HI all,

Regarding my topic, I quite confuse why there are many type of op amp.
Such as LM741 (Operational Amplifier), TL084 (JFET input Operational Amplifier), LM 311 (Voltage Comparator), LM358(Single Supply Operational Amplifier), TL034 (ENHANCED-JFET LOW-POWER LOW-OFFSET OPERATIONAL AMPLIFIERS ).

Those OP-Amp still have the same function, right?

 

They all have the function called, "operational amplifier" but they are optimized for different specialties. Some have very high input impedance, some very low input voltage error, and several other parameters like high speed or high current or low price.

One problem is that every manufacturer is allowed to name his chips whatever he wants. This makes knowing all of them impossible for any person. We just learn a few every time we research for a job and the best ones become our favorites.

Wait. A comparator is not an operational amplifier. They usually lack the bipolar output stage required for a true op-amp. They start with a differential amplifier for their input, but their outputs are different.

 

This mean, LM311 (Voltage Comparator) does not suitable to operate besides as a comparator?

 

The LM311 has an open collector transistor for its output. You can arrange the circuit to act like an op-amp and run in the linear region, but that is not the best use for a comparator. You can also arrange certain nand gates to act in the linear region, but that is not the job they do best.

 

OK. This clear somethings. I appreciate you taking the time to answer my questions. Thanks

 

Hello,

Have a look at this thread:
Components Selection Guide
Post #6 has the datasheets for some comparators and
post #7 has the datasheets for some opamps.
Looking at the datasheets, you can compare the devices.

Bertus

 

This mean, LM311 (Voltage Comparator) does not suitable to operate besides as a comparator?

A comparator will operate in linear mode if you roll it back with enough negative feedback - but there are usually op-amps better suited for the job.

A favourite comparator in linear mode circuit is the TL431 programmable zener, which is actually a comparator with a built in 2.5V reference.

Its not too difficult to arrange for a large amount of DC nfb to also set the operating point and hold the input within a whisker of Vref, filter out the AC nfb with a shunt capacitor, and you can get quite a lot of gain out of it.

Not exactly hifi - but its a pretty spectacular pre-amp for a TO92 package and a few small components.

 

The LM311 has an open collector transistor for its output. You can arrange the circuit to act like an op-amp and run in the linear region, but that is not the best use for a comparator. You can also arrange certain nand gates to act in the linear region, but that is not the job they do best.

Since a comparator is not internally compensated it will oscillate nicely when used in typical op amp circuits with negative feedback. You need to add heavy external compensation to avoid that. That's why a comparator is seldom used as an op amp.

Alternately you can usually use an op amp as a comparator. It's just that the internal compensation makes it much slower to switch, compared to a typical comparator.

 

A comparator is not an op-amp. Sure, you can fudge things around to make it act like an op-amp, but it is not an op-amp. If you need an op-amp function, then use an op-amp. If you need a comparator function, then use a comparator.

 

Many thanks to all, for elaborates this issue clearly

 

A comparator is not an op-amp. Sure, you can fudge things around to make it act like an op-amp, but it is not an op-amp. If you need an op-amp function, then use an op-amp. If you need a comparator function, then use a comparator.

Comparators tend to be much simpler internally, and in some cases you can get away with using them for higher frequency and/or less propagation delay. But as Crutschow points out; comparators don't usually have internal compensation, so they can be prone to bursting into oscillation.

Its a playing the pros and cons game, sometimes you can get away with a quick and dirty design - sometimes it causes more trouble than it saves.