# 555 timer and biasing

Why is it that op-amps used as ICs, such as LM741 require a +Vcc and -Vcc for biasing whereas there is no -Vcc for a 555 timer?

What determines the biasing for any IC?

• noorav, I think some of your confusion is that in your head you imagine that there are "absolute value" meanings for "negative voltage" and "positive voltage." There aren't. Everything is relative and you have to pick something as "zero" to get other values. For example, a battery used as a $+4.2\:\text{V}$ power supply does so because someone decided to use the (-) terminal as their "zero" reference. It could just as well have been used as a $-4.2\:\text{V}$ power supply, had they decided to use the (+) terminal as their "zero" reference. It's all relative. No absolutes. Lose the idea. – jonk Apr 10 at 19:53
• @jonk I should probably add that to my answer, but I don't have time to right now. That's definitely a very important thing that people new to electronics often don't really get. – Hearth Apr 10 at 20:20
• @Hearth Well, just add it when you get a chance. When I was reading your dialog with the OP, it just stood out to me as the central problem. (I used to teach at a 4yr university and I developed some modest skill in being able to listen, hear, and then find a thinking pattern that might generate what I was listening to. In this case, I think the OP is stymied by an incorrect idea lodged in mind that the rest of the world has mysteriously assigned magic, absolute values through some obscure science knowledge the OP hasn't yet learned about.) – jonk Apr 10 at 20:26

"biasing" isn't really the right word here; this is the IC's power supply. All ICs require at least two voltage levels to operate, including the 555; in the case of the 555 these are called +V and GND, or Vcc and GND, or Vs and GND, you get the idea.

The reason some chips get powered with both a positive and negative voltage with respect to ground is so that they can, without needing any complicated power circuitry, output voltages that are both positive and negative with respect to ground; it's easy to output any voltage between your voltage rails and difficult to output anything outside of that range.

• So, in simple terms, some ICs require both positive and negative voltages to easily get an output voltage, that too within a given range to avoid errors. Is this right? – noorav Apr 10 at 16:13
• Hmm. Not to avoid errors, really; it's just that the IC itself can use a simpler (and thus cheaper and easier to use) design if it doesn't need to internally generate its own negative power supply. If you look at any op amp, its output limits are going to be related to what its supply voltages are; you might see, for example, "-Vcc + 0.5 V" listed as a minimum, and "+Vcc - 0.5 V" as a maximum. – Hearth Apr 10 at 16:17
• Ah okay, but does the 555 Timer generate its own internal negative power? Because we don't supply a -Vcc right – noorav Apr 10 at 16:18
• It does not. You might notice that the 555 never outputs any voltage below ground; this, combined with the fact that none of its internals require a negative voltage for any reason, mean that it doesn't actually have any reason to have a negative voltage supply. – Hearth Apr 10 at 16:19
• Ah okay, thank you. I'm sorry if biasing was wrong terminology, that was what my teacher used when she taught us about ICs, the 741opamp in specific. Is there any reason why 'biasing' isn't the right word? – noorav Apr 10 at 16:23

A 555 operated from a positive supply and 0V which is its ground. Its inputs are not linear (they are digital) so they are not biased. Any opamp can use a positive and negative supply with its inputs biased at 0V or it can use a single positive supply with its + input biased at half the supply voltage.

• the 555 has 2 comparator inputs which are used for linear analog timing functions not digital inputs – Sunnyskyguy EE75 Apr 10 at 16:25

The 555 uses two comparators with a string of three 5k+5K+5k resistors to create precise ratios of 1/3 *, 2/3 * Vcc, the supply voltage, Vcc ( c=collector ) with 0V as Vee for ease of single supply operation.

The comparators function with these ratios for Trigger and Threshold inputs respectively. Since the various timer function designs are based on the somewhat constant slope of the capacitor voltage/time in this range, from dV/dt = Ic/C, this makes the single supply Vcc voltage error cancel out for timer functions.

There are many App. Design Books for Op Amps. The amplifiers are designed to rejection supply variations with internal constant current (CC) sources for bias to reject supply variations yet limited by the gain and bandwidth.

Like all bipolar (BJT) outputs used in Op Amps, the 555 family output drive is not Rail to Rail unless it is a CMOS variation.