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Old 15th Jun 2018, 8:56 am   #21
Radio Wrangler
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Default Re: Cathode follower anode resistor.

This is one of those hidden problems. Firstly, it's all to do with stray inductances and capacitances, and those aren't shown on circuit diagrams so that makes the problem invisible. Secondly, once you can 'see' the strays it doesn't look like a recognisable oscillator circuit. There is no visible loop, no visible feedback. You need to be familiar with the maths of active circuits transforming impedances - not simply scaling them, but changing capacitances into inductances and simulating negative resistors. These things were best covered in material regarding active filters, circuits using opamps down at audio. It takes quite a mental flip to see them in the strays of an emitter follower. but the little devils do oscillate most willingly and thereby show that the awkward theory is right.

In general terms you have to make sure that when using a 3-terminal device, you never try to have low RF impedances on more than one terminal. An emitter follower with a nicely decoupled collector driving a circuit with a capacitor (could be a bit of coax cable) to ground is at-risk.

Most stopper values are guessed, the fruits of experience, or are the results of a bit of experimentation plus a bit added to allow for device spreads and 'improvements'

The theoretical analysis is the proof that you really do need to take precautions. The stoppers generally reduce the performance a little of the stage in its intended purpose, and they cost a little, so professional designers have to keep justifying these things to people whose maths is only at MBA level

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Old 15th Jun 2018, 9:48 am   #22
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Default Re: Cathode follower anode resistor.

Quote:
Originally Posted by Radio Wrangler View Post
... people whose maths is only at MBA level ...
What a very handy phrase ! I shall file that one away, thanks .

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Old 15th Jun 2018, 10:44 am   #23
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Default Re: Cathode follower anode resistor.

Maths has always been the problem for me in trying to understand the finer details of circuit theory. At the beginning of my working life as a service engineer I took the appropriate City & Guilds course in which we were taught very basic stuff like Ohms law and other simple formulas for resonant frequency and reactance etc. I took the Wireless World magazine for about 30 yeasrs from the end of the 60s and always struggled with some of the mathematical explanations in articles. At the beginning of the 90s I took an HNC course and was taught maths at a higher level but I have never had to use it in my working live and so of course I have forgotten it all. The equipment I worked on had all been (I hope ) designed by people educated in maths to degree level but the service engineer has no need of such knowledge.

Maths of course is the language of engineering and its understanding is essential in our field. I consider I have a very good understanding of analog circuits up to the level appropriate to what I do and will have to be content with my limitations.
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Old 15th Jun 2018, 2:07 pm   #24
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Default Re: Cathode follower anode resistor.

Maths is all very well, but the proof of the pudding is in the eating. With ourselves it is almost certainly better to be aware of a solution if we need it as circuit layout is so variable hence parasitics will be too! It is all very well for big companies producing things in thousands or more who may have computers that can simulate most anything, and specialists for individual concepts. I have always found it useful to have a daily handling of mathematical ideas, but only wade into more advanced things when suck it and see fails. It is a lot quicker, and satisfying, than working from a set of equations, just to find an essential correction factor has been missed at a late stage.
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Old 16th Jun 2018, 12:47 pm   #25
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Default Re: Cathode follower anode resistor.

On another website I recently had an argument with someone about the need for a resistor at the output of a follower. He saw it as merely raising the output impedance and thus a bad thing. He would not accept that it was a routine precaution against instability, and his mathematical understanding of oscillators was too limited to grasp the theory of it.

I think I first saw this issue raised by Bob Pease in his book about how to really do electronics (I forget the actual title). An emitter follower was used to pulse a reset input on a microprocessor on powering up, but the device failed EMC testing because the follower was oscillating at some UHF frequency.
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Old 16th Jun 2018, 1:24 pm   #26
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Default Re: Cathode follower anode resistor.

It does raise output impedance, yes - could the resistor not go anywhere else? As per title of this thread, in series with anode or collector would do - and it would have the benefit of adding some current limiting in case of output short-circuit without screwing up the output resistance!
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Old 16th Jun 2018, 1:57 pm   #27
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Default Re: Cathode follower anode resistor.

Quote:
Originally Posted by ukcol View Post
Maths has always been the problem for me in trying to understand the finer details of circuit theory. At the beginning of my working life as a service engineer I took the appropriate City & Guilds course in which we were taught very basic stuff like Ohms law and other simple formulas for resonant frequency and reactance etc. I took the Wireless World magazine for about 30 yeasrs from the end of the 60s and always struggled with some of the mathematical explanations in articles. At the beginning of the 90s I took an HNC course and was taught maths at a higher level but I have never had to use it in my working live and so of course I have forgotten it all. The equipment I worked on had all been (I hope ) designed by people educated in maths to degree level but the service engineer has no need of such knowledge.

