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Old 9th May 2013, 11:56 PM   #11
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Join Date: Sep 2006
Quote:
Originally Posted by Struth View Post
Hi Guys

Yes, the idle current in the VAS is predictable and stable with the above circuit. The third Q in each mirror, Bob calls a "helper transistor'. This Q establishes the voltage on one side of the mirror. If the mirror balances, then the voltage at the output side is identical, with zero current through the 47k.

In the end, the diff tail current sets the voltage drop across the emitter Rs in the mirror. The voltage across these Rs is identical to that across the VAS emitter R, so VAS current is set by resistor ratios and the diff-amp tail current.

The same applies to the front-end of Bob's mosfet amp from 1983.

Have fun
Kevin O'Connor
Hi Kevin,

Thanks for providing this good explanation.

Stability in a circuit like this is a matter of degree and is in two aspects. First, there is the issue of VAS idle current from one amplifier to the next when no adjustment is provided for each amplifier (something one definitely does not want to do). Secondly is the temperature stability of the VAS current in a given amplifier. The matter of degree is basically referring to what one considers a reasonable tolerance due to these factors. A tolerance of +/-30% is probably acceptable for VAS standing current, but a smaller range is obviously more desirable. The bottom line is that the original Slone circuit had the VAS bias current virtually undefined, while this circuit defines it, but with tolerances.

With regard to temperature stability, if the main VAS transistor heats up and its Vbe decreases, VAS standing current will be increased. This effect is reduced in proprtion to the amount of VAS emitter degeneration. I would not recommend having less than 300mV across the VAS emitter resistor if this circuit is to be used.

Nothing comes for free. This circuit gives up some low-frequency open-loop gain in exchange for the VAS bias stability. The 47k resistor can be made a smaller value if greater stability is desired. Circuits like this are blessed with far more LF OLG than needed anyway if the output stage is a Triple.

VAS bias stability in this circuit will almost always be less than that in a single-ended current-source loaded VAS.

I should mention one more thing in connection with the current mirror "helper" transistor. I am less enamored of it than I used to be, as its use appears to cause a bit of peaking in the open-loop response when used in conjunction with a VAS preceded by an EF. AC-wise, the simple 2-transistor un-helpered VAS is better. DC-wise, for this circuit, the helper can be replaced by the same transistor diode-connected. Alternatively, one can keep the helper and bypass its base to its emitter to kill it AC-wise.

Cheers,
Bob
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Old 10th May 2013, 08:09 AM   #12
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by Bob Cordell View Post
Hi Kevin,
I should mention one more thing in connection with the current mirror "helper" transistor. I am less enamored of it than I used to be, as its use appears to cause a bit of peaking in the open-loop response when used in conjunction with a VAS preceded by an EF. AC-wise, the simple 2-transistor un-helpered VAS is better. DC-wise, for this circuit, the helper can be replaced by the same transistor diode-connected. Alternatively, one can keep the helper and bypass its base to its emitter to kill it AC-wise.

Cheers,
Bob
Bob,

This is an interesting piece of information, as I have noticed this peaking response in simulating my attempt at designing an amp. Never even thought of looking at the current helper. I am using the jfet complementary input stage detailed in your book. My VAS is cascoded and is preceded by an EF follower.

Damn it, now I have another experiment to try in LTspice. Must have spent over 1000 hours simulating this design
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Old 10th May 2013, 08:32 AM   #13
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Bob Cordell View Post

I should mention one more thing in connection with the current mirror "helper" transistor. I am less enamored of it than I used to be, as its use appears to cause a bit of peaking in the open-loop response when used in conjunction with a VAS preceded by an EF.
Hi Bob ,

Your schematic works flawlessly assuming a single VAS transistor non enhanced is used , if enhanced a voltage shift must be created with a
diode in serial with the current mirror to increase the VAS base DC
voltage biaising.

A second point is to connect the helper transistor collector to its
commanding transistor emitter , as such the helper is inserted
in the said transistor local loop and will have limited collector current ,
as when connected like in your schematic to ground it has theoricaly
unlimited max collector current , surely the reason of the peaking.
Attached Images
File Type: gif AWB55 CM.gif (8.1 KB, 254 views)
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Old 10th May 2013, 01:17 PM   #14
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Quote:
Originally Posted by wahab View Post
Hi Bob ,

Your schematic works flawlessly assuming a single VAS transistor non enhanced is used , if enhanced a voltage shift must be created with a
diode in serial with the current mirror to increase the VAS base DC
voltage biaising.

