const. voltage & const. current on output FET ?

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Would const. voltage Vds & const. current Ids on output FET reduce nonlinearity ?
If you mean cascodingthe constant current source, than YES, it basically increases output impedance and thus increases linearity at cost of voltage swing and maybe some other nasty things. But proper casoding may linearize, Pass wrote a nice article on this.
 
Stasis^2

I have a circuit concept ready for a single ended class A output stage that keeps both the current in the output transistor constant, as well as the voltage across it. It's basically an emitter follower (could be source follower) with some circuitry around it. The special thing is that the current remains constant irrespective of the load current. In this way Vbe is constant (or Vgs) and less distortion is expected.

If I only had time to finish the design.

Steven
 
Re: Stasis^2

Steven said:
I have a circuit concept ready for a single ended class A output stage that keeps both the current in the output transistor constant, as well as the voltage across it. It's basically an emitter follower (could be source follower) with some circuitry around it. The special thing is that the current remains constant irrespective of the load current.

I made simulation too: SE classA with constant current & voltage on source follower.

What has Stasis got to do with it ?
 
Sure, there are many ways one could do it, but in the end you need something to provide the load current instead of the follower, so do you really gain anything? Maybe I just can't figure out a clever way to do it, but it seems you would probably make things more complex and probably get worse performance in the end. I'd be interested to see a schematic, though, of what you have in mind.
 
Re: Re: Stasis^2

Bernhard said:

What has Stasis got to do with it ?

Stasis is the name Nelson Pass gave to his early amplifiers where the focus was on keeping the current through and the voltage across the transistors more or less constant. Stasis comes from Greek and means "a standing still", see http://dictionary.reference.com/wordoftheday/archive/2002/08/04.html

Nelson used cascoding to keep the voltage more stable and Sziklai output pairs to keep the current through the driver transistors more stable (in a Sziklai pair the driver controls the output).

I called it Stasis^2 to indicate that my circuit keeps the voltage far more stable, as well as the current, but it is just a nickname; Stasis is a trademark, so I cannot use it.

Steven
 
Hi Bernard,

I have a bit difficult to grasp what you have in mind, it would help if you have a schematic to show.

Constant voltage and current is equal to constant power, how can you have that on the output transistor if you want to play music which is not "constant"... :)

Cheers Michael
 
Constans (a different Stasis)

As this thread is about constant voltage and current output devices (BJT or FET doesn't matter), I present an idea that I had put on paper more than a year ago, but had no time to do something with.

I just have put it into Circuitmaker now to see whether it actually works as anticipated, and it does.

It shows a single ended output stage without voltage gain. Q1 is an emitter follower, but could be a source follower too. The current source in its collector determines the constant current of Q1. If the load current through R2 changes, the current through Q1 starts to change too, but this forces a difference current through Q3. Q3 drives Q2 and the current through Q2 will change in such a way that the output current can be delivered while the current through Q1 remains constant.
The voltage across Q1 is also constant. Because of Q3, the collector of Q1 is approximately at the same voltage as its base, which is one Vbe higher than its emitter. So there is a constant voltage of approximately 0.8V across Q1. This can be made more by e.g. adding a diode into the emitter of Q3.

For now I call this circuit Constans, which means more or less the same as Stasis, but now in Latin.

Steven
 

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Constans waves

The picture below shows the current waveforms. It is clear that the collector current of Q1 is constant (green line). The output current is shown in blue, this is the load current. Never mind the sign, that is something of the simulator; my simulator likes negative currents.
Yellow shows the collector current of Q2. It is clear that the modulation of Q2 is causing the ability to source current into the load or sink current from the load.
The important thing is that the output voltage is determined by Q1 and not Q2. The performance Q1 is crucial and Q1 has an easy life with its fixed collector current of 2A in this example and only 0.8V across it. Not even 2W of dissipation!

Steven
 

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Re: Constans waves

Steven said:
Q1 has an easy life with its fixed collector current of 2A in this example and only 0.8V across it. Not even 2W of dissipation!

As you mention it, this nice side effect means one can live with one :) gain device and there is no need for matching and no source of additional distortion due to mismatch.

I have a whole lot of ideas but lack a little bit of understanding the very basics as there are current mirrors & Sziklai pairs and the like.

Any nice website where those stuff is explained ?
 
I have to draw a schematic because I have implemented it in my distortion killer schematic and it is a little confusing to look at it.

Basically the load current is sensed and the CCS current is corrected so that the gain device current remains constant.

IMHO it is not important to holt I and V absolutely stable, hopefully minimized current swing and Vsd variation will result in reduced distortion.
 
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