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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 9th Jun 2018, 6:51 am   #1
Diabolical Artificer
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Default Cathode follower anode resistor.

Just been doing some work on an old scope. In the vertical amp they use cathode followers to drive the Y plates but instead of taking the anode straight to HT, they have a 100R resistor from HT to anode.

Apart from said resistor dropping the voltage a tad, what would be the advantage if any here?

Andy.
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Old 9th Jun 2018, 7:18 am   #2
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Default Re: Cathode follower anode resistor.

Perhaps the Y amplifier produced a touch of overshoot/ringing on a fast rectangular wave and the 100R damped it sufficiently to tidy it up.

Martin
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Old 9th Jun 2018, 7:22 am   #3
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Default Re: Cathode follower anode resistor.

Followers, whether cathode followers or emitter followers have an instability gotcha. it gets worse if it's driving a capacitive load (Scope deflection plates!) and it gets worse if the anode/collector is well decoupled at RF.

If you look at the stray capacitances between electrodes and the inductances of the connections, you can rearrange things and find you have a Colpitts oscillator circuit made out of the strays, the decouplers and the capacitive load. This is where spurious oscillations at VHF come from.

The resistor in the anode spoils the gain of the oscillator and tames it. It's a more powerful technique for a valve than sticking a resistor in the grid connection but it can waste some signal power and some DC power. In this case it won't be significant.

An alternative way of looking at the impromptu oscillator is that the inductance of a decoupling connection is rotated (active filter gyrator style) into something which looks like a negative resistance, and that can cancel the losses of any stray resonance and turn it into an oscillator.

This vulnerability is used. Many UHF oscillators use the "unstable emitter follower" circuit and it foxes people because there are no visible feedback paths.

You can look at a circuit and from the measures to prevent spurious oscillations, you can form an opinion of the experience level of the designer (unless a good designer got overruled by the accountants).

David

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Old 9th Jun 2018, 10:29 am   #4
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Default Re: Cathode follower anode resistor.

That is very interesting David. Speaking as someone who has spent a working life servicing equipment rather than designing it, statements like "....and it gets worse if the anode/collector is well decoupled at RF" seem counter intuitive.

Perhaps I can prevail upon you to expand your explanation of how a follower can become a Colpitts oscillator. It would be particularly helpful, to me at least, if you could show the paths in (equivalent) circuit form.
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Old 9th Jun 2018, 10:45 am   #5
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Default Re: Cathode follower anode resistor.

Damping cathode-followers can get tricky - I have memories of one which 'rang' spectacularly at HF when feeding down a length of coax into a load which had a strange reactance at a frequency well away from what we were intentionally working with. That one took some finding!

The anode-resistor seems to do the thing: it's sometimes specified as a *wirewound* component too, in order to make use of its hidden-but-very-lossy inductance.
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Old 9th Jun 2018, 11:18 am   #6
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Default Re: Cathode follower anode resistor.

Everything RW says is correct. There is a very simple way to look at this too;

One limiting factor in a triode's frequency response is the grid-plate (Miller) capacitance. Excursions of plate voltage (which are out of phase with the grid) provide negative feedback to the grid which increases as the frequency increases, lowering the upper frequency response. One idea of fixing the plate voltage (as in a cathode follower or in the lower valve of a cascode circuit) kills this negative feedback.

If the plate voltage is allowed a tiny amount of wriggle room, with a small resistor, some frequency dependent negative feedback persists and this affects the signal regardless whether it it taken from the plate or the cathode.

And of course if reactive elements are in the plate wiring (for example the plate wiring inductance) any resonance of this with the plate capacitance is damped by the resistor.

So the stability of the cathode follower stage at very high frequencies is improved with the 100R plate resistor, but it is not large enough to significantly affect (roll off) the upper frequency response of the stage for the application, but it might be if the value was too high.
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Old 9th Jun 2018, 11:22 am   #7
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Default Re: Cathode follower anode resistor.

The 100 ohm resistor in series with the anode trick was used in some old tellies as well.

