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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 17th Jun 2018, 7:57 am   #41
Diabolical Artificer
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Default Re: Cathode follower anode resistor.

" ....to people whose maths is only at MBA level " my playschool level maths makes reading analysis of circuits using lots of formulae hard and I have no idea how to use a spreadsheet, but will try to follow along.

Regarding cathode followers, in amplifiers they are often used to drive difficult loads that can have a capacitive or inductive element like OP valve grids and reverb tanks, hence my original question. I've had no trouble so far though when using cathode followers.

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Old 17th Jun 2018, 8:49 am   #42
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Default Re: Cathode follower anode resistor.

Sorry, Andy, you missed the joke. G-J got it, though. It's a background thing

It was a swipe at managers who can't understand what you are doing, but add up all the costs and want everything justifying. It's very difficult to justify something to someone with no background and no interest in what it does.

David
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Old 17th Jun 2018, 9:13 am   #43
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Default Re: Cathode follower anode resistor.

MBA = Master of Business Administration.
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Old 17th Jun 2018, 10:18 am   #44
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Default Re: Cathode follower anode resistor.

Quote:
Originally Posted by G0HZU_JMR View Post
This can alienate many readers who might think you 'need' to understand the maths in order to extract value from the paper.
I agree that you don't need to understand the maths to get value from an article but you do need to understand the maths to appreciate the whole argument. As I have said I took the Wireless World for 30 years and although I didn't understand the maths in some of the articles I still got a lot from them.

Quote:
Originally Posted by julie_m View Post
If anybody ever doubted the value in words of a picture, they need only refer to the above!
Yes looking at those diagrams that kalee20 has drawn is completely within my comfort zone in terms of understanding what he is trying to say.
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Old 17th Jun 2018, 12:53 pm   #45
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Default Re: Cathode follower anode resistor.

Note that the circuit might not be as simplistic as a simple R and small C in the emitter. Doing my best Tom Jones impersonation "It's not unusual" for the small capacitance at the source/emitter of a transistor to look like a short circuit up at UHF. This might be due to a resonance with some wire or the equivalent.

When this happens the circuit can still oscillate up at UHF but the feedback path will be quite different. With a shorted source pin (at UHF) the Colpitts equivalent circuit we have been considering up till now ceases to function because the capacitive tap is now grounded and the feedback path is killed.

When this happens, it's the drain to gate capacitance that can become the bad guy that forms the critical feedback path. This alternative oscillator feedback is actually easier to model and understand.

Of course, it will get very muddy if you end up halfway between the two cases. The feedback path becomes very difficult to visualise and it's best to try and model it on a simulator.

The circuit below looks very similar but because the source is shorted to ground at UHF the internal feedback path is quite different. Fortunately, adding a series 100R resistor in the drain kills this mode as well.
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Old 17th Jun 2018, 12:56 pm   #46
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Quote:
Originally Posted by Radio Wrangler View Post
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.

David
I'll have a look for that, I've never hear of it before. In the past I've done research work into UHF/microwave oscillators that used defected ground as part of the resonator and feedback path. The idea was to achieve low microphony and a low parts count.
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Old 17th Jun 2018, 1:25 pm   #47
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Default Re: Cathode follower anode resistor.

Barton L McJunkin

David
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Old 18th Jun 2018, 11:24 am   #48
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Default Re: Cathode follower anode resistor.

Quote:
Originally Posted by ukcol View Post
Quote:
Originally Posted by julie_m View Post
If anybody ever doubted the value in words of a picture, they need only refer to the above!
Yes looking at those diagrams that kalee20 has drawn is completely within my comfort zone in terms of understanding what he is trying to say.
Thanks - except that he drew them in such a hurry that the cathode follower is labelled as emitter follower!
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