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Old 5th December 2009, 10:58 PM   #11
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Also, Godfrey, I find your comments on transistor linearity interesting. Are you interested in sharing thoughts?

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Old 7th December 2009, 02:03 PM   #12
godfrey is offline godfrey  South Africa
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Hi keantoken

Actually, I was hoping you could shed some light after your experiments with modeling

Anyway Andy seems to have explained it very nicely, although I didn't understand anything he said. OK, I didn't try very hard since I'm not overly interested in modeling / models. Maybe I'll get back to that when I'm bored sometime.

I was shocked at the results of my little investigation into differences between manufacturers, though. I always assumed a 2n2369 is a 2n2369 and it doesn't matter whose datasheet you look at. Now I'm wondering if solid-state is going to end up like tubes.

You know the kind of thing you see on tube threads:
" ... The best are the Sovtek tubes made between about 1958 and the early sixties, but if you can't get those NOS, some of the GEC tubes are good too, but without quite the same midrange warmth ... " or whatever.

As far as linearity goes, I'm not sure how much wisdom I can contribute, aside from the obvious, but here's my ideas anyway:

In the pictures below, biasing at (A) will be almost perfect with very little distortion because the graph is flat and horizontal.

(B) will give 2'nd harmonic distortion because of the slope.

At (C), the graph is horizontal again, so there won't be any 2'nd harmonic, but now you will get some third harmonic distortion because of the curvature of the graph.

Between (B) and (C) then, you have a choice of maybe 1% second harmonic vs 0.1% third harmonic (I haven't done the calculations, but you get the idea).

In an actual circuit, your choice of collector current will largely be determined by other factors anyway e.g. the VAS has to be capable of at least X mA swing, while input stage current can't be too high due to DC offset and noise considerations.

You can still choose transistors, though, that give good performance at collector currents near what you need (or you would be able to if if you had any idea of their characteristics, without actually measuring the #### things individually ).

Time to take a break. I'll be back with more thoughts later.

Cheers - Godfrey
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Old 8th December 2009, 03:10 AM   #13
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You're thinking the same thing as me, Godfrey!

According to some posts, I've heard it said that Ise is the parameter that varies the most between components, so maybe you can get an idea if you set up a virtual Hfe tester in SPICE and screw with Ise.

Also, I think I'm seeing an increasing trend of PNP's becoming better in general than NPN's, though the NPN's still get the corner on pure Hfe.

Now, regarding Hfe and it's affect on distortion:

There are two factors to consider:

1: I(Rbe). This is the current that goes through the resistor usually placed across the B-E junction in the common VAS configuration.

2: Ib. Base current.

Both these factors are their own distortion mechanisms in this case and should be considered as separate.

Vbe is exponential/square/whatever vs. Ic, which means it contributes mainly monotonic harmonics. (However, when driven too far Rb and other factors cause it to turn upward) (monotonic means that the harmonics decrease in a straight line with frequency)

Ib is just as is presented by Godfrey.

The combination of I(Rbe) and Ib makes up the harmonic spectrum of the distortion. With lower gain transistors, Ib can be up to as much as I(Rbe) (typically, at least). With higher gain transistors, Ib can be so small compared to I(Rbe) at AC that it doesn't have much of an affect. My preference is to bias below the Hfe hump point, which helps avoid strange combinations of harmonics (A monotonic distortion profile where all harmonics are in a downward line, also seems to be an indicator of a well-performing amplifier, one that can't be expected to have hidden flaws and odd issues).

This is what I've observed, though nothing is 100% true and some of it is downright lies.

As for my modeling experience, I don't really have any insight into real-world transistor variations but it seems by Andy_C and Sawrey's comments that both types of transistors exist and, judging by the datasheets, some manufacturers make better devices of the same number. This is disturbing but I've seen no evidence to suggest otherwise.

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Old 8th December 2009, 03:18 AM   #14
godfrey is offline godfrey  South Africa
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There's other distortion mechanisms to worry about as well.

Non-linearity of Vbe vs collector current is important, but it's predictable and unavoidable so you just have to deal with it - there's no magic there.

Variations of Gain Bandwidth Product with collector current will cause some distortion. For the graph below, you'd get the same kinds of distortion at various bias points as with the same shape graph for HFE, but at a much lower level, even at high audio frequencies. It's probably not worth worrying about but I included the picture anyway so we can all admire Fairchild's spelling. (Gandwidth )

Something that DOES bug me is variations of bjt parameters with collector voltage, especially collector-base capacitance.