A second point is to connect the helper transistor collector to its
commanding transistor emitter , as such the helper is inserted
in the said transistor local loop and will have limited collector current ,
as when connected like in your schematic to ground it has theoricaly
unlimited max collector current , surely the reason of the peaking.
Hi wahab,

I thought I addressed your first point in my last sentence, where I mentioned inserting a diode-connected transistor in place of the helper to keep the voltages the same, or alternatively to keep the mirror helper and bypass its base-emitter to make it look like a diode at HF. Did I miss something?

I'm a little unclear on your second point, as it seems to be referring to both DC and AC aspects. Maybe it's just too early in the morning here. Maybe you could clarify by citing transistor numbers used in your schematic that you attached.

Cheers,
Bob
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Old 10th May 2013, 03:43 PM   #15
danny92 is offline danny92  Portugal
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Join Date: Nov 2012
Location: Lisbon
Hello Dear Mr. Bob Cordell,
and thank you very much for your help

I can't notice any peaking in the step response of the amplifier.
Is this normal, or I have some problem with my simulations?
I've to use 2*150pF capacitors, instead of two 2*30pF, because of the phase shift caused by the large gate capacitance of the used lateral MOSFET devices (BUZ901/906 by Magnatec).
I'm a newbie at discrete desgns and I don't know the methods you're telling me to use. I'm sorry, can you post some pictures, if you don't mind.
I'm still with some doubts, I don't know if it's better to make a simple current loaded VAS to start, or something more complex such as a complementary IPS-VAS topology or a differential VAS topology.

One more question, the DC current isn't limited by the 1kohm resistor and by the vbe of the current mirror transistors?

Thank you once more for your great book, and for your models and LTSpice tutorials.

Best regards,
Daniel Almeida
Attached Images
File Type: png Complementary_IPS_bjt_MOSFET_output_test.png (34.1 KB, 220 views)
File Type: png phase_complementary_topology.png (41.2 KB, 218 views)
File Type: png MOSFET2.png (31.1 KB, 212 views)
File Type: png MOSFET_output_diff_VAS.png (36.0 KB, 156 views)
Attached Files
File Type: asc MOSFET2_Comp_IPS_VAS_Current_Mirror.asc (13.7 KB, 9 views)
File Type: txt Cordell Models.txt (11.8 KB, 1 views)

Last edited by danny92; 10th May 2013 at 03:55 PM.
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Old 10th May 2013, 05:15 PM   #16
wahab is offline wahab  Algeria
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Location: algeria/france
Quote:
Originally Posted by Bob Cordell View Post
Hi wahab,

I thought I addressed your first point in my last sentence, where I mentioned inserting a diode-connected transistor in place of the helper to keep the voltages the same, or alternatively to keep the mirror helper and bypass its base-emitter to make it look like a diode at HF. Did I miss something?
Hi Bob ,

Not sure if we are talking of the same thing here , i ll redraw the schematic
later fot this point so we can discuss it accurately.
What i m talking about is to insert diodes in serial with Q1/Q2 collectors ,
the VAS base being connected to the said transistors collectors , increasing
the voltage drop by 0.65V to make room for the enhancing transistor
VBE.

Quote:
Originally Posted by Bob Cordell View Post
I'm a little unclear on your second point, as it seems to be referring to both DC and AC aspects. Maybe it's just too early in the morning here. Maybe you could clarify by citing transistor numbers used in your schematic that you attached.

Cheers,
Bob
In the schematic the helpers are Q15/Q19 wich in your book have
their collectors connected to ground.

As you can see in the schematic above the helper transistors colllectors
are connected to the emitters of the relevant driving transistors.
As such , there will be no added current from ground to drive the
current mirrors , rather the driving current is taken from the current source
and localy controled by the differentials relevant transistors.

We can view Q2/Q15 and Q4/Q19 as being both CFPs arrangements, that
is , the helpers are fully part of input stage local loops.

