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Long tail pair - output offset compensation

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atferrari

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In spite I took the pain to match QA and QB for Vbe and hFE, after testing a just assembled class AB amplifier I found that the output offset is quite high: 125 mV. (The attached .pdf shows the parts of interest).

Based on the measurements I took, with 0V at the input, the reason of the imbalance seems to be the difference in value between Rin and RfA//RfB.

My questions:

1 - Is the above correct? If so, why to bother in matching the pair after all if we have to resort to compensation?

I see several possibilities to reduce the offset voltage to a small value:

2 - Using Pot A to equalize currents. Read that it is bad practice (but I know it works). Not very professional...? In fact it was the first thing I did when playing with a long tail pair, more than 20 years ago.

3 - Using Pot B (or Pot C - one of them only) to equalize the currents in the pair.

4 - Changing the value and ratio of Rf A and Rf B (avoiding to mess with Rin).

5 - Using a compensation in the form of an offset voltage applied to the bottom of Rf B provided by a low Z source.

This idea (totally new to me for a long tail pair) I found it in a project from Dick Cappels, here

I cannot avoid the feeling that matching transistors was a waste of time, vis a vis the necessity of offset compensation.

Yes, my experience in analog designs is very limited and certainly rusty.

Can anyone help with comments?

Gracias.
 
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alec_t

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1) QA has no DC base bias current.
2) Since the tail transistor base is grounded via a diode any current flow through the tail resistor (if indeed that's possible) will reverse-bias the base-emitter junction.
3) Shouldn't the tail resistor be connected to a negative supply voltage rather than to ground?
 

atferrari

Well-Known Member
Have to correct the skect - you are right

1)
3) Shouldn't the tail resistor be connected to a negative supply voltage rather than to ground?
Yes, it is wrong in my sketch. I will correct it as soon as I reach my PC. Sorry.
 

atferrari

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Correct circuit here

Had no chance to replace the original circuit with the right one, in my initial post.

Here you have it.
 
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alec_t

Well-Known Member
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Hola, Agustin
Since the tail transistor base is connected to V- via a diode any current flow through the tail resistor (if indeed that's possible) will reverse-bias the base-emitter junction. Perhaps instead use two diodes in series (or a zener) to V- from the base?
The base current of QD going through RcA contributes to the collector current difference. Have you allowed for that?
 

atferrari

Well-Known Member
Current source works OK in actual circuit.

Perhaps instead use two diodes in series
Hola Alec,

In the actual circuit there are 2 diodes as you say. It works OK; I've just shown it wrong but I managed to change my sketch showing it as a current source to avoid further confusions.

More to it: the pots, are NOT in the real circuit: they are for others to see where I COULD use them.

By now, after more than 24 hours of posting and with certain feedback received I feel more inclined to play with Rf A and Rf B to avoid (minimize, in fact) the offset.

I will revert with the outcome.
 

atferrari

Well-Known Member
Assembled and measuring output offset

This is the result of a minor modification of a circuit appearing, IIRC, in the January 2011 issue from Elektor.

After assembling, with input to ground I adjusted the output offset to +/- 2mV using the 100 ohms pot. The adjustment seems rather critical (not smooth at all) and the variations do not disappear even after one hour of being turned on.

I can see the offset wandering, at times, more or less slowly between +/- 7 mV.

My questions:

How to judge how bad is this amp regarding the output offset?

Could I play any additional trick to stabilize / reduce the offset?

Any coments are welcome.

Gracias.
 

alec_t

Well-Known Member
Most Helpful Member
How to judge how bad is this amp regarding the output offset?
Is +-7mV good or bad in your view? I guess it depends what you want to use the amp for.
Could I play any additional trick to stabilize / reduce the offset?
Ensure that the two BC550's of the matched pair are thermally coupled together so that any variation of Vbe with temperature cancels out. You could also try thermally coupling the BD139 to the BD140.
 

atferrari

Well-Known Member
Improvements

I assembled two of these in a small piece of Veroboard. Circuit attached.

The preset (not a POT as shown) is now a 50 Ohms / 20-turns one.

Q6 & Q7 in contact with a small piece of aluminium and Q1 & Q2 in tight contact to each other. Silicon grease smeared to help.

After adjusting them I measured the values shown on the right.

To adjust the offset I found much easier watching at the trace on the screen than reading the fast changing display of my DMM.

When turned on, after being some time off, one of them exhibits an initial offset of about +5mV while the other starts somewhere around -2mV. In few minutes they come to the values shown in the record.

I have the feeling that I reached a practical limit here with this design. Probably, a (tighter) PCB could improve all this.
 

alec_t

Well-Known Member
Most Helpful Member
When turned on, after being some time off, one of them exhibits an initial offset of about +5mV while the other starts somewhere around -2mV. In few minutes they come to the values shown in the record.
That suggests the thermal coupling is beneficial. Good.
 
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