Output voltage mismatch in opamps

[mamech]

Hello all


I face a problem in dealing with opamp, and I am wondering why it happens. I used LM324 in a square wave generator circuit, but I noticed
that the positive output half cycle of the square wave is less than positive rail . The positive supply was 5V while the positive output was around 4.9

The negative half cycle of the square wave is exactly the same as the negative rail.

I noticed that same thing happened when I used opamp LMX385AKA-T in simulation using proteus . I do not understand, is having positive output less than supply one of characteristics of an opamp ?

 

[FANT]

It is normal.

With a 5 V supply you are lucky to have 4.9 as positive output.
This is because the 324 is not a rail to rail op-amp, but it goes to some mV from the ground and roughly 500 Mv from the positive supply.

You have some few solutions:

1) Use a LMC662 that is a rail to rail ( but has only two op-amp inside ).
2) Use a resistor ( say 1 KOhm ) to reach the positive supply, but you have not to absorb any current
3) Use a slghtly higher voltage for thepositive supply ( say 5,5 V )


I have no other solution.

Consider that the 324 is quite old ( I used it some 30 years ago ) and that another op amp of that period as the TL084 has 1,5 V from both supplies ( positive and negative ).

Mandi

 

[Analog Ground]

Voltage comparators are probably used more often than op amps for square wave generation. There is a differential input similar to an op amp but many have an open collector output stage which can give you an output up to the positive supply with a suitable pull-up resistor. Check out some of the comparator app notes for square wave generator ideas. Checkout adding hysteresis to keep the generation clean. The app notes will show how to do this circuitry.

 

[crutschow]

You should look at the op amp data sheet for the op amp you are using. it will tell you the maximum output voltage you can get and other useful parameters of interest.

If you want the output to go to the supply voltage they you need a rail-rail op amp, of which there are many available.

 

[mamech]

thank you very very much for your help

I will consider this...

also please I want to ask few questions about your suggested solutions

1-what does the term "rail to rail" op amps mean? and how does it differ than non "rail to rail" op amps?
2-you said that I may use 1k resistor to reach positive supply, where should I put it?



thanks

 

[FANT]

Rail to rail means that the output voltge can swing between the two supply limits, the NON rail to rail swing little less.

The 1 KOhm is to be connected between output and positive supply.

Mandi

 

[crutschow]

Just to be clear, "rail" refers to the power supply line (rail).