Monday, June 11, 2018

What difference can the transistor make?

I built the circuit in the simulator and the changed the transistor.

The J112 is a good low frequency transistor but not so good at 100 MHz. It is a 5 cent part so a second stage could be used to pick up the gain.

The 2N3819 is another good low frequency amp. All but a total failure here though.

The good news and the bad news (for me). The best performer is a surface mount. For the difference in performance it is worth a little mounting issue. The good news is Arrow sells a 3K reel for $15 with free shipping.

This was an exercise in making the cascode amp. I would use a BF998 or BF2040w when available. Sadly they are both reaching end of life. Last time I checked Arrow still had some but when they are gone they are gone.

The simple approach used in the above circuit was to set the bias on J2 at 1 volt and the bias on J1 at 4 volts. I wanted about 1 ma current so I adjusted the other resistors to suit.

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