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Transistor Specification Selection Guide
Frequency Range
as an Amplifier
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- What:
- The portion of the radio spectrum over which the transistor has useful performance (as an amplifier). A common definition of "useful" is greater than 6 dB of gain.
- Direction:
- needs to be sufficient to meet application, but not excessive.
- Excessive margin in frequency range usually means more attention needs to be paid to stability when designing, and may make the design considerably more difficult.
- Range:
- 1 MHz to 4 GHz for most BJTs
- 100 MHz to 6, 12 or 18 GHz for MESFETs
- 500 MHz to greater than 20 GHz for PHEMTs
- The lower ends of these ranges are not dictated by electrical performance. At lower frequencies, other technologies are far more cost effective. Also, transistor impedances become open-circuit-like at low frequencies, making design more difficult. Devices may not be characterizzed at these lower frequencies.
- It may not be cost effective to use high frequency analog transistors for applications below 100 MHz.
- It can be extremely difficult to match to MESFETs or PHEMTs below 500 MHz, as these devices have open-circuit like impedances at these frequencies; additionally the broad range over which the product has gain makes designing a stable amplifier somewhat challenging.
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