BJT mixer design

[froten]

Hi all,

I trying to design a simple BJT mixer for AM receiver. I tried many circuits found on the internet but I want to design my own circuit with the LO injected through the emitter and the IF out through tuned circuit on the collector, but I have quision:

What is the non-linearity that should be used? is it V-I of the base-emitter junction, saturation regon or cutoff regon.

this is just a simple circuit, so i'm not worrying about IP3, strong stations ........

any help is very appreciated.

 

[Nigel Goodwin]

If you're worried about overloading, then use a balanced mixer a single transistor is probably the poorest method.

Check out almost any AM radio circuit, they almost all use a single BJT, usually using it as a self oscillator as well.

If you want to go deeper, get hold of either the ARRL or RSGB manuals, your best source for all things radio - old versions are fine.

 

[RCinFLA]

You will get better gain just lightly coupling LO into base.

Rules for BJT mixer is to have 10-20% delta current. As injection is applied the current consumption will increase. You want about 15% increase with L.O. injection. This will give you the optimum modulation of the b-e diode curve.

Base input match should provide lowest possible impedance at I.F. output frequency. Collector output should provide a low impedance at input frequency and LO frequency. This can be accomplished with the IF output tuning cap. Put the tuning cap from collector to emitter ground, not across the IF output tank coil with a IF bypass cap on cold side of IF output tank coil.

The more current run on the mixer stage the more LO power required to modulate the Vbe curve. You can use traditional DC bias stablization circuits.

You should get 12db to 15 db of conversion power gain.