Maths of course is the language of engineering and its understanding is essential in our field. I consider I have a very good understanding of analog circuits up to the level appropriate to what I do and will have to be content with my limitations.
For many aspects of RF design you really only need everyday maths. You do however, need an appreciation of the higher level maths behind a lot of the modern simulation and analysis tools you might use.

When I started work in my current job nearly 30 years ago the tool I wanted most of all (at home) was the Circuit Busters/Eagleware RF simulation software we had. Compared to this the exotic HP and Marconi lab gear we had came a very distant second. In the right hands a decent RF simulator is a very powerful (learning) tool and as long as you have an appreciation of the maths behinds it you really don't need anything more than the simulator and maybe an excel spreadsheet and a calculator/pen/paper to do most tasks.
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Old 16th Jun 2018, 3:42 pm   #28
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Default Re: Cathode follower anode resistor.

Quote:
Unfortunately I found studying the Dennis Feucht article above quite frustrating. This is mainly because it is (necessarily ?) written at a higher level than my education allows me to follow. At the end of the article Feucht refers to a Part 2, but I have not been able to locate it. Part 2 addresses the problems faced with amplifiers that have reactive loads and may be more relevant to our discussion. No doubt I would, in any case, have had the same problems understanding Part 2 as I have had with Part 1..
It's a very powerful article but like many RF design papers it has to back up its work with a maths analysis. This can alienate many readers who might think you 'need' to understand the maths in order to extract value from the paper.

In my case I only lazily skimmed the article and it appears to me that the author is suggesting that you can use his circuit transforms to predict the input impedance at the base when you fit RLC parts at the emitter. This can reveal hidden problems that might lead to instability. eg it's easy to end up with loads of negative resistance at the base with a typical emitter follower circuit if there is a sniff of capacitance at the emitter.

You can use each R, L or C (emitter to base) transform on their own or combined as a form of LEGO. He gives a worked (LEGO) example of an emitter follower where a 2N3904 at a given operating point has been fitted with 470R and 10pF in parallel at the emitter. The idea is you place both of his R and C transforms in parallel (like LEGO) to get an indication of the impedance you would see at the base of the transistor.

Although the article may look like a confusing mess to many readers this worked example is much easier to make use of than you might think. (assuming I've actually got it right myself )

See the attached images of a comparison of a simulation of the base impedance of a 2N3904 s2p data file (at 10mA and 10Vce) compared to the Feucht transform. The circuit shows the circled LEGO elements for the 470R resistor and the 10pF cap at the emitter. They transform to the circled sections at the base. The pink components are negative components. i.e. -10pF and -53 ohm.

It does seem to agree quite well across LF to 100MHz and shows loads of negative resistance at the base. The capacitance seems to agree as well. You would just need to add an external (stray) shunt inductance to make up an oscillator. This could typically be the inductance of a wired connection to a low Z source. A 2N3904 could easily oscillate up at UHF with a short shunt inductance at the base with this circuit configuration.

Note that the NPN symbol used in the attached circuit is really just an s2p data file of a 2N3904 from a VNA and so it is an AC model. This is why there is no PSU connection or any bias resistors or blocking caps in that schematic. It's an AC model of the transistor so it doesn't don't need DC connections or blocks.

Note that I only read the article quickly. I think I've done the transformations correctly but I can't be certain. I think all of it could go into a simple excel spreadsheet to speed up the generation of the circuit values.
So a typical user could make use of this via a spreadsheet and it's up to the user if they want to dissect the maths behind the spreadsheet that generates the values to go into each transform. Note that there's only everyday maths there anyway. The article makes it all look complicated but the theory/sums to make up the model of the BJT and go on to make up the transforms really is just GCSE level maths. I'm sure you would realise this if you worked through it or watched someone else do it

Not that the darker traces are the s2p model of a real 2N3904 BJT and the paler ones are the Feucht transform. So the dark red trace is the series resistance looking into the base of the s2p/VNA model of a real transistor. The dark blue trace is the series capacitance looking in to the base of this s2p model. This data should be quite accurate.

The orange trace is the series resistance looking into the Feucht transform. The light blue trace is the series capacitance looking in to the base of this Feucht transform. So you can see there is fairly good agreement across LF to 100MHz.
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Old 16th Jun 2018, 4:27 pm   #29
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Default Re: Cathode follower anode resistor.

Note that adding a 100R in the collector won't do much to quash the negative resistance seen at the base of that 2N3904 emitter follower.

A more relevant solid state circuit would be JFET based. This is probably much closer to a cathode follower using a triode valve. However, I'm not even in short trousers yet when it comes to design/experience with valves so I can't offer much design info wrt valve circuits

The circuit below with a J310 follower looks harmless enough and the 28nH might be a short 'ugly' wire connecting the buffer to a low Z source (represented by the 2R resistor).

