[SOLVED] OPAMP question C358C

[boylesg]

I have some opamps with "C358C" and "Malaysia" on them.

If I wire one of these up as a stock standard non-inverting amplifier, using all 10k resistors (inverting to out, inverting to GND and signal in to non-inverting).

I also have a 680R resistor and LED to GND from the OUT.

If I then simply use Vcc (12V) as my signal in then the LED does not light up. If I use GND as my signal in the LED does not light up.

If however I replace my C358C with an LM358AN then it works as expected.

So has anyone seen these ICs before and know what they are?

I am assuming they are NEC uPC358C, but perhaps I am mistaken?

 

[KlausST]

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

 

[boylesg]

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

Thanks Klaus, but I am unclear what you are telling me?

Is it that C358C is actually a comparator and not an opamp?

The fundamental problem is that I do not know what the IC is because it does not behave the same as LM358AN when both are set up as non-inverting amplifiers.

If C358C is actually a comparator then I guess that explains the disparity in functionality.

 

[Sarathkumarkj]

Hi,

Please read the data sheet of C358C in the following link
**broken link removed**

 

[KlausST]

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

 

[Audioguru]

An LM358A has better spec's and costs more than an ordinary LM358 which might be called LM358C.
I agree that the inputs must never go higher than 1.5V below the supply voltage.
Since you are using the opamp as an on-off switch then why not set the gain to be higher than only 2 times?

 

[boylesg]

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

Are you able to explain to me what this common mode thing is about - it is another gap in my knowledge that needs filling.

- - - Updated - - -

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 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.

- - - Updated - - -

OK Audio Klaus, as per your advice, I have soldered in a 100k resistor to form a voltage divider with the 10k resistor on my LM358AN non inverting input. So the input voltage will now be at least Vcc - 1.5V.

The LM358AN seemed to be working OK without the voltage divider, but I put it in anyway just in case.

It is related to that emitter follower circuit you have been helping me with Audio.

I decided to put a buffer between the AND gate outputs and the darlington bases in order to keep the base voltage as high as it can possibly be. The 10k resistor between the AND output and the LM358AN non inverting input should keep the AND output at around 1uA.

My version of a UCC27325 seemed to be working perfectly with an LED in both polarities with fixed voltage inputs.

Jsut have to hook it up to my GDT and see that happens on the scope.

 

[Audioguru]

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.

 

[boylesg]

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.

I have LM258AN rather than just LM358....don't know whether that makes any difference.

This is in the datasheet:

GBP
Gain Bandwidth Product
VCC = 30V, f =100kHz,Vin = 10mV, RL = 2k
,
CL = 100pF

0.7 1.1 / LM158A, LM258A, LM358A

0.7 1.1 / LM158, LM248, LM358


MHz

According to this I should be OK at least.

If not what opamp (or other device) would you suggest?

 

[Audioguru]

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.

 

[boylesg]

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.

 

[boylesg]

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.

I see what you mean re LM358AN Audio. I scoped my circuit and I am getting perfect square waves up until the inputs to the LM358ANs. But they are outputing triangle waves.

Strangely I am still getting output from my half bridge though.

So I am going to have to find another opamp which is up to the job, hopefully with the same pinout.

Do you have any suggestions?

 

[Audioguru]

A TLE2141 single or TLE2142 dual opamp from Texas Instruments has many packages available and the 8 pins DIL is the same as the LM358. It is 250 times faster than the LM358 but has the same output voltage loss.