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Old 2nd February 2013, 09:44 PM   #791
smms73 is offline smms73  Portugal
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Kean , the PSRR of this type of circuit is primarily determined by the current sources connected to the rails. And is normally 80db .

I prefer this type of connection over the baxandall pair, because in this way the helpers transistors have a higher Vce and because of that also have a higher Hfe. And the higher the Hfe of the helper transistors the higher the atenuation of the Early distortion in the base current from the output transistors. And also at 0.65V that the baxandall helper transistor works a little variation in vce will produce Early distortion in the helpers themsef.
Yes I have expriment with baxandall but i get better distortion cancel with my type of connection.

I do not know what is the circuit that you refer, can you please comment on this one.
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Old 3rd February 2013, 02:16 AM   #792
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Do you know what the difference in Hfe really is? For the BC5xx and BC3x7 it is very small because quasi-saturation is almost absent. You have the prototype built, why not test it?

I see in your schematic you have the output drivers referenced to the rails which is not true for my schematic. I use a better CCS. This way rail modulations don't modulate the input stage bias.

As I understand you referenced the drivers to the rail to effectively cascode the CCS. However this leaves the input stage open to rail variations. If you decouple R15 with a lytic and delete R10/R24 decoupling, and then improve the CCS, you will lave less overall Iq modulation which I think will result in more PSRR.
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Old 3rd February 2013, 03:28 AM   #793
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Kean yes you right I have referenced the drivers to the rail to make the ccs simpler.

I am aware of the problem of the input stage vulnerability, but that is just a problem if the output impedance of the DAC is low. the pcm1794 have a output impedance higher than 200k , that is the dac i use, there is no PSRR decrease at that impedance.

But as you said the matter is of simple resolution, I wanted to keep the design simpler because the people tend to prefer simpler designs

I have a more complex design that i don´t post here that have a diferent input stage and a better ccs .

About testing the baxandall pair in the output , I only have the capacity with the equipment that i have to measure distortion to 0.0003% , and even this simple I/V converter have less distortion than that .
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Old 3rd February 2013, 03:39 AM   #794
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Here ya go. BC5xx or BC3x7 at 0Vcb is no object.

Mpp
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Old 3rd February 2013, 03:56 AM   #795
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Ok, the bc327 is is better that what i thought, I will try the baxandall pair, but my efforts are for improve the input stage , as i am tryng to design a I/V conv. for the es9018 that have a output impedance of less than 200 ohms.
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Old 3rd February 2013, 04:03 AM   #796
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Quote:
Originally Posted by keantoken View Post
If you decouple R15 with a lytic and delete R10/R24 decoupling, and then improve the CCS
Is simpler to improve the input stage to make it immune to voltage variation in the rail , is better to leave the ccs cascoded by the output stage. Believe-me .
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Old 3rd February 2013, 04:19 AM   #797
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One thing that also increase at low Vce is the value of the parasitic capacitances, I can not find the values for bc327.
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Old 3rd February 2013, 04:33 AM   #798
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The spice model should have accurate values for Cje and Cjc since it was made by Phillips. The OnSemi datasheet shows capacitances:

https://docs.google.com/viewer?url=h...%2FBC327-D.PDF
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Old 3rd February 2013, 04:53 AM   #799
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Sure, i posted on the MPP thread the same. Vce goes down, parasitic capacitance goes up.
That also gives the oportunity to measure Cob simply with a DMM from base to collector.
It is likely that the DMM measures with low voltage so we get a worsed case scenario-
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Old 3rd February 2013, 05:20 AM   #800
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If the meter measures with more than several tens of mV, the value may only be approximate. I would assume different meters would give different results because of different measurement voltages.

One way to circumvent this may be to use a capacitor in series with the transistor junction in question, and then determine the junction capacitance based on series capacitors. The capacitances in series will share the measurement voltage with the larger capacitance having less voltage. This would be easy for power outputs with several nF capacitance. However for TO92 devices like these with only a few tens of pF, it may be very difficult to get any usable reading.
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