I have this prototype:
Input signal is from a sine-wave function generator and circuit required output is 0-4kHz 0-36Vrms (0-100Vpeak to peak). This works OK. But a new requirement is added - must be able to provide 200mA output current.
Would this type of configuration work?
I have two problems - transistor base voltage too high? I can't find a DC-DC module in Digikey that can do +/-100V 200mA?
Any other possible configuration? I can change opamp if needed.
Here is a simple way to do it:
Power dissipation of the transistors is about 2W each with +/-70V supplies and +/-200mA peak output current.
Would this type of configuration work?
There is going to be some cross-over distortion when the op-amp output has to jump between ~ +0.7 V and -0.7 V at each zero crossing --- since the op-amp can't do that instantaneously there will be a "dead zone" in the output waveform at each crossing.
If your application would be impacted by this distortion, you can reduce it with a simple adjustment:
simulate this circuit – Schematic created using CircuitLab
This does cost some power as R1 and R2 must be chosen low enough that they can provide sufficient bias current to the BJT bases and keep the diodes forward biased.
If your requirement is 36 \$V_{RMS}\$ you don't need a +/- 100 V supply. That's 100 \$V_{pk-pk}\$ which is +/- 50 V so you just need that plus a bit more for some headroom. You might want to go with the same voltage as the op-amp is running from so you don't get the base-collector junctions reverse biased.
As for the buffer, you would need not only higher voltage transistors but higher current and wattage ones. 50 V @ 200 mA is 10 W so the transistors are going to need to be a bit beefier than the ones shown. And you need to make sure the op-amp can drive them with enough base current or else use Darlingtons.
The output voltage and power requirement probably means you need a more robust output stage than can be achieved with 2N3904/3906 alone. A second driver stage can help (simulate it here):
Design Notes:
The LTC6090 datasheet](https://www.analog.com/media/en/technical-documentation/data-sheets/6090fe.pdf) gives several applications of "bolting" transistors to the device to get a higher current output. For instance, there's this high current pulse amplifier: -
Or, there's this 100 watt audio amplifier: -
The datasheet even shows you how to current limit the output: -
My reaction when reading the question:
I can't find a DC-DC module in Digikey that can do +/-100V 200mA?
What you want is an amplifier. DC/DC converters are typically power converters, and do not work as amplifiers. Any amplifier that can work with the swing and current you need will work fine. 36V, 200mA, means about 7.5 Watts, so go for at least 10 Watts rated capacity.
In addition to choosing an amplifier with the appropriate rated power and low distortion, beware of output filtering requirements. Most class D amplifiers need heavy filtering, which is implied to be provided by a speaker/driver on the output, but if you're just outputting signal into a resistive load, you will need to add additional filtering yourself in that case.
Main challenge would be to find an amplifier with +/- 50V range. You might need to get something like an LM3886 and bridge it to get where you need to go. Also, limiting to 200 mA current at 36 Volts, you'd need to specify minimum output impedance of 180 Ohms.