Lawrence.
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Old 9th Jun 2018, 8:29 pm   #8
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Default Re: Cathode follower anode resistor.

Colin, the instability of emitter followers and cathode followers is quite well known, but not very well documented in text books. Some analyses crop up in the field of oscillator design.

I've been trying to find something a bit more approachable, I did find this which goes a little into the issue of gyration creating negative resistance. The action in valves is similar. It was a known problem in the valve era, but serious analysis came in the transistor era.

http://audioworkshop.org/downloads/A...T_CIRCUITS.pdf

David
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Old 9th Jun 2018, 9:51 pm   #9
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Default Re: Cathode follower anode resistor.

It's definitely a 'gotcha!'

Any emitter-follower I use, if the base is driven from a capacitor-to-ground, I make sure I allow for a series base resistor, just in case. I've seen one case of instability - 'scoping on the emitter, the voltage looks about right but the scope trace is thickened, due to a few hundred mV of oscillation at VHF (at least).
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Old 9th Jun 2018, 11:00 pm   #10
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Default Re: Cathode follower anode resistor.

Some designers seem much more cautions about unwanted parasitic oscillations. I've seen some examples where there are 100R resistors everywhere on inputs and outputs of discrete stages and OP amps, to the extent where you could wonder if they were just using them for pcb links, until it is realized that it very much helps the RFI/EMI immunity of the whole assembly if its blasted with RF and it also kills the opportunity for various modes of parasitic oscillation in a stage too.

In the Conrac Video monitor I recently worked on, in the vertical amplifier and other places there were many 100R resistors, which one would not normally see in a domestic TV or monitor. Presumably the circuit, being for a military applications, had to be relatively immune from RFI and not produce RFI. The vertical deflection circuits are on page 8 to see these 100R resistors, typically in the transistor bases as Kalee20 points out:

http://worldphaco.com/uploads/The_19...o_monitor..pdf

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Old 10th Jun 2018, 6:00 am   #11
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Default Re: Cathode follower anode resistor.

Most semiconductor makers pepper their datasheets with weasley words like "We have a policy of continued product improvement and specifications are liable to be changed without notice. So there. Yah boo sucks!"

Consequently, the seasoned designer (= well bitten) knows to apply thorough damping, knowing that future years will bring much more frolicsome devices.

Once upon a long time ago, there was a little oscillator chip called MC1648. It was part of Motorola's MECL III logic series. It could go up to about 200MHz and got used in all sorts of things. Motorola improved their processing for later MECL families, but kept the old 1648 on. It performed a valuable function. It may have had a few die shrinks along the way. The bipolar transistors making up the 1648 became much livelier. and later production parts fitted to old PCBs started to think the old resonator tanks looked like decouplers, and that the tracks to the tanks looked like much more interesting resonators. Result: oscillation at a frequency set by the track lengths! People started desperately seeking old stocks of MC1648s, ones which were not so fruity.

Valves got improved a little over time, but by nothing like the factors semiconductors got hit by.

David
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Old 10th Jun 2018, 6:49 am   #12
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Default Re: Cathode follower anode resistor.

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Originally Posted by Radio Wrangler View Post
Once upon a long time ago, there was a little oscillator chip called MC1648. It was part of Motorola's MECL III logic series. It could go up to about 200MHz and got used in all sorts of things. Motorola improved their processing for later MECL families, but kept the old 1648 on. It performed a valuable function. It may have had a few die shrinks along the way. The bipolar transistors making up the 1648 became much livelier. and later production parts fitted to old PCBs started to think the old resonator tanks looked like decouplers, and that the tracks to the tanks looked like much more interesting resonators.
Ha ! I wish you had told me this a year ago. I attempted to use this IC as the 16MHz VCO in my PAL freeze frame machine. I had all kinds of trouble with it and in the end gave up. Instead I made an Xtal oscillator with a varicap diode and a 74HCU04 buffer IC and had no troubles.
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Old 10th Jun 2018, 7:03 am   #13
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Default Re: Cathode follower anode resistor.

Sorry, Hugo.