But this is an oscillator that will hoot away up at about 500MHz. There are loads of ways to dissect this circuit to show why it is unstable and to predict what will work and what won't in terms of trying to tame it. I don't think you need any exotic maths to analyse it but an RF simulator would definitely help a lot. Even an old school/obsolete simulator like RFSIM99 would prove useful here and it is free.

I can show why/where/how this circuit oscillates if you want to see this stuff? Don't be put off by the lack of a decoupling cap at the drain in this simplified circuit. The PSU assumes a perfect wideband voltage source so perfect decoupling is assumed here from DC to daylight and adding a cap won't make it any different.
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Old 16th Jun 2018, 6:56 pm   #30
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Default Re: Cathode follower anode resistor.

Quote:
Originally Posted by G0HZU_JMR View Post
Note that adding a 100R in the collector won't do much to quash the negative resistance seen at the base of that 2N3904 emitter follower.
It won't, no. But what it will do is add 100R in series with Cga, which in the absence of the 100R would go direct to RF ground.

As Cgk, Cga, Cak are same order of magnitude (for a triode), then adding the 100R in series with Cga imposes a lot of damping across (Cgk + Cak). Hopefully enough to stop it hooting!

I can picture the thing in my head, but access to paper and pen is limited now. Later, I aim to share my take on this, which I trust other members will comment on
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Old 16th Jun 2018, 7:20 pm   #31
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Default Re: Cathode follower anode resistor.

Ad doing it in the real world with actual components shows that a resistor in the collector is actually quite a powerful preventative measure. Nothing comes free and it acts to reduce the available positive going swing, but then that may be preferable to an emitter resistor.

Base resistors are also fairly effective, and this is the reasoning behind slipping ferrite beads onto transistor base leads. The chosen beads look like 50-200 Ohms resistive at VHF and at DC you only see the DC resistance of the wire.

On the other hand, deliberate oscillators made from 'emitter followers' with RF capacitors in all the wrong places, and a litttle bit of printed track inductance are very popular ways of making signals in the 200MHz-few GHz region.

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Old 16th Jun 2018, 7:48 pm   #32
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Default Re: Cathode follower anode resistor.

Putting 100R in the collector of that jellybean 2N3904 example only shaves a few ohms off the negative resistance and it only cuts down the crossover frequency from negative to positive resistance by something like 10%. This might be enough for some designs but it seems a bit marginal to me.

It seems better suited as a 'stopper' for jellybean JFET followers and (probably) valve followers. I think the benefits of 100R in the drain of a JFET will be more noticeable here.
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Old 16th Jun 2018, 7:58 pm   #33
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Default Re: Cathode follower anode resistor.

As the collector of a BJT is a less sensitive 'input' than a triode anode or JFET drain I would guess that adding a resistor here would have less effect than adding it at the emitter circuit.
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Old 16th Jun 2018, 8:44 pm   #34
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Default Re: Cathode follower anode resistor.

Quote:
Originally Posted by G8HQP Dave View Post
As the collector of a BJT is a less sensitive 'input' than a triode anode or JFET drain I would guess that adding a resistor here would have less effect than adding it at the emitter circuit.
Probably- but it's less the sensitivity as the device is intrinsically and more the magnitude of the inter-electrode capacitances, as I se it.

See below:
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Old 16th Jun 2018, 9:13 pm   #35
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Default Re: Cathode follower anode resistor.

If anybody ever doubted the value in words of a picture, they need only refer to the above!

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Old 16th Jun 2018, 10:02 pm   #36
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Default Re: Cathode follower anode resistor.

I think (from left to right) pictures 1,2 and 3 are OK but I think you have to refit the Cga cap back across the anode to grid once you add the 100R resistor in the anode. Then it's a case of analysing the effect of the negative feedback it introduces. But like I said before, I know very little about valves.

Valves are huge structures and I have no idea of the stray capacitances and inductances within a typical triode. But with a jellybean JFET it is just about possible to extract the strays and put them into a basic open loop analysis of the circuit. i.e. break the loop open at the source and then do a gain/phase plot to show where it might oscillate.

This is a fairly dodgy approach for various reasons and it requires a few approximations based on datasheet info but it should roughly indicate where the JFET circuit I posted up earlier will oscillate. This will be up around 450-500MHz. However, there are much better ways to analyse the circuit on a simulator that will give more accurate results.

But the open loop analysis is probably the most intuitive for most people. The plot below shows > unity gain and zero phase around the loop at 460MHz. But this type of analysis is very dodgy if it means extracting component strays into the open loop and it's better to analyse the circuit in closed loop with a proper s2p model of the JFET and treat it as a negative resistance circuit. The impact of adding the 100R stopper resistor in the drain can then be analysed with a fair degree of confidence in the results.
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Old 16th Jun 2018, 10:35 pm   #37
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Default Re: Cathode follower anode resistor.

