Constant Current Sources
Reference: Son of Zen article from
https://www.passdiy.com
Question- Has anyone tried to use regulation or active current sources on the SOZ? - PassFan?
Answer By Nelson Pass
You can replace the 8 ohm resistors on the
negative side with a constant current source
and lower the negative voltage rail to 15 volts
or so, but if you put constant current sources
in place of the 8 ohm resistors on the positive
side, you'll have to come up with some way to
stabilize the output, either with feedback or
using load resistance. Nelson Pass
Current Sources Explained by Grey Rollins
Remember when I mentioned asymmetrical rails back when we were talking about current sources for the SOZ? Same thing Nelson's talking about.
Okay, here's the deal...
You're familiar with the idea of a voltage regulator, right? An ideal voltage regulator (as opposed to a real world one) will deliver any arbitrary amount of current from 0 amps to infinite amps while maintaining an absolutely steady voltage. That's why it's a *voltage* regulator--the voltage remains constant, no matter what, but the current varies.
Now, a current source has another name: current regulator. What's a current regulator do? (You can already see where I'm going, I'll bet.) Locks the current...and lets the voltage vary. Kind of like an upside-down voltage regulator.
Suppose we were to say we wanted a steady 1 amp. We then build a current source with this in mind. How would it behave? If you give a 1A current source a 1 ohm load, it will develop exactly 1V across the load. Simple application of Ohm's Law: I*R=E...1A*1 ohm=1V. But what happens if you give it a 2 ohm load? It will do whatever it has to do to force 1A through the load. In this case, it will develop 2V of output. 1A*2 ohms=2V. An ideal current source could develop 1kV across a 1k resistor, simply because you told it to deliver 1A, no matter what.
So let's consider what happens if you put one into a differential circuit (i.e. the SOZ). When there's no signal, each device will conduct half of the current. If a balanced signal is applied, one device will conduct more at the same time as the other is conducting less, but always such that the instantaneous total is 1A, regardless of whether the ratio is 90/10, 70/30, or 20/80 at any given moment. The current regulator *will* continue to push one amp through the load.
If you put an unbalanced signal in, the device receiving the signal will behave in accordance with the signal--just like you'd expect. But...that current source is a stubborn l'il critter. It *will* have its way. As the device receiving the signal decreases conductance, the rest of the current gets forced through the other side, willy-nilly, thus creating a signal through that device, even though there wasn't any input at the gate (note that the same principles hold for tubes and bipolars, I'm just using MOSFETs because that's how Nelson laid out the SOZ). An important feature to note here is that the signal through the other side of the differential is *out of phase* from the original signal, creating a balanced signal, where none was before. As the first side goes negative, the second side goes positive, and vice versa.
Due to the fact that current sources vary voltage so easily, this leads us to the possibility of asymmetrical rails for the SOZ. If you program a current source for 1A and give it, say, a -30V rail, it will develop any voltage it has to in order to supply 1A to the load and--this is the keen part--it's self adjusting! If you give it a -10V rail, it will adjust. If you give it a -300V rail, it will adjust. All because a current source excels at changing voltage whilst maintaining current.
Since the SOZ circuit doesn't need a whole lot of negative voltage underneath the MOSFET's source pin (we're assuming N-channel devices, here), you can get away with trimming the fat. Leave some wiggle room for the signal, add a volt or two for the current source itself, add bias, and stir, and presto! you've got a recipe for an amp with asymmetrical rails. Set things up to where you've got 10-15V under the tail of the thing, and you can put any voltage you like on top, whether it's 10, 20, or 50V. But the lower rail can stay the same.
There's one caveat: Real world current sources ain't perfect. They suffer from capacitance and other woes that slow them down, making them somewhat less than the ideal solution.
Grey
P.S.: And don't nobody start griping about the difference between current sources and current sinks, and such. I'm using the term current source in the vernacular sense because that's what everybody calls 'em. Okay? Start messin' with me about nomenclature an' Santa Clause won't bring you that order of MOSFETs you want for Christmas!
Current Sources Explained Again by Grey Rollins
Explain a current source?
In layman's terms?
Why, sure, glad to.
Let's come at this from the backside. Let's talk about voltage regulators. So, what is a voltage regulator? A voltage regulator is a circuit that takes a raw, incoming DC voltage with all its warts and imperfections and smooths it out into a constant, predictable voltage. No bumps, no wiggles, just pure, clean, steady DC voltage. If you look at it on an oscilloscope, there's nothing there; it's just a straight line, nice and boring, just the way we want it to be.
