It's like we're a little kid sitting in your lap being explained circuits. Excellent explanation!

You have amplified my understanding of differential pairs, current mirrors and active loads.

I can't believe how you brought this small chip that was invented in 1971 back to life and instilled in the heart of every listener the spirit of enjoying this simple invention. Thnx mate

The 555 has so many things to teach us!

That active load is indeed a neat little trick. I used one in the input stage of my homemade op-amp, it not only increases the gain per stage but also the GBW product making the op-amp faster.

The 2 comparators have different operating points, but for both of them the need is to, when the input is out of the mid range, to have a fast charge injection or removal from the flip flop stage, so they are designed, for the upper one, to turn on and supply current into the flip flop stage fast, but recovery when it drops is not as critical, provided it is just fast enough. Same for the lower, it pulls current out of the flip flop, turning it to the other state fast, but recovery again, as the slow edge does not worry the output state at all, is slow. This use of one input to the flip flop being active high, and the other active low, helped with the design, saving the need to put in an inversion stage, with the need for then level shifting of the one, to handle the different levels of the comparators, and also the need for large areas consumed by polysilicon resistors, as the transistor active loads are much smaller than the equivalent in polysilicon resistors, and the 100k pull down on the lower comparator is achieved by having the resistor actually being made using a very small JFET, with the gate strapped to hold it hard off, and thus it varies resistance minimally with voltage, meaning it acts as a weak pull down R6 at trigger, keeping Q15 off, but using minimal board area. The flip flop designed that way needed fewer transistors, and also, as you just have a single output driving the power stages, you made it simpler as well. The different levels of the comparators also interfaced with the flip flop well, saving area, which was a big thing when designed, as making the chip smaller meant a lot more per 6inch wafer, and even more so with larger wafers. Thus no stability capacitors, instead using the inherent capacitors of the junctions to make the comparators stable at transition, and, aside from the output having that cross conduction in switching, made it faster. You got used to having a 555 timer design having the main power supply decoupling right by it, just for that, and the rest of the TTL barely needing any bypass overall. Till the advent of cheap mosfet drivers, you often found a 555 timer being used as an inverting driver and level shifter, simply using a 3k3 resistor from pin 5 to ground, to force it to be 'more or less' TTL compatible with a 12V supply. They say 500mA, but you can easily get the originals to 1A of current drive on a MOSFET gate, though the modern ones are wimpy compared to the old process ones, and for more pull down connect 7 to 3, to get that gate charge out faster. Better have that 1000uF low ESR capacitor right by pins 1 and 8 though, the current would be nasty. Still got a few TO100 can versions, ran them with clip on heatsinks, driving piezo transducers direct in push pull, they ran rather hot without them, with them just merely uncomfortably warm, though the transducers were kind of hard driven, at 24Vpp on them, as ultrasonic deterrents. Worked on the bats, they decided to decamp after a week.

According to the designer of the 555 Timer Hans Camenzind it got it's name because his boss at Signetics liked the number 555. At least that is what he said in his book Designing Analog Chips

Aha. The cell phone says this video was almos two months old. There should be lots of materials in the pipe ;)

I’m always impressed with how clean the PWM signal appears on your scope. Thanks for the video!

Small correction: A darlington pair differential amplifier has less differential current gain than a single transistor differential pair. Collector current ratio for single transistor pair is I0/I1=E^(DiffVoltage/TempVoltage), for darlington pair is I0/I1=E^(DiffVoltage/2 * TempVoltage). I think in case of 555, the Darlington pair decreases input current and allows seeing voltages very close to supply rail.

you touch really close to an idea that I have been thinking about regarding how to teach circuit design. that is the idea that what we should consider teaching first is not transistors, tubes, diodes, etc,... but rather start with those devices you see as root devices in Spice. that is current sources, controlled sources, voltage sources and the controlled version, etc. then AFTER students understand those items you behavior model transistors etc with the sources to explain the configuration/topology of various circuits. --- I imagine that had we been taught in that way - circuit design/analysis would be simpler.

I love this schematic video. Thanks!

I was wrong in my previous comment. The currents are not constant. But they are equal by current mirror. Using KCL at collector output node, the output current can be calculated as 2x the delta Ic of the right side or left side. Anyway the first stage is still a voltage-to-current amplifier.

I think I get the current mirror going into the two sides of the teeter-totter... The extra current on the other side is *blocked* from going through its collector, and therefore has to go *somewhere* which then forces that extra current into the base of the transistor in the *next stage* of the circuit. It wouldn't work well, I don't think, if the next stage couldn't sink it (e.g. if it were Hi-Z input).

Awesome ! a new and improved 555 tutorial...cheers.

Learn a lot. Thanx.

Op-Amps also have a lot of gain but make poor comparators unless one supplies some positive feedback to ensure some hysteresis and prevent oscillation at the threshold voltage. Is the current mirror in the 2/3 V comparator of the 555 effectively fulfilling the same role by injecting more current into the opposite side of the differential pair?

By holding the currents constant at the top, all the delta current will be squeezed out of the collector. They want maximum current, they don't want voltage.

You're the best.

Love the more in depth explanation. Oddly for that last few videos you put out seem to be about just the stuff I been wanting to get more understanding of. I been searching for discrete comparator circuits to understand it further (and possibly make my own comparator for the fun of it). Do you have a discord server?

15:43 I don't understand why you need Q5 and Q8. All 3 pins are connected together. If i pause the video and cover up pin 1 of Q5/Q8, pin 3 to pin 2 is basically a diode. When Q1 is turned on, it pulls the base of Q6 down, turning it on (Q6 is PNP), which allows current to flow out of pin 1 to R11 and Q15.

On many of your circuit/breadboard videos you use small bypass capacitors on your power rails. Could you do a short video on this topic?

Pin 5 can be used for frequency modulation when the 555 is oscillating.

Can someone explain why they didn't use the same buffered current mirror at the trigger comparator stage as at the threshold comparator?

Pin 5 is the mystery pin . You put a cap there but, no one knows why.