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Old 18th July 2002, 05:52 AM   #1
mirlo is offline mirlo  United States
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Default Linearized Source Follower

Has anyone tried anything like this, to linearize a class-A source follower by indirectly controlling its Vgs by controlling the current in the device?
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Old 18th July 2002, 09:49 AM   #2
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An old, old circuit from Wireless World. It doesn't work the way you think it does. The top device is running class AB. On positive waveforms it is driven further into enhancement and the voltage drop across its drain resistor shuts off the current source. As the waveform drives negative the falling current through the drain resistor makes the current source kick in and deliver the negative portion of the waveform. It will work better with BJTs than FETs. you will need lots of loop feedback to overcome the Vgs error. Even though it is a dynamic bias scheme, it would probably work best with FETs when the idling current is set to about half what you would normally set a single ended class A stage to. In other words, bias it like a push-pull class A stage.
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Old 18th July 2002, 10:18 AM   #3
AKSA is offline AKSA  Australia
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Mirlo,

This circuit is interesting, and rather like a Single Ended Push Pull I am presently considering with sliding bias.

However, here's another which more simply achieves the linearising effect you refer to.

It is not original, Self referred to it in a Class AB PP topology in the late eighties.

However, this is a single ended version, and it is very linear. It sounds absolutely wonderful running at around 3A from 50V, and will drive an 8R speaker to around 42Vpp, which is 28W which astonishing impact. I developed an amp using refined versions of this circuit and it is probably my best amp, but oh! the inefficiency is crippling........

There is an asymmetrical distortion (H2, in fact) introduced by the varying Vbe with signal excursion; when Vce is large during the negative half cycle, the Vbe is small, and the step function tends to squash the waveform; alternately sharpening it on the positive half cycle. But the effect is nothing like as pronounced as a straight source follower, and the output could reasonably be called a step function of the input.

Stability can be an issue. A 220R gate resistor is mandatory, and sometimes a 10R resistor inserted into the emitter of the bipolar is required. It is also sensible to decouple the power supply for the resistor from that supplying the mosfet; the two can interact causing intermodulation problems.

Cheers,


Hugh

www.printedelectronics.com
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Old 18th July 2002, 10:48 AM   #4
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Default This is probably the best way to go

Use FET where FET has its advantages
and bipolar where it is superior.

By the division into dogmatic people
of 2 fighting groups,
this approach is rarely seen.

A Happy Symbios! AKSA "The non-dogmatic"
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Old 18th July 2002, 11:04 AM   #5
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Default AKSA - LINKPAGE

Not many links, but......

"The Best"

http://www.printedelectronics.com/pe...indexlinks.htm
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Old 18th July 2002, 11:11 AM   #6
mirlo is offline mirlo  United States
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No, it's not original -- it is essentially a folded version of the "hybrid-Sziklai" (?) circuit you drew, AKSA.

Just out of curiosity, how much current do you run in the NPN of your circuit?

The folded version will have more loop gain, because the PNP will buffer the Vgs of the lower NMOS, but then it also probably has some worse stability issues for that very reason.

I think it won't need the power supply decoupling you mention because of that buffering.

As for efficiency -- that's a problem. It is probably going to be worse than the usual class-A output stage. And maybe thus relegated to things like linestage output or headphone amp.

To solve the efficiency problem, maybe it can be run class-B by paralleling it with the complementary PFET / NPN and

On the other hand, being (almost) all N, it might make a nifty tube circuit...

I'll post some results after I simulate it a bit (I just downloaded Switchercad-III) and if it looks promising in the simulator then try to build some version of it.
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Old 18th July 2002, 11:18 AM   #7
mirlo is offline mirlo  United States
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Default I meant to say ..

To solve the efficiency problem, maybe it can be run class-B by parallelling it with the complementary PFET / NPN and reducing the controlled current in the (now complementary) source followers, letting the bulk of current handling be taken care of by the current sources.

One thing I never quite understood about MOSFET amplifiers is the gate resistor thing. If I drive the gates from a high impedance, what good do they do? My naive thought is that the gates are driven from high impedance, they do nothing, and if they are voltage driven then gate resistors would actually degrade stability by introducing another pole in the system.
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Old 18th July 2002, 11:28 AM   #8
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Default Linearized etc

Aksa,

The original circuit reminds me of Nelson Pass' Active Current Source loading patent. Are you familiar with that?

Jan Didden
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Old 18th July 2002, 12:00 PM   #9
AKSA is offline AKSA  Australia
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Default The Pass Patent

Hi Jan,

No, I have not read the Pass patent. However, much of Nelson's work is familiar to me; years ago, I built a Zen and quite liked it and this started me off on this voyage of discovery. I like his minimalism and fundamental grasp of the strengths and weaknesses. He is also a very decent human being......

The amp using this topology I have built I call the Glass Harmony. The input stage, the voltage amplifier, is a 6SL7, and the output stage is two parallel P types while the current source is two parallel N types, IRFP 150s.

Mirlo, I run only 4.2mA through the bipolar by using a 1K resistor on the collector/rail, and a 0R47 resistor in the source to degenerate the OLG a little.

Your comment about the gate resistor is well taken. The pupose of a gate resistor is to block rapid transfer of charge to and from the gate, so that the infamous self-oscillation properties of a mosfet can be 'poled' out, to use your terminology.

It works pretty well, and you are right that buffering the power supply would not be necessary with a cascode. However, such an approach would limit the voltage swing at the drain of the mosfet, which is not good for rail efficiency.

Cheers,

Hugh

www.printedelectronics.com
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Old 18th July 2002, 01:17 PM   #10
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Default On the other hand

AKSA is a seller.
Seller likes profits.
Be warned.
Compare what you get, and what you give.
given the balance is satisfactory to you-
BUY!
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