Analyzing Internals of Analog Semiconductors - Mystery Component

AnalogKid

Joined Aug 1, 2013
8,706
That is indeed a current source pulling up the quasi-complementary symmetry output stage. Very common in opamps, and in most discrete audio amps. Remember, what you are looking at is not the full schematic of the part, just enough of a representation to give a designer an idea of how it behaves internally.

ak
 

OBW0549

Joined Mar 2, 2015
3,566
Keep up the good work! I fairly taught myself electronics by dissection the drawings of IC innards.
For a clue how important this is, look at this thread and see how little circuits like current generators are the building blocks you will always need in analog design.

http://forum.allaboutcircuits.com/threads/again-led-dimming-and-fading-on.108787/page-7#post-837873
Thanks.

I had pretty much the same learning experience. A pretty significant chunk of my electronics knowledge and understanding came from studying (and scratching my head over) the IC internal diagrams in National Semiconductor's datasheets, and Linear Technology's also. There's a lot of good stuff in there.

Regarding those "little circuits," IMO they're the backbone of being a productive design engineer. Anyone who does analog for a living for very long, I suspect, builds up a mental "library" of scores, or even hundreds, of those basic functional building blocks. And much of the design work I do, comes in selecting the appropriate combination of basic functional blocks and tailoring them for the specific task at hand.

And after 50 years working in the field, I'm still learning new ones and filing them away in my cluttered brain...
 

Thread Starter

MachineHum

Joined Nov 3, 2014
74
Regarding those "little circuits," IMO they're the backbone of being a productive design engineer. Anyone who does analog for a living for very long, I suspect, builds up a mental "library" of scores, or even hundreds, of those basic functional building blocks. And much of the design work I do, comes in selecting the appropriate combination of basic functional blocks and tailoring them for the specific task at hand.
I've tried analyzing it using currents and my transistor knowledge, but it turns into a huge equation that's impossible to solve and doesn't really help the understanding. Do you have a resource for understanding these building blocks? Are most of the transistors operating in their saturated region? IE either fully "on" or fully "off" or in their "linear" region, running up and down the load line?

I'm also getting into transconductance amps, so if the knowledge crosses over (which it usually does) bonus points
 

OBW0549

Joined Mar 2, 2015
3,566
I've tried analyzing it using currents and my transistor knowledge, but it turns into a huge equation that's impossible to solve and doesn't really help the understanding.
That's why SPICE was developed, because trying to understand circuit operation in the "equation domain" quickly becomes useless-- and insanity-inducing-- once you get past a couple of transistors. Linear Technology, Inc. has a free SPICE program, LTSpice IV, that a lot of people use.

Do you have a resource for understanding these building blocks?
Get a copy of The Art of Electronics by Horowitz & Hill. Download and study every application note you can find on the Linear Technology, Texas Instruments, Analog Devices and Microchip websites. Lots and lots of good information there.

Are most of the transistors operating in their saturated region? IE either fully "on" or fully "off" or in their "linear" region, running up and down the load line?
Except when the amplifier is overdriven and its output is clipping (i.e., slammed up against the positive supply rail or down against the ground rail), all transistors in that circuit are operating in their linear region.
 
Last edited:

#12

Joined Nov 30, 2010
18,210
My favorite current generator is called, "jfet" or, Junction Field Effect Transistor.
You connect the gate to the source pin and you're done. One part, one connection.
Jfets are born in the, "on" condition. You can buy them idling from 50 ua to 150 ma.
I expect they are a favorite of the IC designers.
 

WBahn

Joined Mar 31, 2012
26,319
My favorite current generator is called, "jfet" or, Junction Field Effect Transistor.
You connect the gate to the source pin and you're done. One part, one connection.
Jfets are born in the, "on" condition. You can buy them idling from 50 ua to 150 ma.
I expect they are a favorite of the IC designers.
Not if they aren't available in the process you are using. If you are designing CMOS circuits then you are very limited in the devices you can draw upon and JFETs are almost never on the list. Mostly you use enhancement mode PFETs and NFETs. You also have some ability to make resistors and capacitors. The linear capacitors tend to be very low valued and the larger ones, which use the gate capacitance of FETs, are very nonlinear. You can usually made lateral NPN transistors but they aren't very good. They are useful for things such as bandgap references, pad protection structures, and thermometers. You often have the ability to use different implant masks to get different threshold voltages. Other than that it becomes very process specific.
 

OBW0549

Joined Mar 2, 2015
3,566
I doubt the current mirror in the output OBW0549.
What would be the point?
Are you talking about I1, Q8 and Q9 in the model I linked to? That's how I modeled the current source for driving the upper NPN output transistor, Q13. I could have omitted Q8 and Q9 and connected I1 directly to the base of Q13 (in other words, exactly as shown in the diagram in the top post of this thread), but if I'd done so it would have allowed the base of Q13 to be driven above the +Vs rail and therefore Q13 to be driven into saturation, which wouldn't have been realistic.
 

studiot

Joined Nov 9, 2007
4,998
Are you talking about I1, Q8 and Q9 in the model I linked to?
No I was talking about the TI diagram presented by the OP and the current source between the base of the upper LIN output transistor and the + rail.

I further doubt that all transistors are operating in their linear region.
This is definitely contradicted by the TI curve of output v supply voltage in the OP linked datasheet.

At least the output ones are active, yes, but, like all large signal amplifiers, non linear.
 

WBahn

Joined Mar 31, 2012
26,319
It seemed like a good answer while I was looking at post #1
My guess (and I don't know this for sure) would be that the availability of JFETs on a bipolar process would be even harder to come by. Although I know that National developed the Bi-FET process that allowed them to fab reasonable JFET devices on a bipolar process. I believe that nearly all bipolar processes that support JFETs are derivatives of their work (in the 70's). But I don't know how widespread such processes are. Even if the standard process physically permits JFETs to be realized, there would be additional masking steps, which are very expensive, that most people are not going to want to pay if they don't have to.
 

#12

Joined Nov 30, 2010
18,210
My guess (and I don't know this for sure) would be that the availability of JFETs on a bipolar process would be even harder to come by.
I don't know Jack about fabrication. I only know that I love j-fet input op-amps. They enable common mode inputs outside the power supply range, have low noise, high input impedance, and enough speed for audio work. They are a Ferrari compared to an LM741.

Don't bother arguing. I'm emotional about this one. :D
 

WBahn

Joined Mar 31, 2012
26,319
Oh, I agree. JFET input opamps are definitely sweet. For a variety of reasons JFETs have never really captured a portion of analog design that they probably should have from a pure performance standpoint. Mostly it's because their uses, in high volume and/or integrated designs, is enough of a niche realm that other, more widely available and low cost, options are at least "good enough".
 
Top