I keep having issues with creating a unity gain buffer amplifier to isolate a voltage source. My voltage source is very limited in current ~1-2 mA and I need that signal to drive a transformer at 10 VAC. I thought it would be as simple as using an op-amp, but I am finding out that op-amps do not like to source more than a few milliamps. My transformer draws about 20 mA at 10 V.
I thought I could use a BJT or two to help source this current, but I'm finding out they do like my 15V rails because all they do is get hot and output nothing. I have them in a push pull configuration. They are darlingtons rated for much more current than what I'm drawing. I am going to be using an NI cRIO module to create a 10 VAC sinewave and all I need is to find a way to buffer the current output, so that I can drive this unloaded transformer.
Someone mentioned the LT1210 but I seem to be missing something since I lose half of my input voltage. ignore the .01 Resistors. I have them there to measure current through that trace.
Best Answer
You have to use a power amplifier or maybe a line driver. These type of opamps can supply more current than the standard opamp. (You can find some e.g. at TI website under Amplifiers/Line drivers or Amplifiers/Power Opamps).
If you are sure that you don't need more than 20mA, you might also find some standard opamp. Take care, that the higher the output current, the lower the output swing - check that in the datasheet.
Extra tip, because I failed to do this a few times: You also need to take care of the self-heating of the amplifiers. It is usually the sum of the intrinsic heating (quiescent current times supply voltage) and the load dependent heating (output current times the voltage drop on the opamp). Multiply this power with the thermal resistance and check whether you are in the operating junction temperature limits.
Edit: as pointed out by electrogas also check out the TI AN-31 OpAmp Circuit Collection. You find the current sink on page 21.