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Reading transistor datasheet.

Joni

I'm a hot dog
I want to experiment a little bit with this radio transmitter: makezine.com: Super Simple iPod FM Transmitter | Make:
I want to change the LC resonator to get other frequencies, but the used Transistor (2n3904) but it should only work near 300 Mhz. The datasheet has no max. Frequency :/
https://www.onsemi.com/pub/Collateral/2N3903-D.PDF
I searched a little bit in the 2n series and found the 2n6428 transistor: https://www.mouser.de/datasheet/2/68/2n6428-1149869.pdf
This transistor works up to 700Mhz but I know it can't be that easy. For which specifications should I look that it's working?
I don't think it's the right forum and it's probably too difficult to answer, but I hope that someone knows more about that then me.:p
 

TabbyTomCat

Well-Known Member
Simplified answer is that field effect transistor frequency range is limited by the speed of charging/discharging it's gate capacitance.
You can either use datasheet input/output capacitance to calculate transient response of the transistor in your particular circuit or simply use rise/fall time from switching characteristics which is inaccurate because it is valid only for test conditions (usually mentioned) in datasheet.
Your operating frequency for analog design should be below these limits with large margin. Also mind that transistor parameters (not just resistivity) vary with temperature and age, switching gets slower with both.

I'm not an analog circuit designer, so you can get better help from someone else.
 

Canis Dirus

Extinct Pleistocene canid
Add my five kopecks: "AN139A: Understanding transistor response parameters", "The Handyman's guide to understanding transistor data sheets and specifications", "Choosing transistors for radio frequency applications".

This transistor works up to 700Mhz
It's may, but don't have any obligations to work at frequencies over 100MHz (minimum value of gain-bandwidth product from datasheet). Plus gain-bandwidth product just show frequency at which the transistor's hFE drops to 1. In practice, the circuit becomes unusable much earlier.
 

Canis Dirus

Extinct Pleistocene canid

Joni

I'm a hot dog
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