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I want to design a power amplifier to work at 40 MHz. The only RF transistor that has a higher frequency rating than this I can find where I live is the MRF646, but the datasheet defines its parameters at 512 MHz. Can I use that transistor at 40 MHz?

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  • \$\begingroup\$ You could but I don't see how you'd be able to design it properly. The datasheet doesn't provide any characteristics for that low of a frequency, rendering it difficult to design. I would recommend something that would suit that frequency band. You're paying for an overqualified part. \$\endgroup\$
    – Colin
    Apr 15 at 21:42
  • \$\begingroup\$ Yeah, I know, I would want something closer too, but I cant find any where I live, and if I buy them in the US for example, shipping is very expensive \$\endgroup\$ Apr 15 at 21:44
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    \$\begingroup\$ If you can somehow find a SPICE model for this part, I would run a frequency sweep on this. \$\endgroup\$
    – Colin
    Apr 15 at 21:50
  • \$\begingroup\$ Ok, will try to do that. Thanks \$\endgroup\$ Apr 15 at 21:54

3 Answers 3

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One thing you need to be cautious of when using components with gain above your intended operating frequency is the possibility of oscillations or other artifacts at a higher frequency.

For instance, if you really want to use a 512 MHz part at 40 MHz, you need to take pains to ensure the overall gain rolls off outside of your band of interest. If the circuit still has gain at (say) 200 MHz, you could have an oscillator on your hands.

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Sure, you can! There's nothing wrong with not using a part to its full capabilities.

Note, though, that the internal input matching network of the transistor will be ineffective at such low frequencies. You'll need to match it externally.

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  • \$\begingroup\$ Awesome! Thanks. Someone hinted that the amplifier might become unstable, that's why I'm asking. That can't happen right? \$\endgroup\$ Apr 15 at 21:43
  • \$\begingroup\$ It could become unstable, but that can happen with any transistor you choose. It highly depends on the collector load and physical construction of the circuit. \$\endgroup\$ Apr 15 at 21:46
  • \$\begingroup\$ @FranciscoMaggi I suspect the hint was about driving the base without a resistor arranged to rapidly dissipate out oscillations since it is very easy for RF transistors to ring when given half a chance. They are fast. \$\endgroup\$ Apr 16 at 1:44
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The datasheet I found for this, only shows input and output impedance over a small range. Once you get down to 40 Hz and lower, things will be significantly different.
You can use stripline and tuned stubs at 470MHz, but in your desired range it's all lumped element capacitors & inductors. If your impedance matching networks (input and output) are also low pass filters, you should reduce the chances of unwanted oscillations.

Good luck!

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  • \$\begingroup\$ This helps. Thanks! \$\endgroup\$ Apr 18 at 20:18

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