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Old 20th June 2013, 05:55 AM   #3601
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Default cascode amp from the book

Hi Bob
I was thinking about this example of cascode power output stage, I cannot calm it
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Old 20th June 2013, 06:24 AM   #3602
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It is on the page 109 fig 5.8 stacked output stage design
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Old 20th June 2013, 08:17 AM   #3603
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padamiecki,

If it is your intent that we take the effort to read your schematic, please, make it more readable.

Cheers, E.
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Old 20th June 2013, 10:35 AM   #3604
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Quote:
Originally Posted by padamiecki View Post
Hi Bob
I was thinking about this example of cascode power output stage, I cannot calm it
Hi Pawel,

I am on my way to a mini-vacation, so I may not be able to respond for a few days.

Cheers,
Bob
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Old 20th June 2013, 10:42 AM   #3605
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my intention was to send almost pm to Bob
but dear Edmond, if you could suggest how fight the oscillation...
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Old 20th June 2013, 11:15 AM   #3606
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Hi Pavel, this one looks much better. Thanks!
As for oscillations, hard to say beforehand what might cause it. Maybe the collector resistors (200R & 470R) are the culprits. I would start with Bob's example and then going to refine it. Success!

Cheers, E.
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goed verliezen dan dooft het licht…(H.M. van Randwijk)
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Old 20th June 2013, 04:58 PM   #3607
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Quote:
Originally Posted by padamiecki View Post
if you could suggest how fight the oscillation...
If randomly chosen miller caps don't work, try randomly chosen RCs with an Fc below that of your oscillation frequency. I found this stopped oscillation. More RCs might improve phase linearity.
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Old 21st June 2013, 12:48 AM   #3608
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Quote:
Originally Posted by padamiecki View Post
but dear Edmond, if you could suggest how fight the oscillation...
Pavel, what you show is a theoretical circuit with ideal current & voltage sources.

But everything else that connects to the circuit also affects stability.

If you want sensible advice, you need to show us the real thing including details of earthing and decoupling.
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Old 24th June 2013, 01:02 AM   #3609
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I just found this really great paper on ferrites and RFI protection!

Quote:
So far, we’ve talked only about ferrites, but there are other very reliable system installation
techniques that are well known to minimize RFI. They are:

1) Never use parallel wire (zip cord) for loudspeaker wiring. Always use twisted pair
cable for loudspeaker wiring. Why? It’s quite common for audio output stages to
lack the low-pass filters needed to reject RF, so any RF picked up by the loud-
speaker wiring will be coupled back to a driver stage via the feedback loop, where it
will then be detected and amplified. And, of course, it is well known that a twisted
pair provides good rejection of magnetic and electric fields, but we tend to forget
that this still applies at radio frequencies.

2) Avoid the use of shielded cables with a drain wire if the system will be exposed to
strong RF below 10 MHz (AM broadcast, ham transmitters, light dimmers). Any ca-
ble with a drain wire will have significantly greater SCIN below about 10 MHz than
a cable that uses a good braid shield. On the other hand, foil-shielded cables pro-
vide better shielding above 20 MHz. The best cable, if you can find it, is one that
has both a foil and braid shield. Second best are those with very dense braid
shields.
Both of these cable mechanisms are quite powerful -- simply switching from zip
cord to twisted pair for loudspeaker wiring, or from foil/drain cable to a good braid
cable, can easily reduce RFI by 20-30 dB if that is how the RF is getting in.

3) We know that pin 1 problems are a common cause of RFI, so fixing them is always
worth the trouble. The best fix is one that disconnects the cable shield from the
connector pin that goes to the circuit board, and connects the cable shield instead
to the equipment’s shielding enclosure. This is particularly important with VHF and
UHF interference sources. Don’t forget that pin 1 problems can occur with unbal-
anced inputs and outputs too – if the equipment has RCA and/or 1⁄4” connectors
mounted to a circuit board, you can bet your lunch that it’s got a pin 1 problem!
Vol 31 #2 and #3 of the SynAudCon Newsletter has a detailed discussion of Pin 1.
Unfortunately, lots of equipment with pin 1 problems is designed in a manner that
makes it difficult to fix them. That’s where ferrite chokes save our bacon!

4) There’s a tendency on the part of many audio equipment manufacturers to design
excessive bandwidth into their products. This happens for two reasons. First, it
costs a few extra dimes to include the components needed to limit the bandwidth.
Second, some (many?) misguided souls think they can improve audio sound quality
by extending the bandwidth into the MHz range, which in turn can improve the
phase response at very high audio frequencies. While flat phase response is a won-
derful thing, extending the bandwidth of an audio system’s inputs and outputs to
100 kHz should achieve that objective with even the simplest of filters, and extend-
ing it beyond about 200 kHz almost guarantees interference from nearby AM
broadcast stations. The use of more sophisticated filter topologies allow sharper
cutoffs with minimal phase shift in the passband.
Using braid-shielded audio cable greatly reduces the likelihood that equipment like
this will see enough RF for audible detection to occur, but if additional help is
needed, a good input transformer with a Faraday shield (Jensen and Lundahl are the
good brands) acts as an effective low pass filter to block the RF.

5) RF interference often enters equipment and systems by more than one path. You
may eliminate or reduce the interference coupled into one path, but not achieve the
full elimination. Always suspect more than one path, especially with interference
that is especially strong or persistent. Continue implementing all of the “right” tech-
niques throughout the system, even when the first things you do don’t seem to be
accomplishing much. One dominant path may be “swamping” the weaker ones
you are fixing. Eventually you’ll find the dominant one.
http://audiosystemsgroup.com/SAC0305Ferrites.pdf
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Old 24th June 2013, 07:55 AM   #3610
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by keantoken View Post
If you take a diamond and EF driver pair in an FET output stage and bias them at the same current, it will take the same level of loading to send each into class B. So I don't consider a Diamond stage "weak" when used with FET output stages and a large enough gate bridge cap. The gate currents may be asymmetrical, but this shouldn't matter at audio where the drivers should never enter class B.
When you say large enough, what sort of size are you looking at? I have read large caps in a circuit of this type are risky as to what effect they would have on sound (Assuming I'm correctly understanding the term bridge cap to be otherwise known as a speed up cap).
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