The second picture shows Ccb of about:
5V - 3.6 pF
30V - 1.8 pF
55V - 1.5 pF

So in the simple VAS shown, the total feedback capacitance is about 49pF, but when the output swings +-25V, it varies from about 48.5 to 50.6 pF.

That's about a 4% variation in the capacitance that sets the open-loop gain of the amp. Ouch! Distortion will be bad. Also important is that the capacitance variation is very non-linear (despite what the graph looks like - check the numbers above), so it's not all nice 2'nd harmonic distortion either.

You can keep the non-linear capacitance out of the feedback path with e.g. a cascode or baxandal super-pair, but for ultra-low distortion, you might want to worry about the output still having to drive that non-linear capacitance. That's a lot harder to deal with!

The non-linear input capacitance of the output stage is easier to deal with, using e.g. some kind of diamond.

btw: If you haven't seen it yet, GK's got a nice thread dealing with VAS linearity here: My folded-cascode VAS linearity investigation

Cheers - Godfrey

edit - oops, I see you just posted - I'll read yours now
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Old 8th December 2009, 03:28 AM   #15
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by keantoken View Post
1: I(Rbe). This is the current that goes through the resistor usually placed across the B-E junction in the common VAS configuration.
I'd prefer to drive the VAS from a current mirror to get rid of this effect. It also gets you higher gain (fwiw), and less variable DC offset. Of course the mirror has to be decently linear.
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Old 8th December 2009, 03:51 AM   #16
godfrey is offline godfrey  South Africa
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Hi

I decided to learn something about Spice models and found this: excel

Apparently it's a spreadsheet to derive BJT Spice parameters from datasheet info. Haven't tried it yet, but it looks interesting.
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Old 8th December 2009, 04:12 AM   #17
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Andy_C used that spreadsheet to work on his MJL models. I haven't figured out how to use it, mostly because I don't have MSE.

I'm looking at that thread... If you attach it to the right OPS, it should do some miracles...

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Old 11th December 2009, 06:30 AM   #18
godfrey is offline godfrey  South Africa
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Oops, just realized I got it all wrong in post 12. I think the comments there apply to graphs of hfe not hFE.
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Old 11th December 2009, 08:43 PM   #19
andy_c is offline andy_c  United States
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Quote:
Originally Posted by keantoken View Post
Andy_C used that spreadsheet to work on his MJL models. I haven't figured out how to use it, mostly because I don't have MSE.
Here's a bit of history on that. I did the MJL models back in October of 2006. At that time, I found an Excel spreadsheet at the Analog Services web site that gave me the idea of using Excel like he did. Back at that time though, the Analog Services spreadsheet was very simple, and only computed the parameters required to simulate beta vs Ic. So I used a technique equivalent to what was used on the 2006 Analog Services spreadsheet to compute only those parameters.

Since then, it looks like the Analog Services spreadsheet has become much more comprehensive than it was three years ago, computing many more parameters. Also, I've developed my own ways of using Excel to compute the other parameters besides the ones needed to simulate beta vs. Ic. This involves using Visual Basic for Applications (VBA) to compute some of the formulas that are too complex to put into a spreadsheet cell. I'm not sure how the Analog Services guy is doing this nowadays, as I've only glanced at his latest spreadsheet.

So, what you're saying was true back then, but things have changed in the meantime such that we are doing things differently now.

All that having been said, there really is no "turnkey" method for computing all the parameters. One reason is that all the data needed to compute the SPICE parameters is not present on any datasheet, so one must do some educated guessing to determine some of the parameters. Further, each datasheet seems to provide data that are incomplete in a different way from those of other devices. Therefore, each device ends up being treated a bit differently, wherein one tries to make the best of the incomplete data.

There are also a number of annoying limitations of Excel when trying to use for this purpose, but I won't get into that at the moment. The process still ends up having a lot of manual procedure mixed in.
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Old 31st December 2009, 02:19 PM   #20
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally Posted by keantoken View Post
I've been thinking for some time a curve tracer would be a worthwhile investment.
Don't be tempted. I have a Tek 571 and I know of no other piece of test equipment that even comes close in unreliability. The thing has let the smoke out of its collector supply three times (once with a rather bright bonfire) and now its (obsolete) video microcontroller (running unknown code) has died.

However, in the brief moments when the blasted thing was working, I established a few things. PNP transistors always show Early effect far more than NPN transistors. hfe for those MJL beasties genuinely is more constant with current than anything else I've seen. SPICE models do not tell you about those charming little quirks that happen at low VCE.
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