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Old 9th March 2007, 01:26 AM   #1
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Default Opamp PSRR

I noticed that the good sounding opamps all have high PSRR.

Is there any way of using common opamp (NE5532, TL072) to have better PSRR? How to make supply to feed the opamp (+/-15) that can boost the resulting PSRR of the whole CCT?
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Old 9th March 2007, 01:52 AM   #2
jcx is offline jcx  United States
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David, there Is a reason I keep mentioning the Sackinger paper

http://www.diyaudio.com/forums/showt...61#post1146461

read it, at least look at the pictures, read the text explaining them

if psrr is measured properly a op amp with high psrr on both rails must have high loop gain
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Old 9th March 2007, 02:20 AM   #3
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Hi, JCX,

It's a very good paper. With my limited brain, I have not understand this single paper wholy. It will take some time for me to digest it. I'm not very good with math. For now, the usefull picture is fig4 and fig6, this can be implemented in discrete designs. Fig8 is implementable with more complicated CCT.

Since NE5532 or TL072 only have 8pins, all have been used, and we cannot access to the internal CCT of it, I come to conclusion that the "usually overlooked" pins that "may" give better audio performance is pin 4(-suppy) and pin8 (+supply). Giving better supply (maybe?) will extract more reproduction with these ordinary opamps.

But what is to be connected with pin4 and pin8 that can give better audio performance for these ordinary opamps?
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Old 9th March 2007, 02:47 AM   #4
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From STAX patent #4366432, I came with this drawing.

The supply is higher than +/-15V needed (coming from +/-25V), connected to the opamp by CCS (10mA), and put zener 15V to maintain the voltage in the opamp's supply.

The idea is that a CCS will have high output impedance, have smaller disturbance. Pin4 and pin8 will see neglected amount of +/-25V's ripple. Then we will need a zener to set the voltage.

It makes a second loop in the zeners as a second load besides the opamp itself.

But after looking at the drawing one more time, what could be the difference between this and ordinary usage of opamp (which connects pin4 and pin8 directly to +/-15V without zeners)? Will the CCS+zener method make better supply rail for the opamps?
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Old 9th March 2007, 02:58 AM   #5
jcx is offline jcx  United States
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http://www.diyaudio.com/forums/showt...110#post951110

In that sim, if you change U2 to the opa227 model which better models psrr

you can then move the AC stimulus to the V1 and then the V2 power supplies to see the psrr boost by looking at the V(boot) and the V(out) performance in the .ac sweep

this connection increases psrr by the added loop gain, ~ 60 dB extra for a total of 120 dB psrr @ 10 KHz



with the composite op amp circuit of post #1 in that same thread you can filter or sub reg U1 power supply for very high psrr too

Zener shunt regulated supplies can acheive good high frequency regulation, where active 3 terminal regs run out of loop gain and psrr
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Old 9th March 2007, 05:18 AM   #6
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Hi, JCX,

Thanks . So your trick is to add OL gain to improve PSRR?

I found something interesting in your thread. Bootstrapped power supply (Msbootstrap.asc). How good is this method to improve PSRR?

Back again to picture in post#1 in that post. I have 2 questions about this picture :

1. Is it really important to make U2 integrator (feeding back R4-C4 in local loop)? I read somewhere that cascaded opamp design should have the second opamp much faster to inhibit instability.

2. What is the function of R6-R7 (10k to rails). UCD appnote also uses this R, it is said to improve PSRR, but I really don't understand what is the mechanism. In my simple thinking, putting R to rail will worsen PSRR, not improving it.

Quote:
Zener shunt regulated supplies can acheive good high frequency regulation, where active 3 terminal regs run out of loop gain and psrr
Is this what I draw in post#4?
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Old 9th March 2007, 05:58 AM   #7
jcx is offline jcx  United States
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The Sackinger paper makes clear that adding gain or a ac gnd ref terminal are the ways to increase psrr

in the composite amp adding sub regulation of the input op amp pwr can be thought of as adding a ac gnd ref to the composite

V gain in the output stage of a composite amp is useful to reduce distortion, the modified integrator in the compos.asc circuit lets more low frequency gain be visible in the outer loop where it can greatly reduce input op amp signal levels and therefore the distortion

coordinating high frequency gain/bandwidth of the output amp with the loop gain intercept of the input op amp isn't too hard but understanding the use of Bode plots helps

most examples I've seen/built have relied on much faster output op amps so the dominant pole comensation of the input op amp can control composite loop stability


in the compos circuit from post #1 in my thread R6,7 just add bias to the diode clamp bridge so that in normal operation D1,2 are slightly revese biased to reduce leakage current error and diode junction capacitance that would add more distortion to the feedback node if the clamp diodes weren't biased
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Old 12th March 2007, 03:02 AM   #8
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Opamp's supply lead seems really can affect the opamp's performance. I found this article from Analog Device, AN-232 by Walt Jung. The supply bootstrapped towards output lowers the distortion. But why he only put it on negative supply, none in positive supply?
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Old 12th March 2007, 04:00 AM   #9
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i wouldn't know for sure without looking at the internal circuits of the op amps, but it's possible that the neg side is bootstrapped because the input diff current sources and output (which may be class a instead of class ab) pulldown resistors are sourced from the negative rails. quite often the pos and neg rail PSRR on some op amps are very different.
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Old 12th March 2007, 07:26 AM   #10
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Walt's negative supply bootstrap of the AD744 "cascodes" out the effect ot the input fet body-substrate capacitance which is nonlinear with input common mode voltage - and low noise fet input transistors can be large, the AD743 and OPA134 warn of this distortion source with higher source impedances:

(OPA134 datasheet)
"SOURCE IMPEDANCE AND DISTORTION
For lowest distortion with a source or feedback network
which has an impedance greater than 2k[Ohm], the impedance
seen by the positive and negative inputs in noninverting
applications should be matched. The p-channel JFETs in the
FET input stage exhibit a varying input capacitance with
applied common-mode input voltage. In inverting configurations
the input does not vary with input voltage since the
inverting input is held at virtual ground. However, in
noninverting applications the inputs do vary, and the gate to-
source voltage is not constant. The effect is increased
distortion due to the varying capacitance for unmatched
source impedances greater than 2k[Ohm].
To maintain low distortion, match unbalanced source impedance
with appropriate values in the feedback network as
shown in Figure 3. Of course, the unbalanced impedance
may be from gain-setting resistors in the feedback path. If
the parallel combination of R1 and R2 is greater than 2k[Ohm], a
matching impedance on the noninverting input should be
used. As always, resistor values should be minimized to
reduce the effects of thermal noise."



the choice of rail is specific to this particular op amp, psrr can be very high as mentioned when the compensation/VAS is not referenced to this rail

since the VAS/gain/Miller comp is referenced to (the other) one of the supply rails you get major instability if you try to use the buffered output of the op amp to bootstrap away the effect of the compensation - by for instance bootstrapping both op amp supply pins:

Quote:
Originally posted by jcx
G1,C1 op amp model, E1, R1,C2 "unity gain" output stage model:

Click the image to open in full size.

the closer E1 is to 1.00, the less damping due to the bootstrap connection of the op amp which partially cancels the effect of the miller cap C1 - not a exactly trivial compensation situation


LtSpice file:
The Sandman patent in the previous sim I pointed to earlier in this thread shows a more stable approach using "feedforward" bootstrapping

I really need to finish up an article on these issues that has been lying around my computer for years now - by comming up with that ref it makes me worry that David's looking at my hard drive
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