Let's say that we've got a load that draws current in spurts, like the output stage of a class B amp. If there's no regulator in the circuit, just a transformer and rectifier feeding some caps, the voltage on the caps will drop when the circuit demands a large amount of current (unless the power supply is very, very large indeed). This isn't good. It changes the behavior of the amp. So we stick a regulator in.
The regulator works by chopping off all the voltage above a certain preset level (yes, this is wasteful, but necessary). If you set a regulator for 20V, then feed it 30V, it will lop off the 10V that it doesn't need, allowing 20V to come on through. If the circuit demands more current, the regulator delivers more current, but at that same preset 20V. This is the crucial point for our present purposes--a voltage regulator holds the voltage steady, but allows the current to vary according to the needs of the moment.
Okay, let's turn back around and look at current sources. The thing that makes a current source mysterious is that it has multiple names: current source, current sink, and...wait for it...current regulator. Fasten onto that last name and things will become clearer. Whereas a voltage regulator maintains a steady voltage and allows the current to vary, a current regulator (aka current source) maintains a steady <i>current</i> and allows the voltage to vary.
Think of a circuit as a variable resistor. From the current source's point of view, there's somebody out there twisting the knob back and forth, back and forth, with no rhyme nor reason. The load changes unpredictably. That's okay. Current sources are patient little fellows. They're also obedient. If you tell your current source to deliver 10mA, it will do everything in its power to deliver 10mA no matter what the load does. If the load goes to an effective 10 ohms, the current source will set itself to .1V (10 ohms * .010A =.1V, simple Ohm's Law stuff). If the load suddenly goes to 1k, then the current source will set itself for 10V. At all times and in every way possible the current source will do its dead level best to deliver 10mA, simply because that's what you told it to do.
Okay, so where's that leave us in terms of an Aleph output stage?
The bottom transistor is the actual amplifier. It gets its signal from the front end differental (the two IRF9610s back-to-back to the left of the schematic) and amplifies it, in this case adding both voltage and current. The current source is the MOSFET and NPN (plus a few resistors and caps) up above. Normally, and there are numerous commercial and DIY examples of this, you would expect to see a conventional current source up there. The resistor on the upper MOSFET's Source sets the current and the MOSFET delivers it. The NPN is there to oversee the operation and kinda nudge the MOSFET back into line if it does a less than perfect job; this being the real world, the MOSFET benefits from a little help in accomplishing its task. (In fact, you can build an even simpler current source without the NPN, but performance would suffer a bit.)
If we were to leave the circuit here, this is what would happen: signal would enter the lower MOSFET and cause an output signal to appear at the Drain. The current source, seeing that the voltage is varying, and seein' as how varying voltage is second nature to a current source, goes along for the ride. As the voltage varies, the speaker (whose other side is connected to ground--nominal 0V) will see current travel back and forth, depending on whether the signal happens to be swinging above 0V or below. Music will come out of the speaker.
Back to the Aleph part of the deal, which is the crux of the matter. What Nelson did was add a few parts to the current source, so that it's even more attuned to what the lower MOSFET is doing.
As the signal leaves the circuit, the very last thing it does is pass through an array of resistors (R22-25 in the Aleph 5). The fact that these resistors are in series with the load means that they serve to sense the current going out the back door. This signal travels back up into the innards of the current source via R21 and tells the current source that it's okay to vary a little bit, as long as it does so in accordance with what the lower MOSFET is doing.
Seems a little weird to be telling a current source <i>not</i> to be delivering constant current, but as long as it does so in synchrony with the lower MOSFET, it's cool. If you want to see a voltage analog to this, take a look at Part 3 of the current Zen series over at www.passdiy.com, wherein Nelson puts together a voltage regulator that...well...it varies the voltage. Again, in synchrony with the output of the amp.
If you want more of the grisly details, check out the patent. It's not all that difficult to follow.
Incidentally, Nelson likes to point out that although the Aleph current source varies on an instantaneous basis, when taken as an average over time, the current is indeed constant.
So what happens when you remove R21 is that you end up with a "normal" current source, i.e. one that doesn't dance along with its partner. With R21, you give the current source a little bit of wiggle room to dance along with the music in the current domain.
Incidentally, the IRF9610 sitting by its lonesome up above the differential is also a current source. It's one of the conventional kind that doesn't dance, current-wise. The Zener diode and the resistor to ground set a reference voltage. The resistor above its source is what sets the current, when referenced against that 9V. In principle, you could use a similar arrangement for the output current source, using the same 9V Zener and 10k resistor at the Gate of the upper MOSFET, but substituting a 10 ohm resistor below the source (better make it a 5 or 10W resistor), but you'd have to give up the "Aleph" part of the circuit to do it. You're better off with the NPN.
Howzzat? Grey
