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Design >> RF Design >> RF transistor biasing
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Message started by RF_ freak on Jul 15^th, 2012, 12:54pm

Title: RF transistor biasing
Post by RF_ freak on Jul 15^th, 2012, 12:54pm

I have a question concerning RF transistor biasing. For example if I'm going to design a VCO. Should I set the base voltage to about 0.65 volts for a bipolar junction transistor no matter the bandwidth, the type of oscillator and transistor circuit mode?

Sometimes I see there are no resistors in biasing an RF amplifier transistor, for instance, there is sometimes an RFC shunting base to ground.

Can someone give me some practical tips without pointing and referring me to a shelf of RF books??? For I have lots of them, and I get frustrated every time I recourse to them.

They are either too theoretical and superficial or too mathematical and smith-graphical!

Title: Re: RF transistor biasing
Post by RFICDUDE on Jul 16^th, 2012, 5:25pm

From your post it sounds like you are looking for a practical guide to VCO operation without a lot of detailed theory.

This is an open ended question that makes it very difficult for the members to know how to respond.

Can you describe the constraints you are trying to work within?

Title: Re: RF transistor biasing
Post by RF_ freak on Jul 18^th, 2012, 2:14am

RFICDUDE wrote on Jul 16^th, 2012, 5:25pm:

Thanks for the reply!

Yes, exactly, so practical that I can replace the original transistor to any ft similar transistor and still get the circuit to work as well.

In this example I copied the design and I used hypothetical all variable components at the outset . then I kept adjusting this potentiometer, that capacitor ,and so..etc till I got the final result.

This means I have to sit for hours on end in front of the simulator, besides sometimes I leave the circuit working after successful design and when I come the next day to add or simply look at it I find that it doesn't work!!!

Even if it does I don't know how reliable and sound in practice!

Is there a better way using own drawn schematic, a calculator and pen to design a 100% working VCO?

To put it simply: how can I set the transistor out to work as an oscillator? The hartley, colpitts etc. networks are left to me.

How can I bias another transistor to amplify the VCO's signal?

Two stages!