Help with Amplifier Design

Our prof had given a challenge to design given amplifier and I stuck on how to go with power?

Q.) Design a transistor amplifier with f = 2.5 MHz, Pin = 1 mW and we must get Pout = 5 W. (No restriction on components we use, but we cannot Microwave Power Amplifier)

How should I proceed? I also need to simulate the circuit in MULTISIM 8.0 version.
I was thinking to design a CLASS A amplifier but I dont think using CLASS A I will be able to boost 1 mW signal to 5 W.
please help

I think class A would be the simplest. There's no problem boosting 1 mW to 5 W with any class. However class A will dissipate more power as heat than other classes, for the same output power. That isn't a big problem - you just need to use a power transistor and a heatsink.

Before you start with the design you need to know what the source impedance and load impedance will be. As it's an RF amp, they will quite likely both be 50 Ohms, but it's best to check.

You also need to know what bandwidth is required. You said "2.5MHz" but does that mean you can make a tuned amplifier that only works at say 2.5MKz +-20KHz, or should it be a wide-band amp that can work at any frequency from say 250KHz to 2.5MHz, or what?

Well You Can Choose from different class of amplifier. Each Class offers its own advantages and disadvantages.

Class A: This is most linear of all the classes,meaning the output signal is a truer representation of what was inputed. But these amps run hot, as the transistors in the power amp are on and running at full power all the time.Because of this Class A amplifiers are very inefficient; for every watt of output power, they usually waste at least 4-5 watts as heat.

ClassB:In this amp, the positive and negative halves of the signal are dealt with by different parts of the circuit. The output devices continually switch on and off so there is a lot of "crossover" distortion.The input signal has to be a lot larger in order to drive the transistor appropriately. meaning that from the amplifier input, it needs to be "stepped up" in a gain stage; This means more circuitry in the path of your signal, degrading sound even before it gets to the output stage.So in general these amps run cooler than Class A amps, but the output quality is not as pure.

ClassAB:Class AB operation has some of the best advantages of both Class A and Class B built-in. Its main benefits are sound quality comparable to that of Class A and efficiency similar to that of Class B. A benefit is that the inherent non-linearity of Class B designs is almost totally eliminated, while avoiding the heat-generating and wasteful inefficiencies of the Class A design.

ClassD:They are better termed "switching" amplifiers. The input signal is converted into a sequence of pulses whose average is directly proportional to the amplitude of the input signal.The output of the amplifier goes through a passive filter to remove unwanted spectral components, resulting in an amplified replica of the input.Power efficiency is the main advantage of a class D amplifier.This efficiency gain is at the cost of high-fidelity.

Class AB design I would recommended for most general-use applications. Since it is this combination of good efficiency (around 50) with excellent linearity.

I think "boosting" 1mW to 5W is referring to his power gain, not power consumption or dissipation.

What kind of equipment do you have access to? Do you have a network analyzer?

In any case, a class B or class AB is probably the easiest route, especially if you don't have any distortion requirements. You'll probably three amplifier stages. At such low frequencies, the first one can probably be an op amp.

@godfreyl
@ishailesh
@mtwieg

I am extremely thankful for the response, almost solved my problem. Once I design the ampfr, I will post the snap-shot