I need to produce an AWG output (or specifically a sinusoidal signal), using a transformer to implement isolation and voltage amplification. There is an OPA to produce a 20V output (20mA driving ability) connected to input of transformer, how to choose the transformer to implement 100V output on the secondary side with 20mA driving capability?
2 Answers
If you have 20 volts RMS and a peak current capability of 20 mA from your amplifier then that limits the power into a transformer to about 280 mW. Because a transformer doesn't amplify power, that means, after losses are taken into account, the max power from the transformer may be about 250 mW.
You require an output power of 1.414 watts so you need to insert an extra amplifier between your OPA and the transformer to boost current and provide the required power.
The simple answer is that you can't do it. If you take a 20V/20 mA signal and run it through a 5:1 stepup transformer, you will indeed get 100 volts out (20 x 5). However, your current will only be 4 mA (20 / 5).
If you can provide a current booster to increase the transformer input current to 100 mA, a 5:1 transformer will provide you with 20 mA out. I disagree with Andy about transformer efficiency - you should only have losses of a few percent.
You have not specified your frequency range, but if you're talking about audio frequencies you can probably find something close to your needs as a driver transformer. When searching for such transformers, the winding ratio is expressed in terms if input and output impedance, so for a 5:1 voltage ration you'll need to look for a 25 to 1 impedance ratio. Such a ratio is unusual. The closest common ratio you'll find is 600 ohms to 8 ohms. This is a voltage ratio of 8.7. See, for instance, Digikey. Also, when looking at specs, make sure you check the power rating for the transformer.