I'm trying to step a 0.5 V AC to 5 VAC and 10 VAC. I used a transformer, but I got unpleasant results (A very unstable wave, and the voltage is even lower...)

Any ideas on why this happened?

 

What is the nature of your 0.5VAC? Source, frequency, shape, current?
What transformer did you use?
How do you know the output wave was unstable? What is your meaning of unstable?

 

Where is the 0.5v coming from? My guess it is an impedance problem between the source an the transformer. What kind of load will the 5VACor 10VAC be driving? What frequency range are you dealing with... The devil is in the details.

 

Where is the 0.5v coming from? My guess it is an impedance problem between the source an the transformer. What kind of load will the 5VACor 10VAC be driving? What frequency range are you dealing with... The devil is in the details.

So far, the period of the source is about 2.9 ms . The load will simply be a resistor. I don't really know why, but I think a mosfet is needed for some reason.

 

Also, the source of the 0.5VAC comes of a transformer

 

Sorry. We cannot see your setup where we are, at least not yet via the internet.

Where is the signal to the transformer coming from?
What is the make and model number of the transformer?
What is the value of the load resistor?