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Old 6th March 2014, 08:36 PM   #1
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Default Increasing slew rate and stability

The first amp that I built, class A, of course had a extremely high slew rate due to no global feedback, and the sound quality of if it is nice and crisp (highs can be clearly head and are VERY fast).

However, with the class AB amp I just finished building, the slew rate was very slow at first (about 1.6V/us when applying a 20khz square wave for 10V at the output). You can clearly here this at the amp output as the sound sounded a lot more muted and sluggish than my class A.

With a few component value tweaks, I was able to increase the slew rate to about 3.3V/us but at the cost of some stability. Any further tweaks I do such as decreasing cdom, increasing the VAS or diff pair current sources, causes the amp to become unstable. I can also put the same values in spice and it's also unstable, so it looks the same in simulation land and on the real PCB.

Where are the common places you can increase slew rate while maintaining stability? Standard AB amp with CFP outputs.

I have the stability cap on the negative rail of the CFP stage, was 220pf, tried to reduce to 18p and was unstable so I had to put it back to 220pf.
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Old 6th March 2014, 09:09 PM   #2
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3.3 V/s with a bit of a following wind for a power amp, that doesn't sound too exciting indeed.

Please show an accurate schematic.

My guess would be that you may be using an undegenerated bipolar input stage, with resulting LTP current a bit on the low side. This then gets you into slew rate trouble with the Miller cap (things are not symmetrical, I assume?). Could it be that your output stage is a tad on the slow side?
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Old 6th March 2014, 09:16 PM   #3
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Quote:
Originally Posted by sgrossklass View Post
3.3 V/s with a bit of a following wind for a power amp, that doesn't sound too exciting indeed.

Please show an accurate schematic.

My guess would be that you may be using an undegenerated bipolar input stage, with resulting LTP current a bit on the low side. This then gets you into slew rate trouble with the Miller cap (things are not symmetrical, I assume?). Could it be that your output stage is a tad on the slow side?
Don't have schematic on me, I apologize, but will post when I get home.

I don't have degeneration resistors at on the input pair, but do have them on the current source load.

I have tried increasing the current at the input stage, but that causes stability issues WITHOUT increasing the slew rate at all, so I don't think that is the issue. Increasing the VAS current does increase the slew rate considerability though, but that is at the cost of stability, and past a certain point it does nothing.
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Old 6th March 2014, 09:43 PM   #4
kouiky is offline kouiky  United States
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I don't believe that you will ever find that a high slew rate improves the sound of an amplifier. I tend to see a lot of talk about changing operational amplifiers and devices to increase slew rates. The problem is that the people involved are not fully aware that the fastest settling time obtainable from full 0dBU attenuation to nil with Redbook 16Bit/44.1kHz playback is 22uS, equivalent to a 0.85V/uS slew rate. For 24Bit/192kHz playback, it is 3.5V/uS and corresponds to a frequency about four octaves higher than the human hearing extends. Musical audio and even theatrical film audio does not make use of the full conversion rate because it would only be useful to recreate near-perfect square waves spanning the full >92dB dynamic range of digital audio, which are not components of any audio recording. Instead, we use a much smaller dynamic range in recorded audio and thus the rise time and slew rate of any recorded fast transients are reduced, too. Personally, if the amp sounds sluggish, I would be looking elsewhere.
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Old 6th March 2014, 10:15 PM   #5
DF96 is offline DF96  England
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Increasing currents will increase slew rate, but may also increase loop gain. Is is the latter which affects stability, not the former. Hence people use more current, but also more degeneration, so you get the same gain but with better driving ability for capacitance and so better slew rate.
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Old 6th March 2014, 10:21 PM   #6
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Schematic:

Click the image to open in full size.

Open loop response:

Click the image to open in full size.

Step response (0 to 2V input step, 1ps rise time):

Click the image to open in full size.
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Old 6th March 2014, 10:46 PM   #7
catalin is offline catalin  Romania
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Hello Fusion.
Your stages are weak biased now.
To increase the slew rate you need to increase the dv/dt or I/C capability of the voltage amplifier stage .This means to increase the current which flows into the Miller capacitor and also to decrease this cap to mantain it stable.You should decrease R1 to 330 ohms to increase the current through the differential stage.ALso insert degeneration for Q2 ,an emitter resistor about 1kohm ,the same thing for Q3 .After this,you can decrease the C7 to less then 33pF .Doing this you will get a more than 60V/us slew rate .
The current through Q4 is very small in your schematic ~1 mA !You need to increase it to at least 5 mA .You can do this by decreasing R3 to 120-150 ohms .
What supply voltage you have choosed ?

Slew rate test is accurate when your outputs are full swing .
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Last edited by catalin; 6th March 2014 at 10:53 PM.
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Old 6th March 2014, 10:56 PM   #8
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Quote:
Originally Posted by catalin View Post
Hello Fusion.
Your stages are weak biased now.
To increase the slew rate you need to increase the dv/dt or I/C capability of the voltage amplifier stage .This means to increase the current which flows into the Miller capacitor and also to decrease this cap to mantain it stable.You should decrease R1 to 330 ohms to increase the current through the differential stage.ALso insert degeneration for Q2 ,an emitter resistor about 1kohm ,the same thing for Q3 .After this,you can decrease the C7 to less then 33pF .Doing this you will get a more than 60V/us slew rate .
The current through Q4 is very small in your schematic ~1 mA !You need to increase it to at least 5 mA .You can do this by decreasing R3 to 120-150 ohms .
What supply voltage you have choosed ?

Slew rate test is accurate when your outputs are full swing .

Thanks, I'll give these suggestions a try and report back.

I can't really increase the current through Q4 too much unless I use some kind of vbe multiplier instead of the simple diodes due to it over biasing the output stage.

Nominal supply is around +/- 27V.
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Old 7th March 2014, 04:42 AM   #9
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After tweaking values, adding the degeneration resistors, and changing the diodes to a vbe multiplier (can't have 5mA on Q4 with diodes and bias the output devices correctly), I can still only achieve about ~20V/us. Any suggestions or improvements? Or should I just go with 20V/us?

Schematic (ignore the open loop test module):

Click the image to open in full size.

Open loop gain:

Click the image to open in full size.
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Old 7th March 2014, 07:36 AM   #10
catalin is offline catalin  Romania
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I can't see the schematic .May be you can attach somewhere else ,not on tinypic or may be here using the attach files function .
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