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Old 2nd October 2012, 09:20 PM   #1041
wahab is offline wahab  Algeria
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In a previous post i adressed this topology s front end extreme sensitivity
to loading but it seems that it didnt attract much attention...

Anyway , unless the front end i simulated has some nasty flaw ,
the simulations show how much of a problem it can be as soon
as a compensation is implemented.

It s not all to compensate , one must check if the high frequency gain
is effectively used for local NFB or if it is simply dumped by the loading
effect of the compensation network.

To check the thing i simulated the uncompensated OLG and then
loading the FE output with either a resistive or a capacitive load,
the circuit remaining uncompensated so the variation of gain due
to loading can be checked.

Below are the schematic used for sims as well as the graphs
for resistive and capacitive loadings OLGs.
Attached Images
File Type: gif TIS FE.gif (12.2 KB, 238 views)
File Type: gif TIS FE OLG CAPACITIVE LOADING.gif (30.8 KB, 234 views)
File Type: gif TIS FE OLG RESISTIVE LOADING.gif (28.3 KB, 215 views)
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Old 3rd October 2012, 01:22 AM   #1042
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Look at ho those transistors behave at lo Vce.

http://www.datasheetcatalog.org/data...or/mXwvryv.pdf

Much inferior to the BC550C or BC337. At 1V Vce the collector impedance ill be abysmal in the current mirror and the Baxandall drivers. Use Cordell's BC550C/560C. You can save those expensive high-voltage japanese transistors for the LTP and Baxandall outputs here they are useful. You NEED lo-Vce transistors for lo-Vce situations like this; high-voltage transistors ork best for high-voltage but ill just saturate in a cascode or current mirror.

The output impedance and loading derive naturally from the input degeneration and Gm, it is a transimpedance stage after all. It is intentional. Any drabacks ill be made up for in other parts of the amplifier. Your LTP degeneration is 100R, Edmund uses 10R IIRC, hich ould make a difference.

See if these changes make an improvement. Actually I don't kno, but I've pointed out some things and am curious about the results.
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Old 3rd October 2012, 02:04 AM   #1043
wahab is offline wahab  Algeria
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Using 2SC1845/2SA992 ,wich are better than the BCXXX , slightly
improve the thing but still a 10pF load has dreadfull effect on the
high frequency gain while resistive loading allow 100dB OLG
with 1 Mohm load , the gain decreasing as about a linear function
of the resistive load reduction for values lower than about 10 Mohm.
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Old 3rd October 2012, 02:12 AM   #1044
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Did you look at the Vcesat graphs or did you just assume the C1845/A992 are superior because of other specs? I tried them in real life eith my Kmulitpliers. The BC337-25 is best, the BC550C second-best. I don't kno about other gain groups. Again, lo-Vce transistors are best for lo-Vce situations. The 2N5551/5401 are commonly used for current mirrors, but certainly not optimal for that purpose.
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Old 3rd October 2012, 02:40 AM   #1045
wahab is offline wahab  Algeria
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They have lower VBE than the BC550 , the BC337 being a medium power
part not used in small signal but rather in low power amplifiers paired
with its complementary BC327 , while all these transistors have low
VCE max not compatible with a front end that has about 100V
supply voltage.

For the sims i reduced the degeneration to 10r , wich seems
to me quite low considering the currents flowing through
the differentials , at about 5.5mA total current.

Quote:
Originally Posted by keantoken View Post
Look at ho those transistors behave at lo Vce.

http://www.datasheetcatalog.org/data...or/mXwvryv.pdf
.
Did you notice the base current values.?..

Last edited by wahab; 3rd October 2012 at 02:48 AM.
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Old 3rd October 2012, 03:43 AM   #1046
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If you recall I said to save the high-Vce parts for the LTP and Baxandall outputs. So Vcemax as never an issue. I meant to use lo-Vce transistors for the current mirrors and Baxandall drivers here Vce never goes above 1V.

I am talking about mirror output impedance, not mirror imbalance. The C1845 datasheet at 6uA Ib shos a slope of about .5V/1mA hich is the definition of a collector impedance of 500R belo 1V Vce. This is equivalent to 500R shunting your current mirror output. This is because of quasi-saturation. You ill notice that the BC560C and BC337 datasheets sho virtually no quasi-saturation. These transistors have much higher collector impedance at lo Vce. The difference for my Kmultipliers is beteen 48db and 66db of input rejection, or a factor of 8. From a PSRR of 250 to 2k.

Since in this design the current mirrors have a lo-impedance load (Baxandall emitters), raising the 500R mirror impedance may not do much. It may be more critical for the Baxandall drivers hich have just .6V Vce. In a traditional LTP+CM design it can matter a lot because the input of the VAS is a high impedance load. In this position the aforementioned transistors can be a significant improvement over the C1845 or 2N5551.

Hoever most BJT models do not model quasi-saturation. You ill need to check if your models do, otherise they ill not sho you these things.

As a transimpedance stage this circuit has a defined constant transimpedance. The 10R degeneration makes this transimpedance even more constant. They reduce transconductance to about a third. Beyond that I have no explanation offhand for the values.
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Old 3rd October 2012, 04:10 AM   #1047
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Sorry for being testy. I haven't slept for 2 days.
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Old 3rd December 2012, 11:16 PM   #1048
Waly is offline Waly  United Kingdom
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I suppose this thing is dead? Nobody is picking up and give it a shot on a breadboard or PCB?

Another idea destined to remain in the simulator only? I would like to try it myself, but can't afford... too bad.
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Old 4th December 2012, 01:00 AM   #1049
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I am still interested. Just been busy with the speakers it is intended to power. Now have the compression drivers for the surrounds (JBL 2432's) and the centre channel (JBL 2453) should arrive pretty soon. Now to clear out the workshop to have the space to build it all.

Best wishes
David
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Old 4th December 2012, 04:49 AM   #1050
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by wahab View Post
In a previous post i adressed this topology s front end extreme sensitivity
to loading but it seems that it didnt attract much attention...

Anyway , unless the front end i simulated has some nasty flaw ,
the simulations show how much of a problem it can be as soon
as a compensation is implemented.

It s not all to compensate , one must check if the high frequency gain
is effectively used for local NFB or if it is simply dumped by the loading
effect of the compensation network.

To check the thing i simulated the uncompensated OLG and then
loading the FE output with either a resistive or a capacitive load,
the circuit remaining uncompensated so the variation of gain due
to loading can be checked.

Below are the schematic used for sims as well as the graphs
for resistive and capacitive loadings OLGs.
Why don't you just buffer it and be done with it?
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Amplifier Design and Construction for MUSIC! http://hifisonix.com/
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