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I thought about this question when I read an interesting comment by @Simon Fitch in his answer.

In the complementary amplifier stage, in total there are three "pulling" elements - Q2 is a "pull-down" transistor when Vin is negative, Q1 is a "pull-up" transistor when Vin is positive, and the load RL is constantly a "pull-middle" resistor :-)

schematic

simulate this circuit – Schematic created using CircuitLab

Push-pull graph

However, for some reason, this configuration is called "push-pull" stage. So there should also be a "pushing" element.

My question is, "What is it?"

Circuit fantasist
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    Don't get too caught up on words. People use them, often, without precision unless they are writing for a very technical audience and have taken extra time to shape their words well for a formal setting. When speaking/writing colloquially and off-the-cuff, expect any and all abuses of words. That said, capacitors in DC-blocking settings are often nicely *seen/viewed* as pushing and pulling a driven node. Transistors, less so. – periblepsis Apr 23 '23 at 20:54
  • @periblepsis, I see... But it is important to me because I think in this way - figuratively and intuitively. – Circuit fantasist Apr 23 '23 at 21:21
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    RL is a load resistance. – Edin Fifić Apr 23 '23 at 21:29
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    @Circuit In physics there are usually clear demarcations that nature shows us. For example, the concepts of solid, liquid, gas, and plasma arrive readily from phase diagrams about what nature shows us. Words tethered closely to physical nature, like that, carry meaning. On the other hand, words that describe internal mental states used to interpret the world through simplifying analogies may not always have such well-defined, objective, and clear demarcations and therefore much poorer robustness in my opinion. I prefer to see what nature says, than what some human says. More consistent. – periblepsis Apr 23 '23 at 21:33
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    @Circuit In short, I tend to ignore words used by people when "talking simple" about something and instead work at what nature would want to show me about the situation and come at it with my own imagination, not that of someone else. Works for me better that way. And I sometimes come up with something new that others didn't see. Nice when that happens (rarely!) – periblepsis Apr 23 '23 at 21:35
  • @periblepsis, I see that you have a very serious theoretical training and inclination towards science. My ambitions are much more modest. I rely on my intuition, the visual representation of circuit operation and the experience of practice. – Circuit fantasist Apr 23 '23 at 21:49
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    @Circuitfantasist Then my visual explanation should be helpful to you. :D I plan to add a little more (a pull down circuit). – Edin Fifić Apr 23 '23 at 21:57
  • @EdinFifić: RL is also a pull-down element . – RussellH Apr 24 '23 at 13:20
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    @RussellH In a way, it is a "pull-down" from positive supply voltage to 0V and a "pull-up" from negative supply voltage up to 0V, but then add it all up and you get zero sum. Referenced to 0V, it is neither. Referenced to either supply rail, it becomes either. – Edin Fifić Apr 24 '23 at 16:59
  • I wonder what RL is at zero input voltage? – Circuit fantasist Apr 24 '23 at 17:29
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    @EdinFifić: Of course.Silly me. It could be called pull-to-zero ;) – RussellH Apr 24 '23 at 20:07
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    @Circuitfantasist. Sometimes a resistor is just a resistor. – RussellH Apr 24 '23 at 20:15
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    @Circuitfantasist - It's a dead bug. :D You seem to love overthinking things. – Edin Fifić Apr 25 '23 at 09:01

5 Answers5

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Pull-up refers to raising a node to a higher voltage.

Pull-down refers to reducing a node to a lower voltage.

Pull in push-pull refers to pulling current from the load to the transistors.

Push in push-pull refers to pushing current to the load from the transistors.

Pushng a current to a load requires pulling up the output node.

Pulling a current from the load requires pulling down the output node.

RussellH
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  • So your explanation is based on both the mechanical ("tug of war") and fluid analogy ("fish tank")? Very good and comprehensive explanation... – Circuit fantasist Apr 23 '23 at 21:18
12

"Push-pull" refers to the current through the load at the output of a pair of transistors placed as a "totem-pole" or one above another.
EDIT: Thanks to the user Chester Gillon whose answer below contains an actual link to a Texas Instruments page saying the same thing (link).

schematic

simulate this circuit – Schematic created using CircuitLab

  • 0v or GND: In your schematic, the ground is halfway between the positive and negative supply voltages, or at 0V.
  • PUSH: When the upper transistor (Q1) conducts, it lets the current flow from the positive voltage through it and through the load (RL) to the 0V (zero volts) or ground point.
    Since the classic (conventional) direction of current is considered to be from positive to negative, this is considered pushing the current from the positive output to the ground, as well as "pushing" the voltage on the amplifier's output "up" towards the positive supply voltage.
  • PULL: When the lower transistor (Q2) conducts, the (conventional) current flow is from 0V/ground through the load (RL) and through the Q2 transistor to the negative supply voltage.
    Thus, it is called pulling, as it supposedly pulls the current from the 0V side of the load down to negative output of the amplifier, and also the voltage on the amplifier's output side down towards the negative supply voltage.
  • CONCLUSION: The upper transistor does the pushing, and the lower transistor does the pulling in this (type of a) circuit.

