Small Signal Diode Differences

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> offset is largely determined by the bias current in the LTP. Grrrrr.

By the current-match across the pair.

If you take only one output from the pair, yes, you can "trim" the tail current. Like if your boat is bent, you may be able to ballast it to go straight. But it may be more elegant (and lower THD) to make both sides the same.
 
apzx said:
Yeah, the problem with the M100 design is that the "tail" current (that is the current flowing from the collectors of the input pair, yes?) for the non-inverting input goes through a 4k7 resistor to the negative rail and the inverting input goes straight to the negative rail. So, as the LTP is run richer with more current the balance between the two collector currents gets more and more imbalanced. I did not notice this at first.
It would be far easier to trim out offsets. 

The 4.7k and Vbe roughly established the LTP balance for 0V output when base drive of the common emitter voltage gain stage equals the class A current of the pre-output stage. The lower value 680 ohm vs 2.2k local feedback R will affect equilibrium and open loop gain. 
I looked at the JE-918 and that uses a current sink on the non-inverting input and an 8k2 on the inverting input. Granted the transistors are matched for Hfe and Vbe, and it is quoted to have an offset of <5mV. However, for playing around I decided to go a step further. So, on the M100 the current for the LTP and the BC177B on the right that looks like it sets the operating current of the output stage are all set via a 22k resistor. I replaced it with an active current sink. Then for the LTP I added a current mirror, and that really forced the current differences to bugger all. Here is my current schematic. The LTP is running around 960uA with the tail current differences being some 200pA. The output stage re-looking at it is running a bit richer than I'd like at about 3.8mA. I need to configure the Vbe multiplier to drop the current down to about 3mA, just haven't bothered to yet. This is a rather fun exercise though, and I have learned A LOT about how these little devices work on the inside, and it just gives me more appreciation for the work that goes into them. Really, very enlightening stuff!
There are other topology variants that balance easier, but is the complexity worth it?  You may want to look at audio power amp designs that mainly differ in output stage and PS. Should be more examples to look at. 

JR
 
> goes through a 4k7 resistor to the negative rail

In that configuration, counting on thumbs, the "4.7" should really be twice the 2.2k in the tail source. Since 2*2.2 is very nearly 4.7, this should come out very close and avoid driving over to the store to find 4.4k or 2.35k or whatever.

When you get that far you find that thumb-math is imprecise. The Jensen 990 got that far and then found he had to add diode(s) to force Vce equal because Early Effect left a residual mV offset on a zero-offset pair. (And it mattered because he was DC-coupling soft iron output transformer.)

Your mirror not only FORCES current match, it doubles the gain.

Douglas Self and Bob Cordell devote whole chapters to input pair topology and value-picking.
 
Just use a current source for the LTP emmiters and a current mirror on the collectors and move on. Using a current mirror not only has the advantage of balancing the collector currents which in turns lower distortion, but also, the transconductance of the LTP is doubled, the current mirror is acting as a differential to single ended coverter.

Like PRR mentioned, check out power amplifier books, with the exception of the output stage the rest is exactly the same to an opamp, both Douglas Self and Bob Cordell explain things very well in their books. Also check out books on integrated circuit design, the one by Sedra/Smith and the one by Gray/Hurst/Lewis/Meyer are excellent and comprehensive.

Also, if you haven't read it yet, you should, Nelson Pass has a good introductory article on DIY opamps, check it out https://www.passdiy.com/project/amplifiers/diy-op-amps
 

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