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Old 24th May 2013, 11:09 PM   #1
danny92 is offline danny92  Portugal
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Default Differential VAS topology

Hi everyone,

I've made this circuit, because I wanted to test the performance of a differential VAS amplifier. This topology seems to have lower distortion, and lower DC offset at the output. The major problem I've noticed with this design in particular is the lowered output excursion, caused by the Wilson Current mirrors. This design is based on one design of Bob Cordell, but it's not the same. In simulations the behaviour is very linear and has low distortions. I don't know if this amplifier with added S/C protection, DC protection and thermal shutdown could survive in the real world.

Thank you very much for your attention
Best regards,
Daniel Almeida
Attached Images
File Type: png MOSFET_output_diff_VAS2.png (35.7 KB, 315 views)
Attached Files
File Type: asc MOSFET2_differential_VAS2.asc (11.2 KB, 8 views)
File Type: txt Cordell Models.txt (11.8 KB, 5 views)
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Old 25th May 2013, 12:24 AM   #2
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I think you do not properly simulating this distortion, 0,4%-20Khz is much THD

Try this:

Spice simulation
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Old 25th May 2013, 01:44 AM   #3
danny92 is offline danny92  Portugal
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No, that distortion seems real, which is really bad, all the amplifiers I've tried to simulate have similar or worse THD performances, including a circuit directly from Bob Cordell's website, even so this is one of the designs with less THD at 20kHz.


Best regards,
Daniel Almeida
Attached Files
File Type: asc Amplifier THD 20.asc (7.2 KB, 6 views)
File Type: asc MOSFET2.asc (8.9 KB, 3 views)
File Type: txt Cordell Models.txt (11.8 KB, 4 views)

Last edited by danny92; 25th May 2013 at 01:47 AM.
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Old 25th May 2013, 02:08 AM   #4
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Try this:
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File Type: asc Amplifier THD 20.asc (7.2 KB, 11 views)
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Old 25th May 2013, 03:00 AM   #5
danny92 is offline danny92  Portugal
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Wow that gives me a THD of 4%, even higher, thank you very much for your help, the problem is bigger than I thought.

Best regards,
Daniel Almeida
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Old 25th May 2013, 03:14 AM   #6
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Yes the amp was clipping

THD is 0,5%

I made some changes, now with this THD is 0,02%-20KHz
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File Type: asc Amplifier THD 20-1.asc (7.4 KB, 5 views)
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Old 25th May 2013, 03:21 AM   #7
danny92 is offline danny92  Portugal
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Right, so you've added degeneration resistors at the LTP, and lowered the CMiller capacitor, and the result is very good, the only problem I see is the 0.1 VDC at the output, but the amplifier is really stable and has a very good phase margin, CMiller could be decreased to 68pF or 56pF and I think that the phase margin will remain acceptable.

Thank you very much,
Best regards,
Daniel Almeida
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Old 25th May 2013, 03:21 AM   #8
danny92 is offline danny92  Portugal
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Can you help me solving the same problem with MOSFET2 please,

Best regards,
Daniel Almeida
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Old 25th May 2013, 04:53 AM   #9
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I do not know the problem, got 0,023% 20KHz which is a lot for this circuit. Need to review the models Mosfets, I know little about Mosfets.

For you to loop gain, ground the input and plot the graph (right mouse - add trace) -V(Vout)/V(a) and watch your phase margin in 0 dB.
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File Type: asc MOSFET2_differential_VAS2.asc (11.0 KB, 8 views)
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Old 25th May 2013, 10:12 AM   #10
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Very interested in this topic.

Try circuit like Mongrel CX1.3VB U, simple change output stage with SF (Source Follower). And add an active CCS for Vas.
An active CCS for Vas, maybe, not necessary. Vas's Iq controlled by Iq of Ips' CCS.
And I think Wilson Mirror for Ips too.

Pic show THD @20KHz is very low.
Attached Images
File Type: gif CX1.3VB-U_schematic.GIF (60.3 KB, 233 views)

Last edited by Walkalone; 25th May 2013 at 10:25 AM.
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