Last time, I teased you about the aphotic ancillary of op-amp accumulation current, abnormally aback we’re talking about abrogating accumulation pins and how the Single Accumulation Gang are activity about aing all those pins to GND. It’s time to dig deeper.
We’ll allegation some advice from Herr Kirchhoff—but we’ll additionally allegation to accompany him up-to-date. As it happens, Kirchhoff was built-in in the aforementioned year (1824) that Berzelius is accustomed with advertent the aspect silicon. He additionally died afore anyone knew about activity levels, let abandoned semiconductors and op amps.
So let me acquaint you to my booty on “Kirchhoff’s Accepted Law As Applied To Op Amps.” That’s not a decidedly memorable mouthful, though; you’ll accept abandoned it by tomorrow. It needs to complete added like a spy movie. How about… The Kirchhopamp Protocol.
To acquire The Kirchhopamp Protocol, I’ll accomplish some simplifying assumptions. First, that the op amp’s ascribe terminals comedy no allotment in our accepted discussions. Sure, a few devious electrons drag aback and alternating in the circuits we build, but they about bulk to picoamps or nanoamps of leakage, and that’s too baby to anguish about here.
Second, that there are no hidden batteries or added generators central an op amp. Put addition way, it cannot be a antecedent of net activity in any circuit. We’ll avoid the actuality that if the amalgamation were fabricated from a absolute that’s cellophane at some optical wavelength, afresh the pn junctions in the op-amp ambit could act as little photocells aback ablaze hits the chip. There’s added to say about how and aback ablaze can blend up semiconductor circuits, but that’s for addition day.
So as far as we’re anxious for this discussion, then, our ideal Kirchhopamp has three terminals: absolute supply, abrogating supply, and output. Or pin 7, pin 4, and pin 6, as we old guys alarm them, canonizing those DIP8 canicule aback op amps were big abundant to see with the naked eye. I aloof advised in a new Maxim op amp that comes in a amalgamation that’s about 1 x 0.7 mm. About the aforementioned admeasurement as the atom of beach that, in our adventurous acuteness at least, the silicon dent came from.
Revealing The Kirchhopamp Protocol
Here, then, is The Kirchhopamp Protocol in a nutshell: The sum of the currents abounding into the three terminals of a Kirchhopamp is consistently zero.
In blueprint format: Iplus Iminus Iout ≡ 0.
Now, aloof in case you’re about to ability for your accurate actor to validate this claim, let me say aloof one thing: STOP. In the alarming apple of op-amp Spice models, the laws of thermodynamics, attention of charge, and attention of activity frequently do not apply. Op-amp models consistently accommodate non-physical voltage and accepted sources that can accomplish alien ambit currents arise and abandon in abstract ways. It shouldn’t abruptness you to apperceive that the better culprit is GND, which frequently turns up in the netlists of these op-amp models alike admitting a real-world op amp doesn’t accept a GND pin. Don’t alike get me started about non-physical babble behavior…
Our blueprint is absorbing both for what it tells you and for what it does not acquaint you. Let’s alpha with what it tells you: If there’s no amount (i.e. Iout = 0), afresh in this quiescent condition:
Iminus(q) ≡ −Iplus(q)
In added words, the magnitudes are equal: |Iminus(q)| ≡ |Iplus(q)| = (ideally) the quiescent accepted burning amount accustomed in the amplifier’s datasheet. As an aside, Iplus(q) > 0 and Iminus(q)
Note that I snuck in an (ideally) there. I’m blank the absolute achievement voltage of the op amp actuality because it should not accomplish a difference. If you’re not demography any achievement accepted from a well-designed op amp, the accepted that it from the ability accumulation should not depend on what it’s doing.
Yet again, the behavior of some accessories is a disappointment here. I’ve apparent op amps whose accumulation accepted rises alarmingly aback the amplifier’s achievement nears the accumulation voltage, alike admitting the amplifier’s achievement date isn’t accomplishing any advantageous work. Aloof be active to the actuality that the quiescent accepted may not absolutely be absolute of added ambit conditions. Oh, it’ll be temperature-dependent, too, for sure… but we’re abnormal from my advised narrative.
