Common Emitter Amplifier Design S Parameters

[dmowziz]

Hi... Happy new year


Trying to design the amplifier in first image...

Expecting a response like in the 2nd image (More concerned about the S11 response - As I want to use this immediately after a bandpass/lowpass filter in a receiver..)

The 3rd image is the response from prototype...

I added an emitter resistor and bypassed it with 820pF... At the base, there's a choke then to bias resistors


What am I missing?

Please assist... Thanks


2N3904BU 7Vce 10mA

[Robert Gribnau]

Shouldn't the base of the transistor see some sort of dc-voltage? In your schematic there is non.

Your schematic shows both the emitter and the collector connected to earth but since you write "7Vce 10 mA" I suppose the collector is connected to 7 V through L4.

[dmowziz]

Originally Posted by Robert Gribnau

Shouldn't the base of the transistor see some sort of dc-voltage? In your schematic there is non.

Your schematic shows both the emitter and the collector connected to earth but since you write "7Vce 10 mA" I suppose the collector is connected to 7 V through L4.

Hi Charlie, thanks.

The schematic is the AC equivalent...
Maybe you can make out the applied schematic from fig 1, fig 4 and the 4th paragraph in #1 ?

[Robert Gribnau]

Sorry, I missed the meaning of the 4th paragraph in your post #1.

If it were me, I would have posted the as-built schematic. But who am I (besides not being Charlie).

Good luck with your project.

[dmowziz]

Thanks, you are right...

Just that I used s parameter file as the transistor and Vbe is not close to 0.7V so not show the simulation is accurate
But I posted the full schematic now... *3.3mH



I meant paragraph 5
"I added an emitter resistor and bypassed it with 820pF... At the base, there's a choke then to bias resistors "

[Restoration73]

http://www.emagtech.com/wiki/index.p...ching_Networks

[PaulDarzi]

Originally Posted by Restoration73

http://www.emagtech.com/wiki/index.p...ching_Networks

Oooh lovely, many thanks for the link, what a wonderful tutorial.

[G0HZU_JMR]

Looking at the first amp in the original post, the PCB layout has lots of inductance in the emitter connection. This inductance will be in the long PCB trace between the emitter leg of the 2N3904 and the ground via which looks to be somewhere near the input inductor. That is quite a distance.

This will introduce a fair bit of emitter degeneration, and this will reduce the gain of the amplifier and it will also affect the input impedance. I'd recommend you shorten this connection a lot.

[G0HZU_JMR]

I had a skim through that tutorial, and the design is a bit odd in my opinion.

http://www.emagtech.com/wiki/index.p...ching_Networks

Normally, an amplifier is designed to a set of requirements and I'm not sure what the requirements were for that amp design. Here's a few initial observations...

The K factor analysis should really be carried out over the full range of available s-parameters, not just at 1GHz.

On top of this, the designer then put 75pF capacitance in the emitter without revisiting the stability analysis. Any change away from a grounded emitter can affect the amplifier stability. The 75pF cap is a risky thing to add in the emitter anyway, because it will affect the K factor down at VHF. Capacitance at the emitter will tend to raise the input impedance at VHF and this can introduce negative resistance here.

The simulation doesn't factor in the impact of any emitter degeneration. At 1GHz, even 1nH of emitter degeneration will cause a reduction in gain. BJTs like the BFG193 have two emitter legs to try and minimise the impact of emitter degeneration on the amplifier gain. The 75pF cap will have some package inductance and there will be a tiny amount of inductance in the PCB layout and via hole(s) at the emitter.

The 584nH inductors in series with the bias resistors are a strange arrangement and I'm not sure about this. Are they really needed? They will be difficult to model at 1GHz.

The amp has been designed to operate from a 25V supply which seems unusual. The operating point is only 10Vce at 10mA. Looking at the collector network design, I'm not sure the design will achieve a very good IP3 performance despite using a beefy BFG193. I'm not sure what was in the designer's mind here.

Big chunks of the design process are missing, the designer seemed to jump to a list of component values for the amplifier without really explaining how these values were chosen.

I'm not sure what the signal handling performance will be, but the design probably doesn't make the most of what can be achieved from 10Vce and 10mA Ic. That's partly why I queried the lack of design requirements. Overall, it just doesn't look like a real-world design to me.

[dmowziz]

Originally Posted by G0HZU_JMR

Looking at the first amp in the original post, the PCB layout has lots of inductance in the emitter connection. This inductance will be in the long PCB trace between the emitter leg of the 2N3904 and the ground via which looks to be somewhere near the input inductor. That is quite a distance.

This will introduce a fair bit of emitter degeneration, and this will reduce the gain of the amplifier and it will also affect the input impedance. I'd recommend you shorten this connection a lot.

Thankss Jeremy. I'm redoing it


In this thread #30, you did an amplifier at 145 MHz, please do you have the board so I can see... Thanks

https://www.vintage-radio.net/forum/...=187482&page=2




Thanks for the tutorial @Restoration73... Although the link you sent, lesson 8 is "broken".

