OpAmp compensation

[Remco Poelstra]

Hi,

I'm working on a DA converter for audio. I would like to get as close as
possible to the theoretical 24 bits resolution. I opted for an AD1955,
so I won't get any further than 21 bits, but that's a good start I think.
Currently I'm working on a shunt regulator to get very low noise supply
lines, as well as some other advantages.
My design (http://remco.beryllium.net/regulator/schematic.pdf) didn't
quite work in the way it's drawn and I was told it was because of the
frequency compensation that I needed to add. With their help I got the
regulator to work, but I don't really know why it works now and what the
theory behind the correct operation is.
So I would like to know more about that frequency compensation stuff.
What's a good start? What books would you recommend?

Thanks in advance,

Remco Poelstra

 

[qrk]

Hi,

I'm working on a DA converter for audio. I would like to get as close as
possible to the theoretical 24 bits resolution. I opted for an AD1955,
so I won't get any further than 21 bits, but that's a good start I think.
Currently I'm working on a shunt regulator to get very low noise supply
lines, as well as some other advantages.
My design (http://remco.beryllium.net/regulator/schematic.pdf) didn't
quite work in the way it's drawn and I was told it was because of the
frequency compensation that I needed to add. With their help I got the
regulator to work, but I don't really know why it works now and what the
theory behind the correct operation is.
So I would like to know more about that frequency compensation stuff.
What's a good start? What books would you recommend?

Thanks in advance,

Remco Poelstra

A good book on opamps can be found at
http://focus.ti.com.cn/cn/lit/an/slod006b/slod006b.pdf
There is a chapter on compensation.

 

[Remco Poelstra]

MooseFET schreef:

On Nov 26, 1:40 am, Remco Poelstra <remco.poelstra+albas...@duran-
audio.com> wrote:
Why did you start off with such a gawd awful regulator topology in the
first place?

Hi,

I choose this topology, because I would like to keep varying load
currents out of the supply rails. With this topology there is only DC
current on the rails, and the fluctuating currents are only in small
loops close to the consumers (given that the regulator is placed close
to the consumer).
An added advantage is with the use of opamps which have no short-circuit
protection on their outputs, they are limited to the current set by the
current source.

Regards,

Remco Poelstra

 

[Remco Poelstra]

[email protected] schreef:

I still don't like using the op amp. There are shunt regulator chips.
Two gain stages means you need a Miller, while one gain stage is
generally compensated with a cap to ground. The Miller cap can be a
source of poor PSRR. You don't need a microvolt of DC regulation, so
you don't need two gain stages.

How can I do the same trick without the opamp? I need some kind of error
amplifier no?

If you insist on the shunt, be sure that it don't overvoltage the
downstream circuitry (load) on start-up. You need to insure the shunt
wakes-up as low impendance so the out starts from ground and rises to
the regulated voltage. Also ramp the raw supply up and down in a
transient to insure it doesn't overvoltage in that case too. I've had
occasions where I would need to put in a diode to the raw power to
insure that if it is up, then shorted, then back up again, the
compensation cap gets discharge on during the grounded phase.

There is a lot of good engineering in chips that often gets skipped
when someone tries to rolll their own. Protection circuitry is
generally what gets skimped in discrete designs.

The shunt regulators that I found all had quite high noise, at least
when compared to ICs like the ADR440. Is there a specific IC I missed?

Regards,

Remco Poelstra

 

[Remco Poelstra]

MooseFET schreef:

In that case you a PNP as the sink and the circuit will be less
awful. The PNP naturally is low noise and low impedance. The
indirect control of the voltage by controlling the MOSFET is just a
bad idea.

I've tried both with a MOSFET and BJT (shouldn't it be an NPN?). I knew
the BJT had less noise, but at first I couldn't get it to work with the
BJT, so I tried the MOSFET.
I don't get the point of your last sentence; When I use a BJT, the
voltage is still controlled indirectly, no?

Regards,

Remco Poelstra

 

[Remco Poelstra]

[email protected] schreef:

Beyond me. I'm an IC guy. If money isn't a problem, you could use any
shunt then follow it up with that LTC part.

Sorry, I don't get your point. What do you mean with 'LTC part'?

I'm sure you've seen this insanity:
http://www.borbelyaudio.com/psshunt.asp

No, I hadn't

The noise you quote is just that of the ADR440. You haven't computed
the noise of the whole circuit.

True, but I first like to get the circuit operate correctly before doing
the noise math and perhaps improve on that one.

Regards,

Remco Poelstra

 

[Remco Poelstra]

MooseFET schreef:

No. I want you to correctly use the PNP
It looks like this:
Current source
V
!
+---------+-------- Load
! !
-----!+\ R1 !/e !
! >-+-/\/\---! PNP !
---!-/ ! !\ !
! ===C1 ----GND !
! ! !
+--------+----/\/\------------
! R3
\ R2
/
\
!
GND

R1 is something like 51 ohms. It is just to prevent oscillations in
the PNP as some high RF frequency.

1+R3/R2 sets the gain at low frequencies.

C1 and R2//R3 close the loop around the op-amp near the point where
the gain bandwidth of the op-amp is hit so that the op-amp doesn't
oscillate or have a noise peak.

The PNP continues to provide a lowish impedance to the load side well
above the frequency where the op-amp runs out of gain. The PNP is
natuarally low noise so the high frequency noise performance is ok.

I see, the PNP upside down . Seems about time to get a good book on
BJT's, because I don't see why this gives a low impedance path for high
frequencies. I'll try this one though to check whether it works.
Am I correct when I say that the opamp controls the output voltage more
direct in this case, because there is only the constant +-0.7V offset
from the Vbe? Can I then also conclude that the second stage has low
gain in this configuration?

Regards,

Remco Poelstra