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Old 25th September 2007, 02:55 PM   #11
Leolabs is online now Leolabs  Malaysia
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Stability???
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Old 25th September 2007, 04:56 PM   #12
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Quote:
Originally posted by Leolabs
Stability???
AC stability stability should be excellent, based on amp of almost same architecture, but voltage output, and on simulation plots of this current output version.
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Old 25th September 2007, 05:06 PM   #13
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For the reason I have several research prototype modules of the PM-A1 amp, and this one is just a modification of the PM-A1 basic structure, I could try to build it, if someone was interested in results.
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Old 25th September 2007, 07:01 PM   #14
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Without source resistors you leave output stage conrolled by transconductance of output transistors.
1. N and P chanel transistors don't have the same transconductance, so output current will not be symetric.
2. without source resistors your amplifier will blow itself when heated, unless you want to have fireworks it's not a good thing.

So, my point is, if you put there source resistors, you lower transconductance of transistors making them more similar and linear. Also change of bias current will be less noticable with them (I usually use 0R47 resistors in circuits like this)
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Old 25th September 2007, 08:41 PM   #15
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Pavle:

Quote:
. . ., I could try to build it, if someone was interested in results
I would be. I have been experimenting with current feedback (using your modified headphone amplifier), and the results are very encouraging. However, I need more power than the heasphone amplifier provides. Consequently, do you think that you could experiment with biasing/output transistors selection for about 5-8 W into 8 ohms? I do not need more, and do not want to build a heater. ;-)

Kindest regards,

M

P.S. Sorry I failed to visit you in Prague, I had some family issues.

M
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Old 25th September 2007, 08:57 PM   #16
PMA is offline PMA  Europe
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Pavel,

this amp has current output (in the other words, very high output impedance). That means that its output voltage changes with load impedance. I do not think this is suitable for dynamic speaker. For resistive driver, like headphone, it might be suitable, but with much less transconductance. This amp would fire the headphones.

In case you have resistance driven speaker, it might be a chance.

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Pavel
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Old 25th September 2007, 09:09 PM   #17
PMA is offline PMA  Europe
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Quote:
Originally posted by DarkOne
Without source resistors you leave output stage conrolled by transconductance of output transistors.
1. N and P chanel transistors don't have the same transconductance, so output current will not be symetric.
2. without source resistors your amplifier will blow itself when heated, unless you want to have fireworks it's not a good thing.

So, my point is, if you put there source resistors, you lower transconductance of transistors making them more similar and linear. Also change of bias current will be less noticable with them (I usually use 0R47 resistors in circuits like this)
Robo,

for a standard VOLTAGE OUTPUT emitter followers, one always uses emitter or source resistors. These voltage output stages have output from emitters/sources.

In a case of the transconductance amplifier, with current output from collectors, the emitter resistor WORSENS linearity. Then, the transconductance amplifiers operate best into SHORT CIRCUIT (load = 0 ohm).

I have already build and operated VOLTAGE output amplifier with output stage based on difference of two squares, with output from drains. No source resistors are used. Again, they would WORSEN linearity. Make your simulations or build these circuits and you will see.

The amp shown here does not need even these 0.1ohm collector resistors, they can be omitted.

The N and P halves non-equality is a problem, that is solved by current injection into inverting input node. This is a CFB amp with voltage input and current output. I understand, it is not a traditional VFB voltage input/voltage output amplifier.

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Pavel
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Old 25th September 2007, 10:37 PM   #18
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Pavle,

Quote:
. . ., this amp has current output (in the other words, very high output impedance). That means that its output voltage changes with load impedance.
I understand that - or at least I think that I do. However, I believe that in my particular application this is an advantage, please correct me, if I am making an error in my reasoning below.

I am using a compression driver on a constant directivity horn. Such a combination results in sound pressure level corresponding to that of a plane wave measurement of the driver. Now, a plane wave measurement shows, that past certain frequency, a compression driver exhibits a monotonically decreasing sound pressure level with increasing frequency - first due to influence of voice coil inductance. At the same time, the impedance of the driver is increasing with frequency - monotonically within a portion of the operational bandwidth.

Therefore, if, as I understand it, the amplifier provides current proportional to input signal, the product of the current and the impedance will compensate the sound pressure level, due to the opposite slopes of the sound pressure level and driver impedance curves.

This is, how I modified your headphone amplifier - I am forcing a proportionality between the input signal and output current.

Kindest regards,

M
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Old 25th September 2007, 11:26 PM   #19
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Pavel,

A clever design. Can it be satisfactorily used with full range speakers of high sensitivity, of the type used with SETs?

Given the wild impedance changes of dynamic speakers, I agree with you, but this might be suitable with very high voltage supplies for use with electrostatics, no? There are now very high voltage devices - particularly mosfets - which might be useful for the output devices.
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Old 25th September 2007, 11:36 PM   #20
anatech is offline anatech  Canada
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Hi Pavel,
Good circuit. In a signal stage it can provide good linear gain as you've diagrammed, instead of driving speakers. Current sources for the top and bottom resistors supplying the inputs. Match the gains closely and connect them together thermally.

Been playing with these beasties for a few years now. It's fast, great detail and the DC offset is well controlled as configured as an output stage. I am trying to use much less bias current than you do though.

The best part is that it's simple enough to wire up and play with.

Hi Hugh,
I don't know that I would run regular dynamic speakers from a current source. Something with a smoother and predictable response might be suitable.

-Chris
Edit:
Hi Pavel,
Have you tried connecting R15 (ground side) to your output instead? I am sure you have.
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