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Old 22nd September 2005, 11:34 PM   #11
Stabist is offline Stabist  Slovenia
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Hi,

mpmarino - don't stop posting here - it's OK!! Actually it has much to do with my MiniA!!!

Btw - changed some things - so now insted of 0R5 i have 0R33 Source resistors ... Also additional resistors added in output sense field ...

Result:
- around 1,4A of bias (that was my goal)
- but now the difference in the dif pair is going up - it's allread 40mV - and DC offset at the output is now jumping from 30-45mV!!!

SO - I'll do something else - I have some irfp9620 I've purchased about 2+ yrs ago and they were very very good matched at that time - and at such conditions my VGS was aroun 3,70V - that's like almost idela for my present demands

I'll change those 9610 for 9620 and see what will happen ...
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Old 23rd September 2005, 12:28 AM   #12
fcel is offline fcel  United States
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Yes, post away. Next time, I'll remember to buy some spares (9610) to check out this trial and error method to see how it goes.
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Old 23rd September 2005, 12:40 AM   #13
Stabist is offline Stabist  Slovenia
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Gee ....

Now I don't understand it at all ...

I've replaced 9610 with set of 9620 - they should be matched very well - because from same matching session some of them were allready taken for A5. And it works without any serious DC offset ...
Or is that just pure coincidence?? BECAUSE - now I've measured 150mV offset and also around 150mV of difference beetwen VgsQ1 and VgsQ2 ... ?!?!? Hmm ...
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Old 23rd September 2005, 01:52 AM   #14
Stabist is offline Stabist  Slovenia
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Default Please help - need confirmation

OK, so after around 5 different try outs I'm on around 45-50mV DC offset (that's 10+mV from the original pair - but didn't mark ie ) ...

Nevertheless - time to learn something:

(ALL MARKS are refering to original A5 schematic)
If I understand right - measuring voltage drop on R11 gives me 5,5V - so current to Q3 source is 5,5/221=24,8mA
Then R14 (392R) has 5,02V drop - so current is 12,8mA - that's a bit more than half of 24,8mA - and also in real life - Q1 has higher Vgs, etc ...

To compenzate that - I have to calculate a resistance to ADD TO Q2 - correct? If I understand correct - this resistor must be put in SERIAL to Source pin of Q2? Right??

OK, and to get value - total current is 24,8mA - so I want to achieve that Q1 and Q2 distribute this equaly - that is 12,4mA.
What I also know is VgsQ1= 3,60V and VgsQ2= 3,55V - so difference is around 50mV.

SO - I take 0,05V/0,0124A = 4R - and this is desirable resistance?? Right??

THANKS
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Old 23rd September 2005, 01:55 AM   #15
jleaman is offline jleaman  Belgium
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To my reccon.. yuo dont need matched output fet's BUT the IRF610's should be.. IF you need some or help please email me..


Jason
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Old 23rd September 2005, 08:46 AM   #16
Stabist is offline Stabist  Slovenia
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It's truth that for MiniA output fet don't need to be matched; but for A30 they should be ...
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Old 23rd September 2005, 09:06 AM   #17
chip_mk is offline chip_mk  Macedonia
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Default Re: Please help - need confirmation

Quote:
Originally posted by Stabist
OK, so after around 5 different try outs I'm on around 45-50mV DC offset (that's 10+mV from the original pair - but didn't mark ie ) ...

Nevertheless - time to learn something:

(ALL MARKS are refering to original A5 schematic)
If I understand right - measuring voltage drop on R11 gives me 5,5V - so current to Q3 source is 5,5/221=24,8mA
Then R14 (392R) has 5,02V drop - so current is 12,8mA - that's a bit more than half of 24,8mA - and also in real life - Q1 has higher Vgs, etc ...

To compenzate that - I have to calculate a resistance to ADD TO Q2 - correct? If I understand correct - this resistor must be put in SERIAL to Source pin of Q2? Right??

OK, and to get value - total current is 24,8mA - so I want to achieve that Q1 and Q2 distribute this equaly - that is 12,4mA.
What I also know is VgsQ1= 3,60V and VgsQ2= 3,55V - so difference is around 50mV.

SO - I take 0,05V/0,0124A = 4R - and this is desirable resistance?? Right??

THANKS


Hi Stabist

Alephs are such design where the output DC offset depends on the input diff pair matching (like any other amplifier with diff pair at input) as well as the relation between the output MOSFET's Vgs and the the current source that supplies the input pair.

In terms of Aleph 5 it means this:

- At some voltage across R14 (let's name it Ve), Q18-Q20 bundle sinks as much current as Q6-Q8 generates. This is the equilibrium we are looking for.
- Voltage Ve present on R14 means that the current flowing through Q1's drain (let's name it I1) is I1=Ve/R14.
- On the other hand we would like to have balance between the currents flowing through Q1 and Q2 in order to minimize DC offset and to keep the pair polarized at the most linear part of the transfer curve.
- This balance can be acheived by choosing appropriate values for R11 and R14. The values given in the schematics are just approximation. Final result depends on the particular transistor Q3 and Z5 (as you know semiconductors have high spread of the parameters).
- Changing R14 would also affect the open loop gain and the frequency responce. The impact of R11 change is quiet neglectable to anything else except DC offset. Even you change the current flowing from Q3, I1 will remain essentially unchanged (in order to keep the output stage in equilibrium) but the rest goes through Q2.

In your case the disbalance is 0.8mA. Put an 6k8 resistor in paralel with R11. It will produce surplus of 5.5V/6k8=0.8mA flowing through Q3. This surplus will be absorbed completely by Q2 and the diff pair will be in balance. When the current through diff pair is balanced the output DC offset equals the difference in Vgs between Q1 and Q2 (i.e. how well matched they are).

I would not put anythig to Q2 as it would make input stage asymetric and generaly decrease the open loop gain of the amplifier.

In regard to MOSFETs matching, this is a quote from A2 service manual "We match the input devices to each other to within 0.2% and the output devices to within 2%." That means about 8mV for input devices (4V*0.2/100) and about 60mV for output devices (3V*2/100).

BTW Has your nickname something to do with Kuzma's Stabi/Stogi?

Pozdrav,
Chip
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Old 23rd September 2005, 03:29 PM   #18
Stabist is offline Stabist  Slovenia
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Default Re: Re: Please help - need confirmation

test
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Old 23rd September 2005, 03:41 PM   #19
Stabist is offline Stabist  Slovenia
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Test again ... ?!?!?
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Old 23rd September 2005, 03:49 PM   #20
Blues is offline Blues  United States
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chip_mk, if I got your explanation right...Stabist can also either tune his Aleph DC offset by replacing R11 or R14 with a trimpot say 500ohms. He can either tune with R11 so as he can have double the current through R14 or he can tune R14 to have half of the total current through R11. He can best tune either way by monitoring DC offset at the output.

Sorry to do a promotion here but the HotRod Aleph Boards feature this tuning setup...to borrow NP:
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