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Old 18th November 2012, 08:31 PM   #21
Elvee is offline Elvee  Belgium
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If we cheat and use a bipolar cascode, matters improve: the offset is apparently down (but in sim only, in reality the N-P matching issue would remain), and the THD is down too, but it doesn't reach the level of the "optimum" version, where FETs are "helped" by bipolars.

In summary, the optimum number of FETs for this buffer is approximately 0
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Old 18th November 2012, 08:34 PM   #22
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Quote:
Originally Posted by kenpeter View Post
All I was sayin'...
Neat and convenient. Even luxurious: only pairs require matching, not quadruplets
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Old 19th November 2012, 01:59 AM   #23
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Emitter input don't need to match HFE, only VBE and temp...
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Old 20th November 2012, 03:46 PM   #24
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And finally, a jFET input MOS output version.

Not very good, predictably, but here the FETs used are not true complements, they are rather approximate.
In a conventional topology, this would have a catastrophic impact on the DC offset and the even order harmonics, but here the effect is minimal, showing the superiority of this scheme.
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Old 20th November 2012, 04:41 PM   #25
Struth is offline Struth  Canada
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Hi Guys

The circuits in the Japanese link are all optimised for voltage gain as complete amplifiers. Operating them as unity-gain buffers requires some redesign. However, you are still left with all the problems of sourcing and matching complimentary jfets.

Elvee's circuit is best built with a BJT front end, as his numbers clearly show. Like nearly all other complimentary symmetry circuits, some degree of matching of the upper and lower devices will ensure a good THD profile and reasonable real-world DC offset without servos. Emitter degeneration - something Elvee dislikes - makes the circuit more tolerant of device parameter spreads at the expense of some RF performance - distinctly not all RF performance.

A DC path to ground at the input and output of this circuit assists in controlling DC offset. The absence of such parts is a common historical error in opamp app notes and most direct-coupled circuits.

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Old 21st November 2012, 04:32 PM   #26
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Struth View Post
Emitter degeneration - something Elvee dislikes - makes the circuit more tolerant of device parameter spreads at the expense of some RF performance - distinctly not all RF performance.
Degeneration does what it says on the tin: it results in degenerated circuits, ie nice and pure exponentials become ugly transcendental equations, making later compensations impossible.
They are sometimes a necessary evil, but they should be used as sparingly as practical.
It is not so much the RF performance that is impacted, frequently the opposite, but rather the accuracy and linearity.

If RF stability is a problem, ferrite beads are a better fix.

Here is the "double-barreled" version of the buffer.

Surprisingly (and disappointingly), it does not result in better performances, the opposite in fact.
In addition, it also requires intersex matching and thus seems (for the moment) to offer no advantage, except a very marginally reduced BOM
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Old 21st November 2012, 11:57 PM   #27
Struth is offline Struth  Canada
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Hi Guys

Emitter degeneration - a form of local feedback - increases nonlinearity. You are a very funny guy!

Sorry, I forgot that this thread is an expose not a discussion.

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Old 22nd November 2012, 07:43 AM   #28
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Struth View Post
Hi Guys

Emitter degeneration - a form of local feedback - increases nonlinearity. You are a very funny guy!
You are right, in simple cases of isolated stages, that is what it apparently does.
But to attain 1ppm linearity with such a method, the degeneration has to be much larger than the junction's dynamic resistance.
For a 1mA excursion of emitter current, this would require an emitter resistor in excess of 26 Meg, not really practical.

Early transistor circuits used that kind of method to achieve an acceptable linearity, but it required a high supply voltage, and there is a limit to what can be achieved.
For example, I have an old W&G PS-6 frequency synthetizer of the sixties having a general supply voltage of ~75VDC

In translinear and related circuits, with loops of junctions, the exponential I-V relationship is essential in attaining high levels of linearity.
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Old 26th November 2012, 05:22 PM   #29
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Here is an alternative compensation scheme, adding some feedforward components to the mix.

The buffer now tolerates in excess of 330nF direct output capacitance.
The cost is a somewhat increased THD level, from 40ppb to 90ppb, but it is still tolerable.

Also shown is the effect of a realistic source impedance (500 ohm) on the performance.
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Old 26th November 2012, 05:46 PM   #30
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I'm a bit nervous about what I perceive to be a requirement of exponential junctions matching up to eliminate distortions - IF indeed I am perceiving this correctly ?
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