boylesg
Advanced Member level 4
- Joined
- Jul 15, 2012
- Messages
- 1,023
- Helped
- 5
- Reputation
- 10
- Reaction score
- 6
- Trophy points
- 1,318
- Location
- Epping, Victoria, Australia
- Activity points
- 11,697
Hi,
my hint: if youu need an amplifier, then use an OPAMP.
If you need a comparator, then use a comparator.
Using an OPAMP as comparator may be problematic - because of stability, common mode input voltage range, differential input voltage range, latch up issues, phase reversal....
Using a comparator as amplifier may be problematic - because of stability, open collector output, distortion, gain, offset...
It may work, but don´t be surprised if it works not:
* in a year
* in another temperature range
* with another production batch of this IC
* with another manufacturer of this IC.
....
Klaus
Hi,
sorry, I am mistaken. Maybe it was the heat of the last days. ...
I thought you want to use a comparator as an amplifer. Don´t know why.. sorry.
****
about your problem:
Datasheet says "Common mode input voltage range" = VCC -1.5V. So if VCC = 12V then you must not give more than 10.5V at input.
And note2 on page2 says: "The normal operation will establish when the both inputs are within the Common Mode Input Voltage Range of electrical
characteristics."
But with your configuration at least one is out of this range. So it is out of specification. No function is guaranteed. The output state is not defined.
Try to use a voltage divider at +Input, and tell us what happens.
Klaus
I didn't think there was any point to making the gain any higher than that because I figured that any voltage loss would only be small anyway. I.E. The different between amplifying a mV audio signal compared to a 10V square wave for example.An LM358A has better spec's and costs more than an ordinary LM358 which might be called LM358C.
Since you are using the opamp as an on-off switch then why not set the gain to be higher than only 2 times?
I looked for Input Common Mode Voltage Range in Google and found a Texas Instruments document describing all opamp parameters.
If you look at the datasheet of the LM358 then you will see that its output cannot go higher than about 1.3V LESS than the supply voltage if its output current is only 1mA or less. So you do not want it as a buffer.
- - - Updated - - -
Also the LM358 is VERY slow and has trouble at frequencies above only 2kHz.
The 358 made by NEC is the same as the LM358 designed by National Semi and made by all the other copycats.
The LM258 is allowed to operate over a wider ambient and chip temperature range. I have National Semi's datasheet and they do not make an LM258A so I do not know its specs.
It is slow because it is one of the first low power opamps ever designed a long time ago. Modern opamps have a higher output current and are much faster.
I do not know how fast you need a buffer to be.
Audio, the max frequency is going to be less than 100kHz.
I just tried my circuit attached to the GDT and half bridge and it was able to run a small air core inductor I made with a pencil. So I reckon I am OK.
I could just as easily made a buffer from a 555 as it turns out.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?