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Old 21st July 2004, 02:51 PM   #1
Mambo is offline Mambo  Italy
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Default Question on output stage

I'm working on a new board and configured the output stage like the attached image; it resembles the internal layout of a darlington transistor and does'nt use the switchoff network.
Should it work well or there could be problems ?

Simulation seems to suggest a good behavior
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Old 21st July 2004, 03:21 PM   #2
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Hi,

I think you might have stability issues if one transistor of three cundcuts more current then the others, it will apply more local feedback for itself and then inturn for the others making it do all the work.

You might want to put your ballast resistors on the collector side and tie all three output emmiters to one spot and then if you need short circuit resistors on the emitters then add them after you tie all three's together. Then use only one feedback resistor per rail. I'm not sure that even this would help, you might need to just remove the 100's all together but then you just a regular old ouput stage.

Cheers
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Old 21st July 2004, 03:45 PM   #3
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Something like this.
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Old 21st July 2004, 05:21 PM   #4
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Default darlington connection

A true darlington connection like this is inherently unstable, mostly because of temp. coeffecient affecting the DC bias. Yes it is true that some amp circuits use this type of connection, but the darlington devices are integrated. A "darlinton" transistor consists of two integrated transistors that are perfectly matched with each other. If you try to use descrete transistors even of the same type, in a darlington connection you will get nonlinear operation and unstability.

If you are going to use descrete transistors for the output, then increase the power of your driver stage to handle the lower beta.
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Old 21st July 2004, 07:45 PM   #5
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@easy amp:
...cannot follow your points...
1.
Especially I think that direct parallel connection of basis and emitter, will result in poor current sharing!
2.
Stability vs oscillation is a concern all configurations, cannot answer this in a short simple sentence...

@ Cunningham:
Yes, thermal runaway has to be taken into account.
But lots of amplifiers use such output stages and with a
proper thermal compensation and resistors at the emitters, they usually work thermally stable.
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Old 21st July 2004, 07:54 PM   #6
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By the way: Integrated darlingtons have the same issues with thermal runaway.
Any BJT goes to higher current if temperature is increased.
Rule of thumb:
For compensation you have to decrease the basis emitter voltage by 2mV/C. For a darlington you need to compensate two basis emitter paths ==> 4mV/C.
If you have a complementary darlington output stage then your thermal compensation must reduce the bias voltage about 8mV/C.
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Old 21st July 2004, 08:30 PM   #7
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@Mambo:
Some points to consider:

-Your emitter resistors are very low, this makes proper thermal compensation more difficult.

-Also if the current gain is different from the original darlingtons, then the thermal compensation may show an inadequate temp gradient (current which is drawn from the output stage does affect the amount of mV/C of the compensator).
If your new assembly has higher current gain then you may get thermal undercompensation and with this thermal runaway.

-When you say:" ...does not use the switch off network..." Do you mean the reverse diodes which are usually integrated in darlingtons, or do you mean the RC-network at the output?

-General issues with oscillation:.... just wishing good luck!!

Bye
Markus
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Old 22nd July 2004, 03:41 AM   #8
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Quote:
Especially I think that direct parallel connection of basis and emitter, will result in poor current sharing!
It makes little diff where you place the ballast resistor emitter or collector they provide similar functions of curent sharring between devices. Emiter ballast are optimum for use as short circuit networks but work similarly on the collectors for current sharing.

It also was only an option for multi power devices in a darlington output, wich you hit on as being bad from the start, but a possible solution to induce better current sharing my thought was to remove the multiple local fedback lines running to a single point controlling multiple devices which is addressed with my removale of multiple feedback lines, letting a sumation of the three outputs control the LNFB.
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Old 22nd July 2004, 05:40 AM   #9
Jennice is offline Jennice  Denmark
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Mambo,

As I see it, it makes a HUGE difference where you put your 0.1 ohm resistors (collector or emitter). They should surely be on the emitter (where you have put them) when your bases are tied together at the same potential.
If they're on the collector side, you cannot compensate for Vbe differences in the parallelled power transistors, which will lead to the current flowing through only one of the tansistors, frying it.

May I ask... why not use the well tried, and reliable complelemtary output stage with a driver and n x power transistors for each supply rail (NPN and PNP push-pull stage).

With this layout, you put a rather varying load on your driver, and thus a varying load on the Vas stage of your amplifier. to me, this doesn't point in the direction of good linearity.

Jennice
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Old 22nd July 2004, 06:06 AM   #10
sajti is offline sajti  Hungary
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Default Re: Question on output stage

Quote:
Originally posted by Mambo
I'm working on a new board and configured the output stage like the attached image; it resembles the internal layout of a darlington transistor and does'nt use the switchoff network.
Should it work well or there could be problems ?

Simulation seems to suggest a good behavior
I used almost same configuration in some of my amplifiers. It can work well, with proper thermal compensation. The bias transistor mounted to the case of one of the output devices (preferably the hottest one), and the driver on separated heatsink,- to keep it cool-, can be the good solution. If any thermal runaway appears, increase the emitter resistors to 0.22 - 0.33ohms will help. But my point of view: keep the emitter resistors as small as possible!
Oscillations: I found very good solution to avoid it. Just apply small series resistor with the base of the output devices 2.2 - 4.7 ohms is good enoug. With this solution, my amplifier can drive single 10uF load without any series or parallel resistor. No overshot, no oscillation.

sajti
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