I did mention it in the oscillators and synthesisers chapter of the ARRL handbook for the 1995 edition onwards. I think it's still there in current versions. It's a rather nasty gotcha.

David
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Old 10th Jun 2018, 8:24 am   #14
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Default Re: Cathode follower anode resistor.

Oscillation, I should have thought of that, it's a pretty standard fix mentioned in valve datasheets often put on the anodes of OP stages to stop oscillation.

The cathode follower in question is direct coupled in this circuit and also has a 100r resistor on the grid. The same resistors are used quite often in the timebase too, they are bog standard carbon comp.

"If the plate voltage is allowed a tiny amount of wriggle room, with a small resistor, some frequency dependent negative feedback persists and this affects the signal regardless whether it it taken from the plate or the cathode." Does it improve frequency response at AF frequencies or only at VHF?

Andy.
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Old 10th Jun 2018, 8:46 am   #15
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Default Re: Cathode follower anode resistor.

It worsens it a little. The current in the anode-grid capacitance opposes the drive to the grid at higher frequencies, and the reduced grid swing reduces the cathode swing

David
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Old 10th Jun 2018, 9:37 am   #16
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Default Re: Cathode follower anode resistor.

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I did mention it in the oscillators and synthesisers chapter of the ARRL handbook for the 1995 edition onwards.
Mine is the 1994 edition !
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Old 10th Jun 2018, 11:16 am   #17
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Default Re: Cathode follower anode resistor.

That qualifies as a near-miss, then. I'm not on royalties, but I suspect I'd be in breach of a forum rule if not etiquette if I even suggested an update. Or I could bung you one of my old ones in the post. Wonder how long it'd take?

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Old 10th Jun 2018, 11:59 am   #18
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Default Re: Cathode follower anode resistor.

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I'm not on royalties, .... I could bung you one of my old ones in the post. Wonder how long it'd take?
Well after that I think you should be on royalties. Thanks for the kind offer of the updated one, but probably the post would be prohibitive.

On the anode resistor topic, a handy place to insert the resistor for octal based audio output valves, say an EL33 is on the socket from pin 3 (the anode) to pin 6 (N/P). And since pin 4, G2 (screen grid) is connected to HT normally, the wires to the output transformer primary connect to pins 4 & 6. Very convenient. The Gotcha here is don't use pin 1 labelled as N/C for anything other than ground, as its connected to the metallisation on valves like the EL33M or metal tubes, so if you use that for the anode resistor tag, HT potential is on the metallisation and its not very smart.
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Old 13th Jun 2018, 9:18 pm   #19
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Default Re: Cathode follower anode resistor.

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Originally Posted by Radio Wrangler View Post
Colin, the instability of emitter followers and cathode followers is quite well known, but not very well documented in text books. Some analyses crop up in the field of oscillator design.

I've been trying to find something a bit more approachable, I did find this which goes a little into the issue of gyration creating negative resistance. The action in valves is similar. It was a known problem in the valve era, but serious analysis came in the transistor era.

http://audioworkshop.org/downloads/A...T_CIRCUITS.pdf

David
Hi David,

Thank you for providing the link to that article. I'm afraid I haven't been able to give it the attention it deserves yet. I've been helping my elder son with some work in his house and I've only managed a quick read through of the article. It contains concepts quite alien to me and I will need to study it rather than just read it through. I will get to do that and will let you know how I get on and to what extent I understand it (or not ).
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Old 14th Jun 2018, 4:46 pm   #20
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Default Re: Cathode follower anode resistor.

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..

I did get a little bit out of Part 1 however. I already understood that real components are not ideal, for example a conductor will have a small amount inductance and a transistor will have capacitance between its electrodes but Feucht goes on to explain that in the right circumstances this unintended inductances and capacitances can lead to resonance and oscillation at that resonant frequency.

I hope I don't sound ungrateful at your attempt to help me understand this problem David, because I am not, and I certainly don't want anyone to be discouraged from trying to widen my knowledge (such as it is) of this fascinating subject.
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