Power amplifiers

November 2, 2007

Click here to go to our main page on amplifiers

Click here to go to our page on compression point

Click here to go to instructions for our downloadable (and free!) Power_Amp_Desingner_101 spreadsheet

Click here to go to our page on efficiency of active devices

Our apologies, the content on this page is still pretty weak, but we're moving it up the list of "things that suck and we need to improve right away". Where are all the amplifier sponsors out there?

Fellow Microwave Dudes, we sure could use some pictures to go along with this text! Send us a picture and you'll receive a cool Microwaves101 pocketknife! Don't make us draw stuff with Microsoft Word, that gets ugly for everyone.

Power amplifier technologies

Power amplifiers are used to boost a small signal to a large signal. Power is relative… and frequency plays a big part in this. Solid state amplifiers and tube-amplifiers have different connotations when we talk about power amps. The table below represents the state-of-the-art for different microwave bandwidths. The info in this table is easily googled from the world wide web and represents no ITAR-restricted or classified information, Mr. FBI!

We have a page on semiconductor tradeoffs, as well as a page on microwave tubes, check them out!

Peak versus continuous wave power

Heat that an amplifier dissipates can be reduced by periodically shutting it off (pulsing it). The temperature in the active channel follows an exponential decay curve, at short pulses it looks like a saw tooth, at long pulses it looks like a square wave. The short pulse situation results in increased gain and power compared to continuous wave operation. The only truly accurate way to analyze this is with finite element design tools such as Fluent.

Let's propose a rule of thumb here (give us some feedback!) A pulse width of 1 microsecond is considered short and will always result in improved performance. If your pulse width is 100 microseconds, the steady-state channel temperature is reached and you won't get better than CW performance.

Temperature considerations

What happens to an amplifier over temperature? In the case of a FET amp, the gain drops and the noise figure increases, all very predictably. Use these temperature coefficients and a simple Excel spreadsheet and you can model what happens to your design over temperature:

For gain, use -0.006 dB/stage/degree Centigrade

For noise figure of an LNA, use +0.006 dB/degree Centigrade (no need to consider stages in an LNA, the first stage will dominate the temperature effect).

Note that specific amplifiers may behave differently from the coefficients we have provided; if you need to know what happens with extreme accuracy, guess what? You better get out in the lab and start measuring. One thing to look out for: if you reduce the temperature of your DUT below the dew point in your lab, the moisture condensation may cause problems, especially if you are looking at a hybrid amp with the lid off!

We have a page that ties together other temperature and thermal effects, check it out!

Amplifier efficiency

This discussion is now covered on a separate page.

Amplifier classes

Class A

When a power amplifier is running in Class A it is biased at close to half of its saturated current. The output conducts during all 360 degrees of phase of the input signal sine wave. Class A does not give maximum efficiency, but provides the best linearity. Drain efficiencies of 50% are possible in Class A.

Class B

In Class B the power amplifier is biased at a point where it draws nearly zero DC current; for a FET, this means that it is biased at pinchoff. During one half of the input signal sine wave it conducts, but not the other half. Class B amplifier can be very efficient, with theoretical efficiency of 80 to 85% depending on FET I-V characteristics. However, you are also giving up six dB of gain when you move from Class A to Class B, so if power added efficiency is important to you the optimum bias point may not be obvious.

Class AB

Most microwave power amplifiers are operated in a compromise between class A (higher linearity) and Class B (higher efficiency). In this case, called Class AB, the output conducts for more than 180 degrees of the input sine wave, but not over the full 360 degrees.

Class C

Class C occurs when the device is biased so that the output conducts for even less than 180 degrees of the input signal. This can be even more efficient than Class B operation, but the distortion is even worse. And output power and gain also take a hit. Class C is almost never used in microwave amplifiers.

Classes D, E and F

Yes these also exist. Mostly these classes of power amplifier get weirder and weirder, with careful attention paid to harmonic terminations. If you are designing a Class F amplifier, you probably don't need any help from Microwaves101!