Cheers,

wb
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Old 10th May 2013, 06:10 PM   #17
danny92 is offline danny92  Portugal
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Join Date: Nov 2012
Location: Lisbon
Hi everyone,

The circuits purposed by Bob Cordell attached, they worked very fine.

Thank you Bob Cordell

If you have some time, could you please read the previous post, if you don't mind?

Best regards,
Daniel Almeida
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Old 11th May 2013, 04:05 PM   #18
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Join Date: Sep 2006
Quote:
Originally Posted by wahab View Post
Hi Bob ,

Not sure if we are talking of the same thing here , i ll redraw the schematic
later fot this point so we can discuss it accurately.
What i m talking about is to insert diodes in serial with Q1/Q2 collectors ,
the VAS base being connected to the said transistors collectors , increasing
the voltage drop by 0.65V to make room for the enhancing transistor
VBE.



In the schematic the helpers are Q15/Q19 wich in your book have
their collectors connected to ground.

As you can see in the schematic above the helper transistors colllectors
are connected to the emitters of the relevant driving transistors.
As such , there will be no added current from ground to drive the
current mirrors , rather the driving current is taken from the current source
and localy controled by the differentials relevant transistors.

We can view Q2/Q15 and Q4/Q19 as being both CFPs arrangements, that
is , the helpers are fully part of input stage local loops.

Cheers,

wb
Hi whab, thanks for clarifying this.

To the first point, the base-emitter drop across the mirror helper elevates the DC voltage level on both sides of the current mirror so that the collectors of the two mirror transistors will now be at the same potential (2Vbe+VRE of the helpered VAS transistor). If the two collectors of the mirror were not at the same potential, current would flow through the 100k at rest, creating an undesirable offset. Vcb on both sides of the current mirror is 1Vbe.

I see what you mean about connecting the collector of the mirror helper to the emitter of, e.g., Q4. However, I usually like to flow some reasonably determined current through the helper (beyond the base current of the 2 mirror transistors) to keep its ft up. This current flowing in the emitter of Q4 would upset the balance of the diff pair. Connecting the collector of the helper to ground does not allow it to draw large currents in practice because the current of the associated mirror transistor will always increase sufficiently to cause self-limiting to occur (i.e., rise to be greater than the maximum current that Q4 can source, which is the tail current).

Cheers,
Bob
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Old 11th May 2013, 10:40 PM   #19
danny92 is offline danny92  Portugal
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Join Date: Nov 2012
Location: Lisbon
Hello Dear Mr. Bob Cordell,

Could verify the design I've posted previously, please?

Thank you for your attention,
Best regards,
Daniel Almeida
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Old 12th May 2013, 02:09 PM   #20
diyAudio Member
 
Join Date: Sep 2006
Quote:
Originally Posted by danny92 View Post
Hello Dear Mr. Bob Cordell,
and thank you very much for your help

I can't notice any peaking in the step response of the amplifier.
Is this normal, or I have some problem with my simulations?
I've to use 2*150pF capacitors, instead of two 2*30pF, because of the phase shift caused by the large gate capacitance of the used lateral MOSFET devices (BUZ901/906 by Magnatec).
I'm a newbie at discrete desgns and I don't know the methods you're telling me to use. I'm sorry, can you post some pictures, if you don't mind.
I'm still with some doubts, I don't know if it's better to make a simple current loaded VAS to start, or something more complex such as a complementary IPS-VAS topology or a differential VAS topology.

One more question, the DC current isn't limited by the 1kohm resistor and by the vbe of the current mirror transistors?

Thank you once more for your great book, and for your models and LTSpice tutorials.

Best regards,
Daniel Almeida
Hi Daniel,

I'm sorry I've taken so long to get to this post. It looks like you closed loop bandwidth is very small, on the order of 70kHz, if I'm interpreting your plot correctly. I'm not sure why you decided to use such large compensation capacitors with those MOSFETs. I'm not sure what value gate stoppers you used.

In general, for someone less experienced, I recommend starting with a current-loaded single-ended VAS design and really understand that first, and sharpen your skills on it. Moreover, I'd recommend starting with a BJT output stage design. The designs in Chapter 3 of my book are a good start.

BTW, in regard to current limiting, I was not clear on which 1k resistor you were referring to. Can you clarify?

Cheers,
Bob
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