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Old 1st July 2011, 08:55 PM   #2061
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Quote:
Originally Posted by WuYit View Post
Bob,
I see the Vbe multiplier as a disturbance in the VAS load (a nonlinear element). In this way, no need for an in itself aggravating bypass capacitor to improve high frequency distortion and transient response.
Another problem with the triple (when composed of devices with the same polarity) is an alarming rise in saturation voltage.
Hi WuYit,

There lurks nonlinearity in every part of a circuit that includes a semiconductor, so you may be technically correct. However, the key in engineering is understanding the relative size of the contribution compared to other similar contributions elsewhere. That is one of the reasons we do simulation, but simulation is not the only approach to sorting things out.

So, have you simulated the contribution of Vbe multiplier nonlinearity to that of an amplifier?

The rise in saturation voltage caused by an EF Triple is one Vbe. This is a reasonable price to pay for the far better performance. The output BJTs don't perform all that well when they get really close to the rail anyway.

Cheers,
Bob
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Old 2nd July 2011, 01:03 PM   #2062
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Originally Posted by CBS240 View Post
I don't know how many times I've tried to lead people in this direction. I was able to use as little as 150R gate stoppers with the planer stripe mosfets; Cgd could reach over 2nf at Vds saturation!

BTW I can amplify at fairly low distortion over 400KHz with that amp. I attribute this mostly to the extra speed of the output stage due to more effecient filtering/damping of the perceived occilators within the mosfet.
Hi CBS,

These are good points you make about gate stoppers and oscillations. The key is to find the most effective way to kill the Q of parasitic resonances while minimizing the reduction in high-frequency performance.

Cheers,
Bob
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Old 2nd July 2011, 01:38 PM   #2063
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You can also play around with the values of gate stoppers as Troy Huebner demonstrated in National Semi Ap Note 1645.
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Old 2nd July 2011, 07:18 PM   #2064
WuYit is offline WuYit  Sweden
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Howdy,
Quote:
I have found that the decoupling of the drivers/ccs and the choice of CCS is what gives stability in THIS triple. The CCS and what TYPE of CCS makes all the difference, as it does nearly all the work in the predriver stage. A 2Q CCS was not as stable as the red led version - too much gain. I'm sure with basestoppers on a 2Q I would be golden - but I like the warm glow of the 2 LED's .
Without a base stopper the LED CCS is not stable either.
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Old 2nd July 2011, 08:48 PM   #2065
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Originally Posted by WuYit View Post
Howdy,

Without a base stopper the LED CCS is not stable either.
False ! Would "not stable" refer to temperature or current ? Too broad of a statement to be valid. An LED CCS has a lower output impedance and is not as accurate (current wise) , so "less" stable would describe it. Temperature wise , the LED does well , compensating for the transistor's Vbe.

This leaves outright oscillation , very improbable as the only feedback is Re. I have never seen a LED CSS do this , I can't say the same for a 2Q CCS.

OS
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Old 2nd July 2011, 09:31 PM   #2066
WuYit is offline WuYit  Sweden
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Both the LED and the capacitor are a very low impedance source causing some level of oscillation.
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Old 2nd July 2011, 09:46 PM   #2067
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Bob Cordell View Post
Hi wahab,



MOSFETs are more friendly in some ways, and I like MOSFETs. However, they are often more prone to HF oscillations, as pointed out in my book. They are fundamentally faster than power output BJTs, and this is part of the reason for an increased tendency to HF oscillation. Indeed, much larger gate stopper resistors are usually employed with MOSFETs, even when they are not paralleled. We sometimes see 470-ohm gate stoppers with laterals!

In my MOSFET power amplifier with error correction design, I was able to use gate stoppers as small as 47 ohms with verticals, but my use of gate Zobel networks permitted that.

Cheers,
Bob
Hi , Bob

Dont know exactly about the Hexfets , but for Hitachi laterals , unless
the layout or the design or even both are badly executed, values
as low as 10R can be implemented, as done by Hitachi themselves
in a commercial amp.

Personnaly, i use three pairs of Hitachi s lateral in most of my amps,
with 100R gate stoppers , but i admit that i did choose such a value
more by some cautious and also gregarious reflexe , since it s generaly
the lowest value used in many designs.

Indeed, increasing the base stoppers value also dramatically reduce
phase margin past a relatively low value that is well below the designers
habits with laterals fets.


cheers,
w
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Old 2nd July 2011, 09:48 PM   #2068
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So just about all the DIYA AB amps oscillate ? 90% use either diode or led referenced CCS's without a stopper. As I said , I have NEVER seen one oscillate using a 20mhz CRO. Could this be happening at 50+ Mhz ? Many OEM's do the same thing.

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Old 2nd July 2011, 10:33 PM   #2069
WuYit is offline WuYit  Sweden
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Quote:
So just about all the DIYA AB amps oscillate ?
Very likely, for more than one reason.
Quote:
90% use either diode or led referenced CCS's without a stopper.
A bad and regrettable practice. Do base stoppers complicate things considerably?
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Old 2nd July 2011, 10:57 PM   #2070
wahab is offline wahab  Algeria
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Originally Posted by WuYit View Post

Do base stoppers complicate things considerably?
WuYit, with a CCS , a base stopper is counterproductive...
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