, they used isolator after 1600W PA. That is huge in waveguide. Some are coaxial. 1600W is peak power, average power is about 16W.

Element_115 said:

So you will be combining two 1KW PA's to get ~2KW out ? You have to be careful,
because if you have a device in line with even one PA (output side) it can get burned
quickly.

i.e. 1KW = 60dBm, if you have an Isolator in line that has 1dB of insertion loss that's:
60dBm - 1 = 59dBm --> 59dBm = 794.33 W
so that Isolator just absorbed (1000 - 794 = ) 206Watts.

Also I know you have to be careful with what type of resistor you use on the
Isolated port. The Port might be "Isolated", but it will get power sinked to it.
and at 2000W Pin that load resistor is going to get hot !

Click to expand...

Thanks for your reply. I have actualy combining Five of such devices to produce 4KW Peak power. My duty cycle is 1%. So , the average power would be around 40 Watts. I have one circulator which can handle 4KW Peak power with 0.3dB insertion loss.

My problem here is , I cant use a low pass filter here due to Insertion loss issue. If I made the Final combiner which will provide 25 dB rejection for Second harmonic , then I will meet the Second harmonic rejection requirement without the filter.

Here my doubt is whether I need to worry about the Second harmoic signal which is reflected back to the amplifier from the combiner ( In the combiner the returnloss is 0 dB for 2nd Harmonic) or not?

I think no need:
1. You have isolators
2. 2H power is always 20dB less than base.

I know a fellow when developing low power SiGe PA modules he had this problem. The PA got damaged not by the poor VSWR at fundamental (which actually was fine), but by the 2nd harmonic which was reflected back to the die.
The PA package was very small 6x6mm, and internally was the die and FR4 PCB for the output matching network and the low pass filter.
SiGe process is very sensitive to ruggedness due to low break-down voltage, but finally he managed to fix the problem placing 2nd harmonic traps (series LC) at the output. There is a trick how to design these traps, they have to have low-Q, so the L/C ratio shall be lower than usual.

Now, to translate this low power example to your situation (and to fix it), you need accurate models of your transistors to put in a simulator. Unfortunately, simulators don't tell you if the transistor dies suddenly due to various reasons. And for a transistor that cost 500 bucks is risky to try live tuning using load-pull.

Thanks vfone. Unfortunately no model is available for my device. Impedance data is available only for two points (1030MHz and 1090MHz). Can you suggest any method to trap the second harmonic of this spots without scarifying the performance.

The two points of impedance provided by Freescale should be fine for tuning the 2nd harmonic traps. The reason of harmonic tuned technique is to find the optimum device harmonic terminations (usually up to third harmonic) to improve the fundamental output voltage, increasing in this way the PA performance as output power, gain, efficiency, and ruggedness.
The 2nd harmonic trap could be series LC, or stubs. At higher frequencies preferable is the open stub, quarter-wavelength transmission line at 2nd harmonic, which provides about 10% bandwidth at 2nd harmonic (BW depending by layout and PCB material).
Tuning the harmonic traps should be done in the same time when tuning the in/out impedances, checking all the time if PA parameters are still ok.
As a tip, tuning the 2nd harmonic trap for best fundamental efficiency (PAE), could be also the the best impedance point getting good ruggedness due to reflected 2nd harmonic. This in case the PCB layout doesn't have unpredictable mistakes.

Did anybpdy of you have such problem - when you install amplifier top cover, it becomes unstable and starts to self-osclilate?
What are methods to fight this?

Use aborption materials, that is popular in RF field.

Materials like which ones - do you know examples ?

that should be some microwave absorption materials, you can search, such as 3M, I guess so.