Now let’s go aback to the blueprint to see what it does not acquaint you: We don’t apperceive annihilation about the amount of either Iminus or Iplus aback Iout 0. This is the complicated bit that makes The Kirchhopamp Protocol such an absorbing adventure, not aloof a dry equation. But why is this?
The acknowledgment is to be begin in the output-stage topology. Of course, an op amp has several added stages, but by and ample these don’t booty abundant current, and that accepted doesn’t depend abundant on alien conditions. About all the affliction you’ll acquaintance comes from that achievement stage. It’s area the elastic meets the Rload, as it were.
Note that actuality we’re talking about achievement stages that can both antecedent and bore the achievement current; in added words, both Ioutout > 0 (it’s positive, abounding into the pin) are permitted. This is binding for any affectionate of general-purpose op amp.
Staging the Achievement Stage
There are absolutely a few abeyant output-stage topologies to consider. Each one presents a altered claiming to the arrangement and lath designer. Getting advantageous bidirectional currents out of amplifier circuits has consistently been of abundant absorption to audio engineers, so it’s no abruptness that abundant of the absolute you’ll acquisition out there on the interweb has a able audio focus.
There’s not about abundant amplitude accessible actuality to alight either into the definitions of, or the about claim of, alleged Chic A, Chic B, and Chic AB output-stage forms. In essence, you allegation to apperceive two things about an achievement date to amount out what supply-current issues it will account you.
First, attending at the accord amid the accepted achievement accepted and the achievement stage’s own quiescent current. Broadly speaking, if the achievement date has a quiescent accepted decidedly greater than the accepted amount current, it’s referred to as operating in Chic A. If the best output-stage accepted (of either polarity) is abundant greater than the quiescent current, that indicates Chic B operation. And Chic AB fills in a rather ailing authentic ambit amid these extremes.
Second, are you ambidextrous with a “push-pull” achievement date or a single-ended one? This can absolutely be a little catchy to actuate from a simplified schematic. The acumen is this: In a single-ended stage, alone one of the transistors that affix the achievement pin to either the absolute or the abrogating accumulation pins is absolutely controlled by the signal. The added one aloof sits there demography a adequately affiliated current. By contrast, in a push-pull circuit, both the “push” transistor (the one affiliated amid the achievement pin and the absolute supply) and the “pull” transistor (between achievement and the abrogating supply) can be controlled, so that the accepted in both is able to vary.
Let’s accede the aberration in evidently arresting behavior amid what I’ll alarm an ideal push-pull date (audio guys alarm this a Chic B achievement stage) and the two accessible variants of the single-ended achievement stage. The ideal chic B push-pull date can be anticipation of as a current-steering block. If Iout is abrogating (i.e., advancing out of the pin), afresh Iout is fatigued from the absolute pin while the accepted in the abrogating accumulation pin is unchanged. In added words:
Iminus = Iminus(q)
Iplus = Iplus(q) – Iout (remember, Iout is abrogating so Iplus > Iplus(q))
Analogously, if Iout is absolute (i.e., activity into the pin, apologetic to accumulate emphasizing this), then:
Iplus = Iplus(q)
Iminus = Iminus(q) – Iout
What happens if Iout is declared to be a nice apple-pie sinewave? It should be appealing bright (even after a diagram) that the accepted in the absolute accumulation pin is the quiescent accepted added the absolute half-wave rectified $.25 of the sinewave. The abrogating accumulation accepted has the quiescent allotment added the other, negative-going half-wave parts. So, both currents are angrily distorted.
If you’re aggravating to body a high-linearity circuit, well, would you appetite those awful baloney apparatus abounding in your arena alike and ability traces? I anticipation not. But that’s generally what will appear on your lath if you let the Single Accumulation Gang affix the abrogating accumulation pin to GND alike admitting it does accept to be at that potential!
This is why some designers (especially ones advantageous abundant to get to architecture their own op amps and amplifiers) like a single-ended achievement date design. Here, there are two choices to make:
Sadly, there’s still a catch, a allurement that abounding designers abatement into alike if they accept anticipation the output-stage affair through carefully. Can you atom what it ability be? Hint: Think capacitors. As always, ping me at [email protected] with questions, comments, complaints, and (especially) with groundbreaking new ambit account (see what I did there?).
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