True, the anode/collector is a less powerful place for a stopper than the emitter, but then adding a resistor to the cathode/emitter has a very much stronger effect on the circuit's intended operation.

Also the effectiveness in the anode/collector seems greater with higher bandwidth RF transistors than with audio types.

Incidentally, some very low phase noise oscillators are made with low noise audio type transistors. It seems that some RFy devices are rather noisy at low frequencies and this acts to modulate the oscillation..

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Old 17th Jun 2018, 12:29 am   #38
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Default Re: Cathode follower anode resistor.

Yes, I think you have to treat every design case as an individual, even if it just involves changing the operating point of the same device. By the way, I did find that article by DL Feucht to be quite interesting. The C to -R and L to R (emitter to base) impedance transforming effects are well known and adding some emitter/source degeneration in the form of inductance is a known way to add resistance at the input of an LNA. However, it's useful to actually see and play with the individual equations for the transforms. Thanks for posting this up

Yesterday, I had a go at building the J310 oscillator and I did it under a microscope using SMD parts to minimise any stray inductance. But this is very hard to do so in order to make a fair comparison I added some stray inductance to the VCCS based model I used for the open loop analysis.

See below for an image of the real oscillator and this uses an SST310 in SOT23 and there is also an image of where it oscillates on a spectrum analyser (about 450MHz).

There is also a simulation comparison of the capacitance and negative resistance seen looking back into the gate of the JFET for the VCCS model and the real circuit when measured with a VNA. I used a Coilcraft 27nH inductor in the real circuit and this measured 28nH at 450MHz on my VNA.

However, the simulation is (temporarily) without the 28nH inductor so I'm looking for negative resistance in series with a capacitance with a view to resonating this with 28nH. The real VNA 1 port measurement into the gate of the SST310 circuit is represented by the little SP1 (1 port) symbol on port 2 in the simulation. I had to include the little pink inductor of -2nH to cancel the approx. 2nH stray inductance in the VNA connection.

But you can see that the data all agrees quite closely. The only disappointing thing was that the real JFET only managed to generate about -6.5 ohms of negative resistance at 450MHz and not the expected -10 ohms predicted by the model. But it got the capacitance OK at about 4.6pF.

So when the shunt 28nH is added the circuit can be predicted to oscillate at the resonance of 4.6pF and 28nH = about 445MHz. The spectrum analyser shows that this is a very good prediction!

If anyone wants to see the simulation plot in more detail then I've attached it in a word doc below:
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Old 17th Jun 2018, 1:39 am   #39
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Default Re: Cathode follower anode resistor.

To show the benefit of adding a 100R in the drain I added this to the datasheet based VCCS model and also the real circuit on the PCB. I used a small 0805 100R chip resistor for the real resistor as this has low stray inductance and capacitance.

I then remeasured with the VNA looking into the gate and also simulated the result with the VCCS model and the 100R drain resistor. The simulator result and the VNA result of the real circuit is as the plot below.

This shows that the 100R now protects down to about 260MHz as this is where the resistance turns negative again. So to form an unwelcome oscillator this would require an 82nH inductor to resonate with 4.7pF and the plot below shows there should be a few ohms of negative resistance here. This needs to be enough to offset the ESR of the 82nH inductor. Sadly I could only find a regular Coilcraft SMD 82nH and not one of their high Q inductors and these are a bit lossy.

But see below for the plot and when I tried fitting the 82nH inductor it would only oscillate briefly. I had to turn up the supply from 12V to 13.5V to keep it running. It oscillated at 259MHz and this agrees with the prediction. But it is right on the oscillation limit just as the simulation predicted.

So this proves that the 100R adds enough protection to prevent negative resistance down to about 260MHz. Before it went up around 500-600MHz before the resistance at the gate turned positive. It's much less likely that a board layout would have this much (>82nH) stray inductance so this circuit is a lot safer now I think.

The dark traces are the VCCS model and the paler traces are the real SST310 JFET circuit. You can see that the model still generates slightly more negative resistance than the real circuit but it's still a reasonable agreement considering this is a bit of a hasty lashup PCB and a crude VCCS model.

Are you all bored yet? I'm beginning to wane a bit and I'm off to bed I think
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Old 17th Jun 2018, 7:53 am   #40
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Default Re: Cathode follower anode resistor.

Well, now you're getting well into UHF oscillators, how about a wild one?

Have a look for a patent by Bart McJunkin and his 'cartwheel' oscillator. It amounts to a vert short length (0.062 inches) of very low Zo coax, resonated with an awful lot of paralleled varactors.

They work very well indeed.

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