[G0HZU_JMR]

I'll try and find it, but it wasn't built using a regular through-hole PCB like yours. I tacked it together using the classic ugly construction method. The aim was just to demonstrate the 15.5dB GMAX result at 145MHz for the 2N3904 as plotted in the graph below.

The GMAX curve shows the 2N3904 can achieve about 15dB gain at 145MHz when biased at 10Vce and 10mA Ic when arranged as a common emitter amplifier. It also shows that 8dB gain is possible at 433MHz. In reality a small amount of gain will be lost due to losses in the external matching components. I did also build the 433MHz amplifier to try and show it can deliver about 8dB gain at 433MHz. I can't remember how close I got to 8dB gain but I think it was fairly close. I'll try and find it as well.

[dmowziz]

I'm done with the board...few delays with some things but etching now... will start soldering shortly

Thanks Jeremy

Please what SMD size do you use for ugly construction?

[G0HZU_JMR]

I managed to find both boards. The 145MHz version was a bit crushed so I've tidied it up a bit. Neither amp design uses thermally stable biasing because I really just wanted to demonstrate the GMAX agreement. This means both amps have directly grounded emitters. This was done to get as close to the theoretical GMAX as possible.

It looks like I used a trimmer to set the input tuning. This probably causes a bit of loss, but the amp still delivered about 15.1dB gain at 145MHz. On my old simulation I think I put 10nH inductance in series with this trimmer to model the wire connections.

The 145MHz version still works but I haven't checked the 433MHz version. You can see the 433MHz version uses better quality parts and has a much tighter layout.

Sadly, I can't spend much time on this stuff as I bought a snazzy new i9 computer a few days ago and I'm busy setting it up downstairs. I'm trying to get to used to using Windows 11. At some point the new PC will replace this one.

[G0HZU_JMR]

I'm not sure you needed to remake your PCB. You could have just drilled an extra via hole near the emitter of the 2N3904 and fitted a ground via pin here. You could then have fitted a very short ground connection to the 820nF cap at the emitter of the 2N3904.

My demo amps in the previous post were really just built to demonstrate GMAX behaviour. Ugly construction works really well at VHF and even at lower UHF although I generally only use it for research prototyping.

[dmowziz]

Originally Posted by G0HZU_JMR

I'm not sure you needed to remake your PCB. You could have just drilled an extra via hole near the emitter of the 2N3904 and fitted a ground via pin here. You could then have fitted a very short ground connection to the 820nF cap at the emitter of the 2N3904.

.

Yeahh!

Just that I changed the resistors to SMD parts...

I'll drill the hole. Thanks

[dmowziz]

Originally Posted by G0HZU_JMR

Sadly, I can't spend much time on this stuff as I bought a snazzy new i9 computer a few days ago and I'm busy setting it up downstairs. I'm trying to get to used to using Windows 11. At some point the new PC will replace this one.

Nicee..But hopefully you spend some time in this forum... I love learning from you

[G0HZU_JMR]

I'm afraid my new PC is quite a seductive machine. It's the first new PC I've bought in many years. I usually just salvage an ex-works Dell desktop every few years.

I spent quite a lot of money on this one and it also has a very expensive Nvidia graphics card. So it's tempting to play games on it. It's currently downstairs, hooked up to a big old Pioneer kuro plasma TV and this works really well as a gaming monitor!

[dmowziz]

Originally Posted by G0HZU_JMR

I'm afraid my new PC is quite a seductive machine. It's the first new PC I've bought in many years. I usually just salvage an ex-works Dell desktop every few years.

I spent quite a lot of money on this one and it also has a very expensive Nvidia graphics card. So it's tempting to play games on it. It's currently downstairs, hooked up to a big old Pioneer kuro plasma TV and this works really well as a gaming monitor!

Hahaha...Please what games do you play? guessing Fifa since you watch soccer


I made the ground close to the emitter... Only s21 improved by about 2dB

I removed the bias resistors and grounded the emitter then used 2 channels from psu (twisted wires)... It's the same response as the change made previously..
No improvement in S11... I'm "Working" on the input inductor..
Please any suggestion?

Thanks Jeremy

[G0HZU_JMR]

It looks like the gain has now increased to be closer to the simulation.

Your input inductor doesn't look to be very friendly in terms of adjustability. Can you rewind it using thinner wire such that you can easily stretch the windings to alter the inductance? The Q will go down a bit but I'm not sure that matters st the moment. Adjusting this inductance may help here.

Also, try adding microstrip sections in your circuit model to try and make the simulation look similar to your real PCB. Try detuning the inductance of the input inductor on the simulator to see if you can replicate your VNA result of the real circuit.

[G0HZU_JMR]

Hahaha...Please what games do you play? guessing Fifa since you watch soccer

So far, I've tried GTA 5 on it and that looks amazing. I haven't played any computer games like this for many years though. I've also loaded an old copy of COD4 MW onto it and tried that too.

I'm probably going to buy the latest COD MW2 at some point. I've not tried the FIFA games. I could try downloading an older version of FIFA from Steam and give it a go.

I think the graphics card is a 3080 and the processor is an i9 12900K with 32GB of RAM. It seems to be able to play GTA 5 quite fluidly on the plasma TV.