A lot of people can use operation amplifier when comparator now and then. Generally speaking, need a simple comparator only when you, and you are enclosed in 4 operation amplifier in when still having amplifier of operation of redundant ” of a “ , this kind of practice is feasible.

 

The phasic compensation that stabilizes operation amplifier to run place to need is meant its rate will be exceedingly low when using comparator as operation amplifier, but if be opposite speed demand is not high, criterion operation amplifier can satisfy requirement. Now and then this kind of use method that meeting somebody asks about our operation amplifier. This kind of method has sometimes effect, anticipate as people however sometimes in that way the effect is good. Why can you appear this kind of circumstance?

 

A lot of operation amplifier have voltage clamp between input end, its great majority uses back to carry diode on the back commonly (use two sometimes or more series connection diode) will carry out. Transistor of these diode protection input are avoided suffer its base node the harm of retrorse puncture. Differential input appears to be met when 6V about a lot of IC craft puncture, this meeting is changed greatly or attaint transistor. Next graphs showed NPN inputs class, d1 and D2 provided function of this kind of protection.

 

 

In application of amplifier of most common operation, input voltage all is 0 bend over about, its are essential cannot open these diode. But apparently, to comparator move and character, this kind of protection became an issue. In the input procrastinates drag another input (the means that is fed up with with a kind pulls its voltage) previously, differential voltage limits (make an appointment with 0.7V) suffer be restricted.

 

Nevertheless, but we still can use comparator as operation amplifier. But, when we are done so must scrupulous.

 

In a few circuit, this kind of method may be accepted completely. The problem is us (include firm of other operation amplifier) always did not show these clamp existence. Although explain somewhat, we also won’t do detailed explanation possibly to perhaps be elaborated. Perhaps we should say: “ uses as when comparator, please scrupulous! The authors of ” product manual just also assume you are met for certain normally regard operation amplifier as operation amplifier is used.

 

Recently, we in American Arizona city ministry of graph dark product held a meeting. Conference decision, after us will more show this kind of case clearly. But, how had produced manufacturing operation amplifier to do now?

 

Following guidance suggests to may help somewhat to you: Generally speaking, amplifier of operation of ambipolar NPN transistor has input clamp, for example: OP07, OPA227 and OPA277. UA741 is an exception, it has NPN to input transistor, the add series connection that and have a few provides inherent protection for NPN is transverse PNP.

 

 

The current operation amplifier that uses transverse PNP to input transistor did not input clamp commonly, for example: LM324, LM358, OPA234, OPA2251 and OPA244. These operation amplifier are type of ” of “ only power source commonly, its mean them to have to expand to negaive electricity the source is carried (a bit low perhaps) in all modular limits. The input slants when buy electric current is a negative, state the input slants buy electric current brings a foot to pour out of from the input. At this moment, we can maintain them to be amplifier of this kind of operation normally. But, those who need an attention is, the amplifier of high speed operation that use PNP inputs has input clamp commonly, and these PNP are a few perpendicular PNP that have lower puncture voltage.

 

 

Taller voltage (be more than 20V commonly) the JFET of next jobs and CMOS amplifier, the likelihood has do not have clamp possibly also. This kind of uncertainty, ask you have more and careful examination. What use the character of craft and transistor type, decided whether its interior is put in clamp. Amplifier of operation of most low-pressure CMOS does not have clamp. Clear automatically or behead wave implement the type is a special case, its may have similar clamp behavior expression.

 

If you consider to use comparator as operation amplifier,the bottom line is …… , it is certain to ask scrupulous.

 

Read product manual carefully, do not want leakage to drop a bit information, include to apply content of a few annotate. In circuit breadboard or its behave the test and verify in prototype, examine an input voltage to input the influence of voltage to another. Do not depend on SPICE grand model. A few grand models may not include pair of clamp a few additional component that build a model. Additional, move operation amplifier ham-handedly to the likelihood when another course to appear from a course when you a few otherer phenomenon, we cannot build a model accurately possibly to these phenomena.