PULL-UP and PULL-DOWN typically refers to bringing an INPUT of a circuit UP to the positive supply voltage or DOWN to the negative side or 0V (zero volt) side to have a defined voltage level when there is no input signal and/or to change the voltage range of the input signal.
One example is an Arduino or ESP32 output only giving from 0V to +3.3V PWM signal, but you might need from 0V to +10V to drive a MOSFET. An ESP32 output drives an NPN transistor which is connected in an open-collector circuit where 0V remains about 0V, but the upper voltage or logic 1 now extends all the way to the positive supply voltage to which the pull-up resistor is connected.

AN EXAMPLE SCHEMATIC:

schematic

simulate this circuit

Edin Fifić
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  • Great explanation... Just to ask, "And which way does the RL load "pull" - "up" or "down" :-)? I'm putting a smiley face because it might look quite strange to the reader... – Circuit fantasist Apr 23 '23 at 21:43
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    @Circuitfantasist The potential on the left side of the RL (towards amplifier output) is "pulled-up" towards the positive voltage side when Q1 is on, and it is "pulled-down" towards the negative voltage side when Q2 is on. The ground or 0V side of the RL is the "reference point". Also, the RL or the load ITSELF is not doing the pushing or pulling, it is actually resisting the pushing and pulling of a current through it. – Edin Fifić Apr 23 '23 at 21:55
  • But RL also "pulls down" in the first case and "up" in the second one, right? So, in the first case, a positive-supplied voltage divider Q1-RL is formed, and in the second case, a negative-supplied voltage divider Q2-RL is formed. When Vin = 0, third "voltage divider" Q1-Q2 is formed; RL does not play any role since the voltage drop across it is zero. – Circuit fantasist Apr 23 '23 at 22:04
  • And to add something to your last edit (bottom picture). Don't you think that R2 is exactly the same "pull-up" resistor as R1? And that R1 + Q0 is the same as R2 +Q1? I.e., R1 converts the *open collector resistance output* into an ordinary *voltage output*? And to take this opportunity to express my admiration for your explanations; I really like them because they show a real understanding of circuits. I follow them with interest... – Circuit fantasist Apr 23 '23 at 22:13
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    @Circuitfantasist You're right, I overdid it! :D Oops! The open-collector was supposed to be inside a sensor, but then I added MCU and confused myself.. – Edin Fifić Apr 23 '23 at 22:14
  • @Circuitfantasist The PULLING-DOWN means pulling BELOW ZERO VOLTS or GROUND. So, no, the RL is not doing the pulling down, because the lowest it can "pull-down" to is the midpoint, ground or 0V. Same for its "pulling up". If you connect its ground side to the positive supply voltage, THEN it will be pulling up, and vice versa. – Edin Fifić Apr 23 '23 at 22:22
  • Really, extremely useful (both input and output) applications of the humble 19th century Ohm's resistor... – Circuit fantasist Apr 23 '23 at 22:25
  • That's why I called it a "pull-middle" resistor :-) I'm stopping here so we don't create a problem for the moderators; but if you want to talk about something else, we can continue in the chat (only tomorrow, because it's 1.30 past midnight here). – Circuit fantasist Apr 23 '23 at 22:31
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    @Circuitfantasist Sorry, I have too many things to do, and I just sometimes catch a few moments to flex my brain and skills here. If my answer is acceptable to you, you should mark it as accepted answer. – Edin Fifić Apr 23 '23 at 22:37
  • Edin, It's very hard to choose between two very good things... I really like your answer but I chose the @RussellH answer because it is very precise and concise. I hope you understand me... – Circuit fantasist Apr 24 '23 at 05:49
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    @Circuitfantasist You're the one who asked and you're the one who decides which answer is more acceptable, helpful or better helps you understand, and that's the whole point. – Edin Fifić Apr 24 '23 at 08:58
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From the point of view of the load \$R_L\$, Q1 pushes current into it, and Q2 pulls current from it.

I would not call an emitter-follower (which both transistors are, in this circuit) a pull-up or a pull-down, but terminology can vary.

Spehro Pefhany
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Well, AFAIK, that's terminology that developed during the history of electronics with no coherent source. That is, it is not the result of the effort of a single author who wrote the basics of electronics terminology.

FWIW, I always assumed the meaning of "pull" in pull-up/down referred to the node voltage which was pulled toward a higher/lower voltage relative to ground.

OTOH, "push" and "pull" in "push/pull" refer not to voltage, but to current in the load, so a whole different context. What is relevant here is that the two kind of BJTs make the current flow in two opposite directions. Which is "push" and which is "pull" really doesn't matter, and I don't think the original "inventor" of this terminology meant something more than that.

In the end, I don't find it strange that the two meanings of "pull" are not in sync, since they refer to two different quantities.

LorenzoDonati4Ukraine-OnStrike
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Found [FAQ] What's the difference between logic output types (push-pull, open-drain, 3-state)? from Texas Instruments which says:

A push-pull output can source current in the high state or sink current in the low state.

Without further searching not sure if any standard terminology is in use by semiconductor manufacturers.

Chester Gillon
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