Hey AAC,


I was doing a freq response experiment on the LM358 op amp and it came out all good.

Sorta. The output waveform came out sine-like but I got flat lines all throughout the experiment where it transitioned from +ve/-ve to -ve/+ve respectively- I have included a few diagrams that all had this anomaly.

Note the diagrams include pretty much everything you need to know unless the upload stuffed up.

I have no idea what this could be and I am interested in knowing what it is.

Diagrams are here:
http://i273.photobucket.com/albums/jj218/sajuukkhar/C.jpg
http://i273.photobucket.com/albums/jj218/sajuukkhar/B.jpg
http://i273.photobucket.com/albums/jj218/sajuukkhar/A.jpg

Schematic:
http://i273.photobucket.com/albums/jj218/sajuukkhar/SchematicFreqResp.jpg

Application: The 358 is used as a signal amplifier for ultrasonics transducers at approximately 40 kHz (at this point in time) and transmitting and receiving signals from/to the transducers, which can act transmitters as well as receivers.



Also first post

Thanks in advance

Edit: Just putting Schematic in
Edit 2: Putting in its application

 

We also need to know the circuit diagram. Please post. Can only guess at what's happening without it.

 

Oops. Its just a Non-Inverting Amplifier. Pics soon in OP.

 

Schematic Added to OP.

 

The LM358 is only slightly better than a 741 in frequency response. What you are looking at is crossover distortion, when the transistors to the +V side switch over to the -V side. The faster the waveform, the more noticeable the switch. All of the older op amps have this problem to some degree or other.

There is a slew of newer devices you can try to see if they would work better, better slew rates, higher frequency responses, basically better parts in all ways.

If you give us your application we might be able to suggest better.

Bill's Index

High Speed Op Amp Query

The LM358 has one spec that is very special. It can get very close to the - power supply buss on the input and maintain excellent functionality. Many op amps stop working when they get to a volt or so of either power supply.

 

Its for use involving ultrasonics transducers at approximately 40 kHz and transmitting and receiving signals from/to the devices.

You know it would probably help me if I said all of this at the start

I will include this in my OP

 

You need a better op amp like one intended for high performance audio pre amps. A JFET amp like an LF356 would work much better than an LM358 for bandwidth. The 5532 is also a good one.

http://www.onsemi.com/PowerSolutions/product.do?id=SE5532AD8R2

 

You could use the 555 for the output, it has decent specs in that range.

The Radio Shack TL082 is also a decent little part.

 

You could use the 555 for the output, it has decent specs in that range.

The Radio Shack TL082 is also a decent little part.

The TLO8X is TI's equivalent of the NS LF356. They are pretty good out to 100kHz.

 

This has been really informative guys so far guys, so before I go and read all of the links provided, you all have my thanks