How does the preamp work?
The preamp input circuits couple the signals in, and transform the impedance of the input, to the impedance of the amplifying device. The amplifying device (bipolar-FET-MESFET-PHEMT) will be biased to operate as a linear amplifier. The impedance match from the input to the amplifying device will usually take the form of an L (inductance) C (capacitance) circuit. Other input circuits include Coaxial Cavities, or Helical Resonators.

The amplifying device will amplify the incoming signal, and pass the signal on to the output
stages. The output stage will match the impedance of the device output, to the impedance of the load (output).

What types of preamps are avaiable?
There are many types of preamps available. A quick look around the Web, finds units ranging from $4.95, all the way to $500.

Lets break it down to the two primary types of preamps: Receive only, and Receive / Transmit.

A receive only preamp may only be used in a receiving system. You may not transmit through it. Doing so will damage or destroy the preamp. To be utilized in a transmit and receive system, steering relays will be required. A sequencer which automates the transmit and receive switching of the relays may also be required.

A receive / transmit preamp may be used in line with a transmitter and receiver. You may transmit through it, provided that you follow the manufacturers instructions regarding
maximum transmit power, and other switching requirements. These types of units
contain steering relays or PIN diodes to switch the transmitted signal around the preamp.
They may be hard switched (by a sequencer) or RF switched, where the unit detects transmit RF, and then steers transmit power around the preamp, and to the antenna. Although they are simply installed, the circuits associated with RF sensing, the small relays, and circuit board losses can degrade noise figure in this type of preamp. Not to mention the built in switching delays, and power limitations. Sooner or later you will increase your transmitter power, and the RF sensed preamp may become damaged.

What is noise figure, and why is it important?
Again, in simple terms, the noise generated by the preamp is the major factor in noise figure specification of a preamp. Noise figure is calculated by measuring the difference between two noise levels. The output level with a noise source driving the input of the preamp, and
then the output with the noise source off. In both cases the preamp is properly terminated.
A calculation is required to determine noise figure. You should investigate and learn more about “Y” factor, Noise Factor, and Noise Figure if you want to learn the details!

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