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Old 19th May 2014, 05:39 PM   #1
adiaz is offline adiaz  Spain
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Default Improvement in VAS

Has anyone tested improvements in VAS?

I've always had doubts. What produces better sound? a transistor in cascodo mode or one transistor in darlington mode. See schematics images, figure C or D.

If has tried this, could tell his experience?.

When I mean better sound, I think it is very important slew-rate, distortion and linearity.

I read the schematic C do not often make great improvement in VAS, and usually a single transistor usually do the job well.

The schematic D, I think improves the slew-rate, linearity and distortion, I'm right?

Thanks.
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File Type: bmp VAS type C.bmp (149.2 KB, 174 views)
File Type: bmp VAS type D.bmp (154.9 KB, 120 views)
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Old 19th May 2014, 07:47 PM   #2
jxdking is offline jxdking  China
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In my experience, C is better than D.
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Old 19th May 2014, 08:02 PM   #3
tvrgeek is offline tvrgeek  United States
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I would agree C is a big improvement. Self discussed this at great length in Self on Audio.

C is not Darlington, it is an EF. Cascode only helps if the load is buffered.

A single transistor does not have sufficient gain to be used for feedback and does not isolate itself from the varying impedance load. Toss them into SPICE to see what I mean. We are taking orders of magnitude between a single and "C".

Two pole or TMC will also give some big improvements.
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Old 19th May 2014, 08:24 PM   #4
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As so often, it would depend on the ifs and buts - particularly the kind of devices you have available, closed-loop gain and input stage current.
C loads the input stage less, but the second transistor must be a good VAS type for best performance. It may also introduce significant amounts of common-mode distortion unless the emitter resistor is replaced by a current source, though this isn't as much of a problem at the 20+ dB gains of speaker power amps (a headphone amp would be a different story).
D has the usual benefits of a cascode (i.e. you can combine a high-hFE lower transistor with a higher Va, higher power upper one), but is likely to require more base current.
Both approaches can also be combined, but obviously, the more transistors, the more likely you are to get into trouble with parasitic oscillation.

I am pretty sure that the clipping behavior of the various types has been discussed here in the past.

BTW - .bmp? Now that's a throwback to the '90s. I would suggest going to the trouble of converting to PNG, which shrinks these to about 11-12K each (and would have them immediately visible here).
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Old 19th May 2014, 09:09 PM   #5
adiaz is offline adiaz  Spain
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Quote:
Originally Posted by sgrossklass View Post
As so often, it would depend on the ifs and buts - particularly the kind of devices you have available, closed-loop gain and input stage current.
C loads the input stage less, but the second transistor must be a good VAS type for best performance. It may also introduce significant amounts of common-mode distortion unless the emitter resistor is replaced by a current source, though this isn't as much of a problem at the 20+ dB gains of speaker power amps (a headphone amp would be a different story).
D has the usual benefits of a cascode (i.e. you can combine a high-hFE lower transistor with a higher Va, higher power upper one), but is likely to require more base current.
Both approaches can also be combined, but obviously, the more transistors, the more likely you are to get into trouble with parasitic oscillation.

I am pretty sure that the clipping behavior of the various types has been discussed here in the past.

BTW - .bmp? Now that's a throwback to the '90s. I would suggest going to the trouble of converting to PNG, which shrinks these to about 11-12K each (and would have them immediately visible here).

Is true, put schemas in png. I'm talking in power amplifiers that will drive power lateral mosfets.

I thought that was the best option D (mode cascode).

I am carefully reading their reviews.
Attached Images
File Type: png VAS type C.png (10.8 KB, 262 views)
File Type: png VAS type D.png (12.0 KB, 258 views)

Last edited by adiaz; 19th May 2014 at 09:19 PM.
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Old 22nd May 2014, 04:32 AM   #6
CBS240 is offline CBS240  United States
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You are talking about the VAS. Voltage amplifier stage is not meant to drive power devices, mosfet or BJT. This is why you must have a driver stage.

Ignoring the type of global loop compensation used, the buffered VAS (first pic) will provide the IPS with a higher Z load which makes it happy. The cascode VAS will have a higher output Z than the buffered VAS. Cascode VAS required lots of driver current gain from the driver stage because it has a much higher output Z. However, linearity of the cascode (common base) VAS is much better, just like a common grid triode amplfier. It depends on the surrounding circuitry as to which one will provide better results as a whole. For every action there is a reaction. For every pro in a particular circuit there is a con. Personally I like the cascode VAS, but there are drawbacks. It requires a larger voltage to the VAS circuit in order to provide full output voltage swing to the OPS. Combining the two circuits, a buffered cascode VAS, is not a bad solution either but there is still the penalty of driver current gain to suffice.....such as a tripple output EF. This is where a local error correction circuit within the output stage might come in handy because it has within the output stage it's own feedback (feedforward) nested loop surrounding the output stage. Besides, most of the non-linear distortion comes from the output devices. Generally adding an extra stage of 'small signal' devices within the global loop is of little consequence of propagation delay in an audio amplifier when considering the improved linearity of the VAS.
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Last edited by CBS240; 22nd May 2014 at 04:42 AM.
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Old 22nd May 2014, 06:48 AM   #7
BV is offline BV  Slovakia
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Combination of both (with Hawksford cascode), works great. Compensation not showed.
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File Type: png VAS.png (4.3 KB, 215 views)

Last edited by BV; 22nd May 2014 at 06:55 AM.
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Old 22nd May 2014, 08:16 AM   #8
Vostro is offline Vostro  South Africa
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Hi BV theres been discussion about 3 transistor VAS like in your post and the problems of getting it stable.

Can you please show how you would compensate this arrangement, and have you found this arrangement to be reliable and stable yourself.

Also in picture you use a 1mA current source, is this the typical current you use for this application ?
Thanks
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Old 22nd May 2014, 08:59 AM   #9
BV is offline BV  Slovakia
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I am using TMC compensation, no problems with stability in realization. LED current is not critical and 0,5-1mA are sufficient . But current source is a must for good PSRR. Or use 2 resistors in series with midpoint blocking to -Ub, (e.g 2x22k, 47u).
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Old 22nd May 2014, 09:38 AM   #10
AndrewT is offline AndrewT  Scotland
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D.Self tells us that both are very good.

It seems he chooses the EF because it uses fewer transistors, when a buffer is required after the cascode.

If one has already committed to a pre-driver output stage, then both EF and cascode use the same number of transistors.
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