ABCs of amplifiers

Updated October 15, 2010

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What is more fun than a microwave amplifier? Nothing. Everything in the natural universe spells loss for your microwave signal, that is the consequence of entropy. Only the sheer genius of an amplifier designer boosts your signal back from the abyss!

While we're building up more content on amplifier design, here's a good link to Queen's University web site that has a lot of information on the topic. Thanks to Sibasis!

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.

Below is an index to all of our material on amplifiers. In expanding our discussion on amplifiers, first we had to break it into several separate pages so it won't become a pain to load.

Low noise amplifiers

Log amplifiers (new for February 2009)

Magnetrons (new for February 2009)

Power amplifiers

Feedback amplifiers

Balanced amplifiers

Distributed amplifiers a.k.a. traveling-wave amplifiers

Active directivity of amplifiers

Pulsed amplifiers

Doherty amplifiers (new for July 2008!)

Amplifier classes (new for July 2008!)

Below are some leftover amplifier topics that we haven't written about yet. Want to help us write about any of these topic, for fame or fortune? Contact us!

Push-pull amplifiers

A push-pull amplifier uses two amplifier stages in parallel. The input to the devices is 180 degrees out of phase. The FETs are biased in Class B, where they conduct in alternating cycles.

Complimentary amplifiers

A complimentary amplifier uses opposite polarity (PNP and NPN) transistors to operate class B yet still provide the waveform. There is distortion, because both of the transistors have threshold voltages to overcome. In an HBT implementation, biasing the base at 0.6 volts can reduce this form of distortion.

Wideband amplifiers

Microwave amplifiers can provide ten's of GHz of bandwidth, even operate down to DC!

Tube amplifiers

Gain block amplifiers

A gain block is usually not a great power amp and not a great low-noise amp, but a pretty good wideband amp. Gain blocks are convenient, the designer merely drops them in where gain is required, so long as the application can afford the noise figure and low efficiency.

Feed-forward amplifiers

Feed-forward amps are used to reduce distortion such as second intermodulation products.

Matrix amplifiers

Coming soon!

Pulsed versus continuous wave (CW) amplifiers

By pulsed amplifier we mean one that is electrically switched off when it is not amplifying. Most CW amplifiers can be used as pulsed amplifiers, but not all pulsed amplifiers can be used CW.

Pulsing the amplifier on/off reduces DC power dissipation, which means that it operates at a cooler temperature. In some cases operating a pulsed amplifier continuously will cause damage due to heat.

They length of the pulse has an effect on how hot the pulsed amp will get, if it is long compared to the thermal time constant.

Because pulsed amplifiers run cooler, they often can provide higher power and efficiency compared to similar CW amplifiers.

Pulsed amplifiers require special power supplies to maintain the DC bias point during the pulse; often large charge storage capacitors are needed.

For very short pulses, you need to consider the bandwidth of the amplifier, so that it maintains the pulse shape.