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Old 8th October 2014, 07:39 AM   #1
Elvee is offline Elvee  Belgium
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Default EZ-Dump: dump your current without really trying

Here is a new project, virtual for the time being, but I'll certainly give it a try one of these days.

It is a current dumping amplifier, but with an originality: the class A amp is based on a boosted-rails opamp. This means that the signal part of the amplifier is minimal, and that any standard opamp having a maximum supply of 36V (or even less) is suitable.

The pic shows the circuit and its operation, with the supply rails moving in a "telescopic" fashion about the output voltage, and keeping their difference ~constant.

The basic principles of the technique are detailed here: http://joebrown.org.uk/images/DualPS...pingOpAmps.pdf

Unlike a conventional opamp amplifier, two resistive dividers are involved in the gain, and the actual gain is determined by the difference in the ratio's of the dividers.
If we call K- the ratio of the divider of the (-) output and K+ that of the (+) input, the gain can be written as (1 - K-)/(K- - K+).
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Old 8th October 2014, 12:49 PM   #2
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Elvee, just for my understanding, you use 'current dumping' in a general sense right? No intention to use the Quad Current Dumping (bridge) topology?

Jan
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Old 8th October 2014, 01:12 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by jan.didden View Post
Elvee, just for my understanding, you use 'current dumping' in a general sense right? No intention to use the Quad Current Dumping (bridge) topology?
It is (will be) actually a current dumping "ą la Quad", but the above circuit is just the first building block, the low power class A amplifier.

The remaining function blocks will follow soon...
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Old 8th October 2014, 02:14 PM   #4
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Why use a superfast rectifier for D1 and D4? Why not BD139s / BD140s rigged as diodes? At least the errors in the SPICE model of the diode would track the errors in the SPICE model of the emitter follower. Looks like there used to be 1N4001 diodes in the circuit, why remove those?

Why connect C3 across the emitters of the bootstrappers? Why not connect it across the bases?
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Old 8th October 2014, 03:14 PM   #5
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Quote:
Originally Posted by Mark Johnson View Post
Why connect C3 across the emitters of the bootstrappers? Why not connect it across the bases?
A 220n should provide low impedance well into the VHF region - I wouldn't trust an EF with fT = 50 MHz transistors to do the same. You could add another one across the bases though.
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Old 8th October 2014, 04:08 PM   #6
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Mark Johnson View Post
Why use a superfast rectifier for D1 and D4? Why not BD139s / BD140s rigged as diodes?
You have to remember that at this stage, the circuit is still virtual: I just wanted to bias the BD's properly without complication, but when a prototype is built, this kind of detail will be refined

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Why connect C3 across the emitters of the bootstrappers? Why not connect it across the bases?
This is basically the reason:

Quote:
Originally Posted by sgrossklass View Post
A 220n should provide low impedance well into the VHF region - I wouldn't trust an EF with fT = 50 MHz transistors to do the same. You could add another one across the bases though.
At first sight, a capacitor across the bases looks more effective, as its value is multiplied by the Hfe of the transistors, but in fact, the opamp doesn't require a low frequency bypass: it is only at HF and VHF frequencies that instabilities due to an excessive supply impedance can show up.
The impedance seen from the emitter of an EF is not something clean and real like |0.026/Ie| at high frequencies, it much nastier than that, hence the local decoupling.

I wouldn't add a cap between the bases, at least not without base-stoppers: a low impedance path between the bases and the emitters could also favor instabilities.
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Old 8th October 2014, 04:31 PM   #7
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Then why not insert base stoppers too, which add to the real part but not the imaginary part of the emitter impedance?
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Old 8th October 2014, 09:00 PM   #8
Ketje is offline Ketje  Belgium
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Aren't you making an oscillator ?
Negative feedback 100k+100k and a posirive feedback 100k+91k+Rsource.
Ones the source is more than 9k I think oscillation starts.
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Old 9th October 2014, 08:34 AM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Mark Johnson View Post
Then why not insert base stoppers too, which add to the real part but not the imaginary part of the emitter impedance?
Simply because I do not see the need for it: with modern opamps (I mean post-1975) it will make practically no difference.
When the time comes to split the last ppm, we may come back to it, but at the moment, I don't find it necessary

Quote:
Originally Posted by Ketje View Post
Aren't you making an oscillator ?
Negative feedback 100k+100k and a posirive feedback 100k+91k+Rsource.
Ones the source is more than 9k I think oscillation starts.
Mona
Clever observation, there is indeed something there.
It will be adressed later.

OK, it's time to progress towards the real dumper: the current dumpers, Q1 & Q2 have been added, as well as the bridge components: R11, R12, L1, C6.
Some values have also been adapted.

Sout is a test probe scaling the output voltage by exactly the gain value, allowing comparisons with the input signal. This is necessary to fine-tune the bridge components, because artifacts are too small to be directly visible on the output signal.

Next pics show the 1K & 20K THD. Not too bad considering the simplicity and crap components used.

The next two pics show the error residue signal between Vin and Sout; even with the probe, it is necessary to zoom to see anything: the amplitude of the residual artefact is ~50µV

The opamp bootstrapping scheme hides some subtleties: Mona has spotted one, it is the input impedance: it is actually negative, and if the source impedance becomes too large, the feedback becomes positive.
Here, this issue is dodged by shunting the input with a resistor whose value is slightly lower than the negative input impedance: this eliminates all risks of positive feedback, and magically raises the resulting impedance.
The uncorrected impedance is -2K, and when paralleled with +1.8K, the result is +15K, as the input current demonstrates.
Attached Images
File Type: png EZdump2.png (54.5 KB, 220 views)
File Type: png EZdump3.png (130.2 KB, 211 views)
File Type: png EZdump4.png (136.9 KB, 198 views)
File Type: png EZdump5.png (97.4 KB, 197 views)
File Type: png EZdump6.png (94.2 KB, 47 views)
File Type: png EZdump7.png (98.3 KB, 60 views)
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File Type: asc EZdump.asc (10.3 KB, 12 views)
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Old 10th October 2014, 07:40 AM   #10
Elvee is offline Elvee  Belgium
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Another issue arising from the interaction of the current dumping scheme with the bootstrapped opamp also needs to be fixed: the bridge component values are OK for the dumping section, but they aren't compatible with the normal operation of the VAS.

The first pic shows the frequency response: the -3dB point is at ~15KHz, which is clearly unacceptable. What's going on?

The problem is that the value of the reactive components of the dumping bridge are multiplied by the bootstrapping bridge gain, resulting in a heavy rolloff.

The theoretical solution is pretty obvious: the boostrapping bridge needs to be compensated. This will change nothing to the dumping characteristics, but it will correct the frequency response.
There are two problems with this approach though: unlike the capacitor, the inductor will be difficult to compensate using practical, realistic components. This means that the compensating cap will need to be much larger, because it will also have to compensate for the inductor, and this will exacerbate the second potential problem: in this case, compensation means tying a capacitor between the output of an amplifier and its (+) input, something I feel rather uncomfortable with... Looks like a recipe for an oscillator.

In sim, it works and it looks stable, but I wouldn't trust one second such a result without making a reality-check.
With 24pF, the frequency response can be extended (in theory) to ~36KHz, which is not large, but sufficient.

This will need to be tested in reality, but fortunately there are also safer solutions.
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File Type: png EZdump9.png (113.0 KB, 36 views)
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