I have been searching in a lot of RF design books, application notes, data sheets, and internet sites for a comprehensive and empirical approach to bias RF transistors to be able to design a multistage RF amplifier but still in vain. I miss an approach in reference to certain transistors and their specifications.
These transistors are mainly from Philips (NXP) for example:
BFG520, BFG540, BFR92, BFR93 etc. and AVAGO AT42086 among others.
After some experimenting and learning I could design circuits depending on certain criteria depicted from the data sheet.
Though essential for stability, I intentionally overlook the mathematics concerning stability as it is beyond my comprehension at the moment. I started learning about impedance matching, smith charts and Sparameters, but sometimes I get stuck in the abstract of confusion.
The approach I adopted (adapted) in designing my circuits is as follows:
1. I refer to the GUM and relevant curves in the data sheet of a transistor and adopt that Ic, Vce, and gain (marginally!).
For example the GUM for the BFR93A is 13dB when Ic=30mA, Vce=8V and f=1GHz.
2. I calculate Ib by dividing Ic on the typical value of Beta from the data sheet.
3. Then I multiply that by 10 for Ib and Ibb.
4. R1= (Vcc-0.65-Ve)/ Ib+Ibb
5. R2= Ve+0.65/ Ibb
6. Re= Ve/ Ic (for a small signal transistor I let Ve= Vce/ 10.
7. Rc= Vce/ Ic.
Imagine I built two identical stages using the BFR93A, and a medium power stage using the BFG135, or the BFG196, or BFG198 etc.
For the BFG198, I follow the above mentioned steps but sometimes I make Ve=0 to get more power and thermal coupling to PCB through the emitter leads.
I use an LM317, or a pnp transistor (active bias) to limit current for this transistor.
After building the three stages and for interstage impedance matching (trial and error) I connect variable capacitors to tweak and monitor for maximum peak on the spectrum analyser.
Once I achieve the best performance , I replace the varicaps with fixed ones.
In order to cultivate this approach for hobbyists and while on the way to simplify Sparameters etc. I need some suggestions and feedback to improve and answers to some questions.
1. In RF design books, the transistor design steps are rarely based on specific RF transistors and their specifications.
2. The beta is "assumed" 50, why not that maximum or the typical!?
3. There are no explicit examples tackling wide-band amps or narrowing amps to varieties of frequencies!
4. There are no subjects tackling trade off of power and thermal stability and how to cool the RF SOT223 transistor!
5. In the Philips medium and high power transistor data-sheets, there are suggested circuits for the intermodulation distortion, can one adopt them as suggested amplifier designs for that transistor (considering the BFG135, ..etc.)
6. These circuits are for specific frequency, how can one modify them?
7. The Vcc and Vbb are never given for these circuits, how can I tell the values?