Why am I getting a positive phase shift - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th December 2009, 01:05 PM   #1
sigpl is offline sigpl  Europe
diyAudio Member
 
Join Date: Dec 2009
Default Why am I getting a positive phase shift

Hi, new at the forum taking up an old interest.
Spending my xmas holidays sorting a shoe box full of old circuit sketches.
Also, came across the TopSpice simulator which I downloaded and just for fun tried out on some of my old circuits.

This one I have problem with. Can't understand what is causing the positive phase shift making it an 25 Mhz oscillator. Don't mind the (missing) details, just look at the idea. Neither mind the component choise, it is what I had to play with in the demo version.

Q3 and Q4 is a standard diff amp. The load, Q6 and Q7 is a standard current mirror leaving ( / taking) its excess current into Q9 and further into Q10 for amplification. Since Q10 and Q11 share a current source, the R5 will create a voltage swing for the output, from the current swing in Q10.
Attached Images
File Type: png circuit.PNG (21.6 KB, 183 views)
File Type: png phase.PNG (50.4 KB, 166 views)
  Reply With Quote
Old 27th December 2009, 02:49 PM   #2
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
is compensation one of the missing circuit details?

I really wouldn't expect that many cascaded gain stages to stable without explicit Cdom

also never start with .AC analysis - always demonstrate that a realistic operating point has been found with a .TRAN and "debug" based on .TRAN circuit values
  Reply With Quote
Old 27th December 2009, 03:00 PM   #3
sigpl is offline sigpl  Europe
diyAudio Member
 
Join Date: Dec 2009
No, compensation is not one of the missing details.
The compensation would of course had made it if I just had the normal negative phase shift and making the gain hit unity before the phase hit -180 deg.

All biases are OK.
  Reply With Quote
Old 27th December 2009, 04:44 PM   #4
sregor is offline sregor  United States
diyAudio Member
 
Join Date: Jan 2006
Location: massachusetts
If you follow the signal through, the out put is inverted (180 degrees) from the non inverting input. the signal is inverted at the collector of q3, the non inverte4d at the collector of q9 and emitter of q10, then inverted at the output stages. Many ways to correct this - I would reverse the current mirror on the input. Also, as mentioned, no compensation.
__________________
Steve
  Reply With Quote
Old 27th December 2009, 05:07 PM   #5
sigpl is offline sigpl  Europe
diyAudio Member
 
Join Date: Dec 2009
Thanks for your idea to reverse the current mirror, but sorry, the same result.

Compensating, yes, but how? Since my positive phase appears already at about 50kHz a compensation had to start at a redicously low frequency. So to really know where to compensate, my question "why" is still valid.

There are of course plenty other methods to create the voltage swing, but this caught my interest since it gives a really nice transient answer, due to the current controlled current amplification.
  Reply With Quote
Old 27th December 2009, 06:46 PM   #6
godfrey is offline godfrey  South Africa
diyAudio Member
 
Join Date: Nov 2009
Location: Cape Town
I don't know where the positive shift comes from, but maybe you can stabilize it with a capacitor between collector of Q11 and base of Q9 to set the dominant pole. Something like 100pF - maybe more.

There's huge open-loop gain in that circuit though, so it could be hard to tame.

Emitter resistors for Q3 and Q4 (say 100R each) would help a few things:
a) Better input-stage linearity.
b) Higher open-loop input impedance.
c) Allows you to reduce the compensation capacitor mentioned above at least 10-fold (maybe to 10pF), giving much better slew rate.

btw: Why are R11 and R12 mismatched?

Also: R11, R12 and R13 probably have nasty effects at HF. Reducing or getting rid of them might help.

To get a better idea of what's going on, try modeling it open-loop i.e. with the feedback disconnected.
  Reply With Quote
Old 27th December 2009, 08:03 PM   #7
sigpl is offline sigpl  Europe
diyAudio Member
 
Join Date: Dec 2009
Thanks godfrey, the Q3, Q4 emitter resistors did the trick. Do you know why ? Always nice to learn the reasons behind. Maybe there wouldn't have been a problem if I had used a real current source (with limited input impedance) instead of the ideal one. I will test this to but then I have to use another SPICE copy. I have downloaded the LT Spice and it seems a lot better, allowing me to use zener diods too.

The R11 , R12 mismatch is due to the fact that a current mirror is not totally symmetric and the resistanse diff is needed to zero-bias the output. In reality there is a pot there to trim the output offset (typical solution).

As mentioned initially this was just a principal solution. Maybe I will use it in my upcoming MOSFET power amp project. I am testing out a few different solutions for each stage.
  Reply With Quote
Old 27th December 2009, 08:41 PM   #8
sigpl is offline sigpl  Europe
diyAudio Member
 
Join Date: Dec 2009
Maybe I was a little to quick there. When adding the emitter restistances I got totally out of DC bias on output. When correcting this I am back with the positive feedback.
  Reply With Quote
Old 27th December 2009, 09:04 PM   #9
godfrey is offline godfrey  South Africa
diyAudio Member
 
Join Date: Nov 2009
Location: Cape Town
Quote:
Originally Posted by sigpl View Post
The R11 , R12 mismatch is due to the fact that a current mirror is not totally symmetric and the resistance diff is needed to zero-bias the output. In reality there is a pot there to trim the output offset (typical solution).
The mirror's probably good enough. Most of the DC offset will be due to Q4's base current flowing through R14. If you use ac coupling e.g. as shown, then the voltage drops across the 39K resistors will match to some extent to improve DC offset. You need Q3 and Q4 to be fairly well matched, though.

Your 10mA LTP current looks quite high, especially with R14 = 39K. The voltage drop across R14 must be quite high - maybe a volt or so? Reducing the current and/or resistance would help the DC offset a lot. Remember even if you trim it right, it will drift with temperature.

I don't like the idea of trimming the mirror because that will change the gain as well, and also cause distortion - it's much better to adjust so Q3 and Q4 have equal collector currents. I included the 1K resistors as stupid-proofing in case the trimmer goes open-circuit.

The 100pF is not just for RF filtering. It's main job is to keep Q3's base at low impedance relative to earth at high frequencies, especially when nothing is connected at the input. Otherwise you risk oscillations again.

I'm off to the pub now for a quick one before closing time
Will post about the HF stuff later (sobriety permitting) or tomorrow.
Attached Images
File Type: gif ckt.GIF (12.1 KB, 118 views)
  Reply With Quote
Old 27th December 2009, 09:41 PM   #10
sigpl is offline sigpl  Europe
diyAudio Member
 
Join Date: Dec 2009
Yes you are probably right about the mirror and the base current on Q4. The 10 mA was not intentional, just left there since earlier simulations. Changed down to 2 mA. But the positive phase is still there.

Thanks for your effort, have one beer for me too. I'm headed for the sofa watching snooker on Eurosport.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Phase Shift PWM Andrew Eckhardt Class D 6 14th November 2010 07:56 AM
phase shift of second order filter bondo Multi-Way 9 8th April 2009 02:24 AM
Phase shift smps topology darkfenriz Power Supplies 12 10th August 2008 11:30 AM
Zero phase-shift gareth Solid State 11 9th May 2008 04:45 PM
Creating positive phase shift? Pabo Class D 31 14th January 2005 06:44 AM


New To Site? Need Help?

All times are GMT. The time now is 09:51 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2