Power phase output for a signal amplifier is not clean and has a negative voltage offset

Thread Starter

Patrick151515

Joined Jun 16, 2016
9
Hi everyone, I am a first time poster, a CS graduate, and Not an EE but I always attempt to learn. I don't seem to be getting where I want to be with this circuit design and figured I would post what I have and what I am trying to do. Hopefully, someone can provide guidance.

I have a 3 phase system...a split power supply coming off a wall wart; an LC oscillator; and finally the power amplification stage.
The oscillator phase is currently providing/outputting a steady ~9Vpp at ~300kHz. Looking at the output in the oscilloscope without the power stage and all is good. I get a beautiful sine wave at the required frequency! When adding the power phase things go south. I have added images for my amplification stage with an input that matches the output I see on the implementation of the oscillator phase.

The output in the simulator looks great. But, the output of the implementation is very messy and has an offset on the negative side.

The opamp is a LM6181 and I believe it to be the correct choice.

Let me know if there is something I can provide that would make this clearer/easier to understand. Thanks in advance for your help.
 

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crutschow

Joined Mar 14, 2008
26,419
So the simulation is fine but the actual circuit output is not?
What is "messy"?
Is it oscillating?
Post a picture of the actual output.
 

crutschow

Joined Mar 14, 2008
26,419
one thing that really messes me up is that the frequency shows in the 8MHz range with the opamp and transistor in the loop.
Then it would appear that the circuit is oscillating.
Do you have a ground plane for all the common connections?
Do you have decoupling 0.1uF ceramic caps from the op amp supply pins directly to ground?
Do you have decoupling capacitors from the power lines to ground?
You are dealing with high frequencies and a sloppy layout and poor decoupling won't cut it.
Post pictures of the layout.
 

Thread Starter

Patrick151515

Joined Jun 16, 2016
9
yes, it is oscillating and perfectly prior to the amplification stage. Crazy after that. Not sure I know enough to answer all of these but here goes.
All the grounds are common.
I do not have the decoupling .1uF ceramics on the supply pins
I do have large capacitors between ground and v++ as well as ground and v--. 1000uF across each. not sure that is considered decoupling...but ok. lol Im a CS.
I will add the .1uF caps when I get home tomorrow and report back. And thank you very much for your help! Much appreciated.
 

crutschow

Joined Mar 14, 2008
26,419
All the grounds are common.
Of course.
But what is the quality of that ground?
It needs to be more than just a wire that meanders through your circuit.
It should be a solid copper ground plane.

How is the circuit built? What are you using for a breadboard?
A plug-in type breadboard won't work for this.
 

Thread Starter

Patrick151515

Joined Jun 16, 2016
9
Of course.
But what is the quality of that ground?
It needs to be more than just a wire that meanders through your circuit.
It should be a solid copper ground plane.

How is the circuit built? What are you using for a breadboard?
A plug-in type breadboard won't work for this.
Uhm...Wow...really!?!? Yes, just your standard plug em in breadboard. Should I skip this and just go straight to the pcb? I can do that. It just takes time for me to make the pcb and it is less flexible/forgiving. I was thinking I could get the design correct first. It would take about 3 hours for me to make the pcb and solder everything. I suppose I can do the phases on separate pcbs and just stack them.
 

crutschow

Joined Mar 14, 2008
26,419
You could do it on a vector type board with a ground plane but if are making your own PCBs that would work also.
You want the complete circuit on one board.
And, as I noted, you want a large piece of copper for a ground plane, with all decoupling caps going directly to it with short leads.

Post your layout before you make the PCB and we'll critique it for you
 
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