Hobbyist RF amp design approach - Page 1
Forum section: electronics / rf-microwave

author: Teknow
author_rank: Regular Contributor
timestamp: May 16, 2017, 09:45:46 am
content: 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+Ibb5. R2= Ve+0.65/ Ibb6. 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?

author: dmills
author_rank: Super Contributor
timestamp: May 16, 2017, 08:46:14 pm
content: Setting the DC operating point is the same as it is for any other transistor amp, use a beta on the low side in the usual way, higher beta should not seriously impact the Q point if you are using a sane biasing scheme.Those 47 ohm resistors in series with the tuned collector loads are not doing you any favors, I would get rid or replace with a ferrite bead or such, maybe with 100n to ground from the top of the tank circuits in stages 1 and 2 (All that resistor does is to steal available collector voltage swing).You might find that a few ohms or so of base resistor is helpful from a stability standpoint.Base impedance for a common emitter stage is usually very much lower then collector load impedance, a capacitive divider works, an L match is better, or tap a turn or so from the end of the tuned circuit and feed a cap divider from that.Bipolar biasing for a stage with the emitters strapped hard down (Such as you might do for cooling) is a pain in the arse, think about well thermally coupled current mirrors, but the bias needs to be stiff for low distortion, which is a pain (There is a reason most use fets for this at any power).EMRFD has some useful notes for the experimenter.73 Dan.

author: Teknow
author_rank: Regular Contributor
timestamp: May 17, 2017, 09:32:32 am
content: Thanks a lot Dan! I appreciate your very informative reply.I checked the reply last night after spending hours in the quest of success while working on similar design to this one. I am going to make the changes you suggested and hope things will be better.Unfortunately, the circuits I have tried so far barely amplify the input signal (1mW @ 433MHz) or make a difference!

author: Unknown
author_rank: Unknown
timestamp: May 17, 2017, 10:17:54 am
content: Have you read 'RF circuit design' by Chris Bowick? It is a good book for explaining the basics and addresses most of the questions you raised in a quite approachable way. Building amplifiers in the high MHz / GHz range is an art, parasitics can kill an otherwise technically sound circuit.

author: dmills
author_rank: Super Contributor
timestamp: May 17, 2017, 04:59:01 pm
content: Yep, UHF can be all kinds of fun, but there should be plenty of acceptable 70cms band designs already kicking around.Up there you will want to do real impedance matching most likely the base impedance will be anything but resistive, or go Pi network with two trimmers.I think you isolating chokes on that bit of copper clad are almost certainly operating above resonance and are probably doing you no favours.With 1mW input to a three stage amp, I hope you have a license as that should manage several hundred mW at the output, and you will want decent output filtering as well.Depending on your modulation, I might make the third stage zero bias class C (Fine for FM, probably ok, more or less, for off on keying, not suitable for QAM).Regards, Dan.

author: MikeLogix
author_rank: Contributor
timestamp: May 20, 2017, 09:37:05 am
content: Quote from: Z80 on May 17, 2017, 10:17:54 amNo, it is not a good book, it is Great book for RF design.

author: kazam
author_rank: Regular Contributor
timestamp: May 20, 2017, 01:39:07 pm
content: As frequency goes up you will need to understand S-parameters and the Smith chart. At the very least!I use Keysight ADS for RF power amplifier design and its a good tool, unfortunately not feasible for amateurs.If you want to change the frequency of your amplifier you need to present a different impedance to the transistor. Or perhaps the same impedance at a different frequency which means your matching network needs to change.This is a complicated subject and a few rules of thumb simply will not be enough to do anything outside of replicating an already built and tested circuit.

author: cdev
author_rank: Super Contributor
timestamp: May 20, 2017, 02:36:08 pm
content: RFSIM99does a basic RF amp and will import s parameters and it's free.And easy to use.

author: cdev
author_rank: Super Contributor
timestamp: May 20, 2017, 07:50:52 pm
content: You're right, it is abandonware. I got my copy ages ago and simply keep copying it from one machine to another.I think the site I got it from was thisengineering class home page- affiliated with the University of San Diego but it came with a disclaimer that they were not affiliated with it. The firm that wrote it seems to be long defunct.Its very simple to use, it does just a few things but it does them well. You can also design simple filters with it.

author: cdev
author_rank: Super Contributor
timestamp: May 20, 2017, 11:50:30 pm
content: I have a question, why aren't RF amplifiers cheaper by now? Very small MMICs exist and make amplification for receiver hardware easy to implement. Why aren't there more, similar modular options for transmission?Or am I missing something?

author: Teknow
author_rank: Regular Contributor
timestamp: May 21, 2017, 08:22:50 am
content: Lots of thanks for all contribution to enrich this thread with ideas and knowledge!Sorry I might not reply to every contribution as I use an archaic IPhone version and super sluggish internet speed and aging eyesight! But I even appreciate all critics!Now I post those words before I have read the new contributions!First I assure you there's always a dummy load.70cm schematics on the Internet is mostly high power modules. I prefer custom "art circuits and designs", and adapting knowledge to my needs and my transistors. Because my Math, knowledge of trigonometry and AC physics are mediocre, it's a must I learn by doing to get along.Two books are of interest and already on the order list: Sayre'sComplete Wireless Design and Bowick's RF Circuit Design.I tried all the possible suggestions and impedance match but I had no progress!I have the feeling that the circuits don't amplify, on the contrary they attenuate! I measure the signal source separately connected to a dummy load, its -60 dB or so. After amplification it's more than -70 dBm!I think the VCO (signal source) I use produces power in the pico Watt!Last night I improvised a step up using an ATF21186 followed by MSA0886 (application note 1064) I waste milliamps and I get heat but no amplification! Now I am in the early steps of how to boost a tiny winny signal!I think beating around the bush of impedance matching is too early and is yet after I learn to boost faint RF signals!