Transistor Specification Selection Guide
as an Amplifier
- for a receiver, the noise figure is an indication of the ability to discriminate the desired signal from background noise.
Best noise performance is only achievable with careful impedance matching, and is typically only achievable over a narrow bandwidth (10% variation in frequency or less). Very low noise figures (< 0.5 dB) are extremely difficult to achieve in practical circuits because of the losses associated with the impedance matching components.
- lower is better. However, for receivers always receiving relatively strong signals, a lower noise figure offers little real advantage, and the higher cost of a very sensitive transistor may not be appropriate.
- bipolars at 1 GHz: 0.8 - 3 dB
- FETs at 1 GHz: 0.2 - 1 dB
- FETs at 4 GHz: 0.3 - 2 dB
- FETs at 12 GHz: 0.5 -3 dB
(note: for FETs the 3rd digit of the part number typically indicates a NF sort; with lower numbers indicating lower NF)