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Old 22nd November 2003, 06:31 PM   #1
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Default bias trouble in pnp output stage

I have a bunch of 2n6109's I got off Ebay for about a penny a piece and had intended on buying some 2n6292 to go with them but now it doesn't seem cost effective.

So I decided to build an amp with a pnp output stage using the 2n6109's and it has turned out very well except when I use a 6.8uf (or any value) cap across the vbe multiplier it's impossible to adjust bias or turn on the amp with out first removing the cap. I would like to use the cap do to it's frequency compensation and I believe I can even hear an audible difference in quality but this could just be me.

Here is the Schematic http://www.easyamp.com/Schematic/pnpamp.gif

The problem is the bias just shoots to .3 ~ .5 A between the final output stage when I adjust the pot from zero bias up and I do it super slow and bam right up the current goes, with no warning or a rise at all in current it just shoots up. If I have trimmed the bias correctly then add the cap it works fine but when I power off and then power up again same thing happens the bias current goes up to few hundred mAís and at first I thought it must just be a flawed design and was oscillating but it wasn't. So I poked and prodded but I can't figure it out.

Iím not an expert by any means and just recently stopped blowing up transistors in my face every time I sat down. So if you find something wrong I would greatly appreciate your input. Iím actually very impressed with this amp itís simple but sounds great and produces a good amount of clean power considering the output devices.

I havenít done any real tests on this amp but it does just start to audibly distort at 10v into a 4ohm resistor from a +20/-20 supply built from a 25v center tap Radio Shack transformer (it was just the closest thing to my hands when I needed some power) which is about 12 or so watts RMS.

Sorry, I know Iím long winded and I ramble but if you think of anything that might solve my bias issue Iíd appreciate your help.


Thanks
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Old 22nd November 2003, 11:13 PM   #2
Eva is offline Eva  Spain
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Q25 is a potential source for various problems :

- Instead of connecting Q25:E, Q22:C, Q31:C directly to the output rail try connecting them toghether and then connect them to te output rail through a 0,22 ohm resistor similar to the ones used in series with the emitters of output transistors, this should make biasing easier and enhance the linearity and frequency stability of the higher side of the output stage

- A resistor should be added between Q25:C and Q22:E or to the positive supply rail in order to bias Q25 properly with 1mA or more to have it allways turned on and with decent hfe and Ft before Q22 turns on

- 2N3904 has a Vce breakdown voltage of 40V and with +-35V supply this means it may break down and start to pass currenent at any time when output voltage goes more than 5V negative, I think this may be causing the circuit to start distorting the output badly at +-10V, I suggest using something fast and rated at 100V or more, like BD139 or similar

- Q25 makes the high side of she output stage perform as a triple darlington in terms of current gain and frequency stability so you should add an adittional current gain stage to the lower side [another PNP emitter follower similar to Q25, ie : BD140 or similar] in order to get some open loop current gain symetry between high and low side, after this you may have to change frequency compensation capacitor

I would also replace Q6 and Q28 by BD139/BD140 or something faster, with lower capacitances than TIP31C/TIP32C and capable of withstanding 2*35V or more

I suggest adding two resistors in series with Q4:E and Q5:E respectively to have some control over the gain of the long tailed pair and to give it some range of linear operation, resistors in the range 22-220 ohms are usual [gain can be adjusted by adjusting the value of these resistors and modifying bias current through them]

Also, the problem limiting output to +-10V may be parasitistic oscillations, they are fully dependent on output voltage and current so your amplifier may be stable giving +-6V +-1A but unstable giving +-12V +-2A and it may oscillate only when sinking current or only when sourcing it, you have to test stability carefully with oscilloscope and diferent loads from 0V output to clipping

A good design should not oscillate even when coming out from clipping with various loads
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Old 23rd November 2003, 02:04 AM   #3
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Default Much to think about

Thank you very much Eva for your very detailed response.

I new there was a problem with my design but you have given me much to think about and detailed points to look at.

I did the math on this circuit and although I didn't leave much wiggle room I didn't have any voltages crossing any of n2390* devices higher then 35.5v.

I believe adding the resistors you have mentioned will add much stability to the circuit and this maybe what it needs but the circuit I displayed does works fairly well and sounds good.

I have switched to a 35v +/- supply as described in the schematic and now can get 18v out into a 4 ohm load with marginal distortion.

I will try your recommendations and retry a compensation cap across the vbe multiplier and see if that doesn't fix the problem.

Thank you for your help.
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Old 23rd November 2003, 02:12 AM   #4
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Default Much to think about

Thank you very much Eva for your very detailed response.

I new there was a problem with my design but you have given me much to think about and detailed points to look at.

I did the math on this circuit and although I didn't leave much wiggle room I didn't have any voltages crossing any of n2390* devices higher then 35.5v.

I believe adding the resistors you have mentioned will add much stability to the circuit and this maybe what it needs but the circuit I displayed does works fairly well and sounds good.

I have switched to a 35v +/- supply as described in the schematic and now can get 18v out into a 4 ohm load with marginal distortion.

I will try your recommendations and retry a compensation cap across the vbe multiplier and see if that doesn't fix the problem.

Thank you for your help.
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Old 23rd November 2003, 02:46 AM   #5
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Default OOPS

I dont know.
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Old 17th December 2003, 06:36 PM   #6
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Default Revised quasi pnp

Have tinkered and tested my original design which now only resembles it in it's pnp quasi output stage.

http://www.easyamp.com/img/revised-quasi.gif



I only have the student version pspice so I was unable to put it all together and simulate all parts at the same time, so I had to build it to test and other than adding a 30k pot in place of r5 and 2k in place of r9, r10 it worked.

Although this amp is impractical to actually produce from the use of pnp's instead of npn's it is now stable enough and with enough power into 4 ohm loads to be used as sub amp.
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