Current Source <- how to optimize?

[Ultima Thule]

Originally posted by mikeks


See below:

Cheers Michael

[amplifierguru]

Hi Ultima Thule,

Your TL431 and BJT CCS is actually a current sink, which begs the question, What do you complement it with?

Cheers,
Greg

[Ultima Thule]

Originally posted by amplifierguru
Hi Ultima Thule,

Your TL431 and BJT CCS is actually a current sink, which begs the question, What do you complement it with?

Cheers,
Greg

Hi Greg,

The exclusive version of my CCS not shown in my link is with an additional opamp, which uses for instance a TL431 as a voltage reference, will do.

Cheers Michael

[mikeks]

Originally posted by amplifierguru
Hi Ultima Thule,

Your TL431 and BJT CCS is actually a current sink.........

That depends..on whether you're thinking electron flow, or conventional current flow....

[pb]

One possible reason I see for better sonics of resistor is that any CCS using
active components will have an associated capacitance parelleled to its output resistance.

When one consider this very high dynamic resistance of CCS of more advanced CCS, lets say 10Mohm,
it is easy to realize that with 10pF load capacitance (active element junctions) it will cause 15kHz cut off frequency.
This is what limits the VAS high frequency response.

This capacitance is nonlinear. Using more complex CCS with the feedback (Wilson, ANF) may lead to some more complex behavior at higher frequencies, slew rates.

Especially ANF does contain deep NFB. I simulated the ANF configuration for relatively fast voltage transitions on the output (10V/us) and in the bottom transistor the current looked pretty ugly. It had to compensate for transient current flowing in CB capacitance.

That would explain why the simple CCS are probably the best compromise. I would follow good suggestions from AKSa regarding degeneration resistor.
For VAS stages it may be better to use cascode with low capacitance transistor (perhaps high voltage video amp BF922 etc)

Cheers,
Przemek

[forr]

PB
"When one consider this very high dynamic resistance of CCS of more advanced CCS, lets say 10Mohm,
it is easy to realize that with 10pF load capacitance (active element junctions) it will cause 15kHz cut off frequency.
This is what limits the VAS high frequency response."


I suppose it works like this :
Even if there is a 15 kHz cut off, the constant current source behaviour is still there : the initial 10 MOhm dynamic resistance falls to 1 Mohm at 150 kHz, 100 kOhm at 1.5 Mhz and 10 kOhm at 15 Mhz where the amplifier may have not any forward gain.

For a long tail pair, I would add a resistor in series with the CCS whose value is such that the current across it makes a voltage drop equal to half the power supply voltage. If the current through each device of the differential pair is 3 mA, with a degenerative Re of 100 Ohm, this resistor could be a 2.2 kOhm. This is about twenty times higher than the values determining the Gm of the input stage, and it is valid at all frequencies of interest. And we still keep all the advantages of the CCS.

The often advocated suppression of the CCS in non inverting amps is a mystery to me. Its immediate effect is to increase non linearity of the input stage, which means more distorsion.

Due to the Miller effect, the output impedance of the Vas main transistor is always much smaller than the CCS. Typically, it starts to decrease at 200 Hz. I've never seen a resistor in series with the CCS of the VAS, but, experimentally, it may not be a bad idea, at the expense of sacrifying the full voltage swing.

~~~~~~~~~Forr

§§§

[mikeks]

Originally posted by pb
Especially ANF does contain deep NFB. I simulated the ANF configuration for relatively fast voltage transitions on the output (10V/us) and in the bottom transistor the current looked pretty ugly. It had to compensate for transient current flowing in CB capacitance.

I suspect there is more to this than meets the eye....info on SPICE set-up...and models required....

In fact the output impedance of the ANF current source (my prefered name for this arrangement) is of the order of 500M-1G depending on the control elements current gain.....

Loop-transmission local to the current source ranges from 10dB to 40dB up to 200KHz.....

Stability margins are vast....typically >80 degrees....

The ANF current source's transient response is the equal of, and indeed superior to that of any other configuration...

[mikeks]

...note that the excellent stability of this arrangement is due to the fact that the dominant singularity in it's loop-transmission frequency response is an innocuous LHP zero....

[pb]

Mikeks,

In ideal world ANF should be the best.
The SPICE is of course not a proof of anything. I actually was quite suprised to see any overshots in the very simple case:
see attached image comparing cascoded CCS and ANF.
I used LT SwitcherCAD and stock models.

The oscillation visible in ANF response decays within 10ns.
You might be right that there is more than meets they eye in that case (simulating engine artifact)

The pretty ugly stuff I have seen comes from, I admit, quite an extreme case: CCS for a class A single ended amp.
The power transistor was a MOSFET. When run with a more detailed SPICE model (I think it was one of IRFs with package inductances included) it would actually generate quite severe ringing.

This just proves that when parasitics are added in equation the behavior might be quite different.

BTW how do you know the phase margins? Care to share your results?

I was thinking of some more SPICE experiments but what really stops me is the knowledge that it may be a moot excercise since I have no access to a good quality models and simulator.

I guess we may need someone with network analyzer to build the real thing and measure it

Cheers,
Przemek

[amplifierguru]

Bit like SEX isn't it?

You can simulate it or enjoy the real thing! Sim all you like won't get you closer.

Cheers,
Greg