Clarification required in designing a RF amplifier

[boiler95]

Hi,

I had posted this at the Analog Circuit Design. This forum is more appropriate hence reposting here.

I'm planning to design an RF amplifier. My basic purpose is to increase the range of an existing radio link of 415MHz.

There are several types of transistors, by types i mean "BJT" and "FET".

I know i need to select the transistor based on my requirement and transistor characteristics(no doubts on that) but, which type of transistor to use "BJT" ? "FET"? "MOSFET" ? (remembered i used FET for switching and BJT for amplification when i studied engineering) please advice.

Transistor Biasing. CB, CC, CE these are which i know. But which one to be used? CE is mostly used for amplification ( please correct me if I'm wrong)

Next, Class of amplifiers. We have class A , Class B, Class C. Which one is more suitable for RF amplification.

What is a power amplifier(this amplifies the RF signals to drive something like an antenna, correct?) Is this a separate category than the above or Power amplifier fits into one of the amplifier class ( A, B, C )


lots of questions. Please advice.

Thanks and Regards

 

[chuckey]

First of all, what sort of power output are you wanting, 10mW or 100W?, it can limit your choice of device. Does the amplifier need to be linear? i.e the output power follows the input power EXACTLY. If the modulation is FM (frequency modulation) then the op/input need not be linear and class C can be used. Class A and B are "linear".
The term "power amplifier" is a hang over from the days of the vacuum tube ("valve"), all the manipulation of a signal within a transmitter were done with signals that were high voltages across high impedances, so they had little power in them. The old tube power amplifiers, would take a signal of say 100V across a high impedance tuned circuit (P = .1W?) and amplify it in one stage to a level of 1kW, which is REAL power!!! With BJTs and their low impedances and low gains, its not so obvious. A 100W BJT amplifier might need 20W into it and this stage might need .5W into it. . . FETs can offer loads of power gain, but because of the requirement of stability (they love to oscilate at UHF frequencies), tend to have low input impedances with lots of added resistive damping so they are half way between tubes and BJTs.
Frank

 

[vfone]

CE has the highest power gain compared with other configurations.
Power class of the amplifier depends mainly by the modulation of the signal that needs to be amplified (e.g. Class A is the most linear, when Class C is the most nonlinear).
All the power amplifiers (for RF, audio, etc.) use power classes.
A power amplifier by definition is an amplifier which main characteristic is the power gain.
The load of an RF power amplifier usually is an antenna, but also could be other circuits depending by the application.

 

[boiler95]

Thanks for all your replies.

For the generation oh my RF signal i;m using an integrated module(Long Range module from Linx technologies Inc). The modulation scheme is ASK/OOK (This would narrow my search to Class A and Class C amplifiers, Let me know if i'm wrong)

One more question. . . . While i was reading through the data sheet i came accross the following line.

The LR transmitter is a low-cost, high-performance synthesized ASK / OOK transmitter, capable of sending serial data at up to 10,000bps

What do they mean by synthesized?

 

[vfone]

Means that for generating the RF signal is using a PLL synthesizer.