Some 990C Qs

[Samuel Groner]

The Jensen 990C schematic ([removed]) shows a few selected parts; I'm not sure how I should select some of them. First the parts where I think I know what do to:

* R3 is selected for correct Q1/Q2 collector current
* R4 and R5 are selected for lowest input offset voltage
* Q3 and Q10 are selected for matching Vbe (and perhaps high hfe)
* D9 and D10 are selected for correct voltage drop for the output stage bias

Now how about Q8/Q9? What is the need to select or match these transistors?

While we're at it, may I ask a few details on the circuit?
* What is the function of D3? I would have made R16 larger and skipped D3?
* How do D13/D14 improve DC offset?
* In the according AES paper, the author mentions that he chose a separate diode pair for the two CCS to avoid interaction. How would such an interaction look like?

Thanks for the appreciated answers!
Samuel

 

[bcarso]

[quote author="Samuel Groner"]
Now how about Q8/Q9? What is the need to select or match these transistors?

While we're at it, may I ask a few details on the circuit?
* What is the function of D3? I would have made R16 larger and skipped D3?
* How do D13/D14 improve DC offset?
* In the according AES paper, the author mentions that he chose a separate diode pair for the two CCS to avoid interaction. How would such an interaction look like?

Thanks for the appreciated answers!
Samuel[/quote]

Clearly Q8, Q9, D9, and D10 interact to determine the output quiescent current. As well the Qs may have been selected for adequate beta.

I'm guessing that D3 is for temperature compensation for the overall tempcos of the currents into the Q4 and Q7 collectors. There are a bunch of junctions plus the zener tempco involved.

D13 and D14 are for making the voltages at the collectors of Q1 and Q2 equal, hence equal dissipation and operating point, hence lower input offset voltage.

What would such an interaction (modulation of a common bias for Q4 and Q7 by one or the other) look like? Model it and see. For openers the big swings are apt to be at the Q7 collector, presuming a fair amount of closed-loop gain, thus modulating the Q7 base current. The static analysis is pretty easy and probably small, the dynamic probably more telling (a lot of miller effect for Q7, fair amount of self-heating shifts etc.). It is pretty tweaky but they must have convinced themselves of the advantages. I might have thrown in a cascode stage above Q7 but this would reduce output swing.

 

[PRR]

> How do D13/D14 improve DC offset?

http://www.groupdiy.com/index.php?topic=5439

Comments from idiots as well as the gurus of the 990 tradition.

To any normal standard, Q1 Q2 collector voltages do not affect input offset. But in fact, collector voltage throws-down about 1/500 in the base-emitter port. Without the added diodes, there is ~0.5V-1.0V difference between Q1 and Q2 collector voltages, which reflects about a milliVolt of input offset even when the Q1 Q2 pair is perfectly matched and equal current. In the AC-coupled world of 1978, this was neglected; with the fad for DC-coupling and 20+ years of testing hundreds of samples, the idea to shift Q1 collector voltage came up.

> R4 and R5 are selected for lowest input offset voltage

Probably for exact-equal Q1 Q2 currents; i.e. correcting for Q10 Q3 offset. That plus the D13 D14 trick "should" give vanishingly low input offset. I suppose the final trim is to null input offset, trim-out Q1 Q2 residual offset when working equal-current/voltage. But any significant residual suggests swapping-out Q1 Q2: if their currents are not equal, THD won't bottom-out.

> Q8/Q9? What is the need to select or match these transistors?

Q8 Q9 should have similar (complementary) Beta curves to minimize 2nd harmonic in low-Z loads.

The part-tolerance is large, the parts are cheap compared to the module: they may throw-out all the low-gain parts (and a few hyper-gain parts).

D9 D10 Q8 Q9 must be selected as a "set" to give the desired bias current. This is not hyper-critical; you may be able to use nearly all of the 1N914s, MJE181s, and MJE171s in a bag, but if you change suppliers you may not find any good sets and need to change suppliers again.

> What is the function of D3?

Work out the temperature drift when the 990 is jammed in a corner of a chassis driving a heavy load on a hot day. I dunno, but I suspect the Zener drifts and the double-junction compensates it.

> separate diode pair for the two CCS to avoid interaction. How would such an interaction look like?

Q7 collector swings from +22V to -22V, a 44V swing. Early Effect suggests over 60mV of equivalent swing at the Base, which would pass to Q4 and cause some variation in current.

It is a frill. But the goal is an amp with a BIG output and a precision input. In that context, it is a useful frill.

Cascoding Q7, as bcarso suggests, would also reduce the sneak feedback. But it would not save parts, and would reduce output swing a little. As I said, when you feel like a cascode, lay down a while and the urge may pass.

 

[Samuel Groner]

Thanks for the answers! Almost everything clear now!

Q8 Q9 should have similar (complementary) Beta curves

So if we would like to make things perfect, we would need to hook up the curve tracer and select the devices there? Reminds me that I have a mic pre whose manufacturer claims to do that (and yeah, they charge for it!).

Samuel

 

[bcarso]

[quote author="Samuel Groner"]Thanks for the answers! Almost everything clear now!

Q8 Q9 should have similar (complementary) Beta curves

So if we would like to make things perfect, we would need to hook up the curve tracer and select the devices there? Reminds me that I have a mic pre whose manufacturer claims to do that (and yeah, they charge for it!).

Samuel[/quote]

Thing is, you are just not likely to ever have NPN and PNP devices match all that well in all parameters. In this app, as PRR points out the equality of beta is going to be helpful since the loading on that preceding big-swing node will be determined by the betas and the output load.

Complementary DMOS outputs are getting used here and there with the appeal of no d.c. input current to speak of. But they still show an output load dependence as far as their load on the driving source, since the heavier loading reduces the bootstrapping effect on the gate-source capacitance. And getting a match between a P channel and an N channel is even less likely than matching bipolars.

As well, DMOS will in general reduce output swing unless fancy tricks are employed since they require a healthy gate-source forward bias to turn on.

 

[PRR]

> So if we would like to make things perfect, we would need to...

We would need to move to another universe.

"Matching curves" nulls one distortion. But it is still curved, just curved the same on both sides.