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Internal Circuit of the 555 Timer in Lesson 1454

Q. I am studying the internal circuit of the 555 timer in Lesson 1454. It talks about two comparators inside the IC. They don't seem to work like comparators, the way they are described in the digital lessons. A comparator is just an exclusive-OR gate with an inverter on the output. I don't see how the comparators in the 555 timer can work to change the state of the flip-flop.

A. There are two kinds of comparators. One kind is the digital kind, which you have described. The other kind is an analog comparator. It is basically an op amp.

The two different voltages to be compared are applied to its two inputs. If the voltage on the noninverting (+) input is positive compared to that on the inverting (-) input, the output of the op amp goes positive. If not, it will go negative. Usually, one input of the op amp is connected to a "reference" voltage, and the Voltage to be compared to it is connected to the other input. This is done in the 555 timer.

The outputs of the two voltage comparators are either high or low, depending on what voltage is on their inputs. With a regular op amp, if the voltages are too close to being equal, the op amp can become unstable and oscillate. So the voltage comparators in the 555 timer are designed to prevent this, and are thus alittle different from regular op amps. For more information about voltage comparators, see Lesson 1436, Operational Amplifier Characteristics.

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