Author Topic: RF transistor biasing issue  (Read 1540 times)

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Offline TeknowTopic starter

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RF transistor biasing issue
« on: November 13, 2017, 11:34:37 am »
Considering ,among others, an AVGO RF transistor biasing Application Note 1293, some questions arise for which I could find no answer.
The Vce of a variety of RF transistors at hand is 10 Volts. If I strictly follow the instructions of biasing RF and general purpose ones setting Vce to be 0.5 Vcc then Vcc has to be 20 Volts.

However I have been biasing RF transistors according to the Data sheets where Vce is 10 Volts but I used a power supple for Vcc 12 or 13.5 and sometimes 15 Volts and never 20 Volts as this is sometimes not practical (especially when car battery operated). Does the RF transistor also go into saturation this way even when the circuit design is for FM applications?
In some data sheets for Class C power amplifiers Vce is 13.5 Volts for instance, should the Vcc then be 27 Volts or is Vcc the same voltage as Vce in this case?
So should Vce always be half Vcc. Is this a rule for RF transistors too?
 

Offline rfeecs

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Re: RF transistor biasing issue
« Reply #1 on: November 13, 2017, 05:49:57 pm »
So should Vce always be half Vcc. Is this a rule for RF transistors too?

That only applies to biasing with a collector resistor.  Typically for RF circuits you bias through an inductor, particularly for power amps.  So Vce will be approximately equal to Vcc.

Here's another biasing article:
http://www.qsl.net/va3iul/Bias/Bias_Circuits_for_RF_Devices.pdf
On page 4 you will see a class C circuit biased through an inductor.
 

Offline TeknowTopic starter

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Re: RF transistor biasing issue
« Reply #2 on: November 14, 2017, 07:03:18 am »
Thank s a lot RFeecs

Actually I looked at this one some time ago and its almost in agreement with Bowick's and Sayre's ideas on the subject. But I happend to find some designs which support my understanding of the data sheet, mainly adopting the suggested Vce and Ic without the necessity of applying a Vcc which is 2 times Vce.
Here is an example which supports what I mean.
Vcc is 10 volts Vce is 8 volts


http://www.qsl.net/yu1aw/LNA/bfp196peng.htm
 


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