Electronic – Opamp output current

Rather than searching for an op-amp that can source 25mA on a rail to rail output (although your supply is apparently 3V above the desired output of 5V), use an outboard boost transistor.

The circuit design is straightforward:

1. Determine the maximum current from the op-amp across temperature that will not generate excessive heat in the device (most general purpose devices have a rather high \$\theta_{jc}\ and \ \theta_{ja}\$).

2. Calculate the required minimum transistor \$\beta\ as\ \frac {I_{out}} {I_{opamp}}\$ across temperature and choose a suitable device. The BC846 I have used is well known for its high current gain characteristics.

3. The power in the boost transistor determines the package, primarily: so for your circuit we have 75mW dissipation in the transistor. For one of the packages for the BC846 I have used, that equates to a temperature rise of 37.5C; if your environment is at \$\le\$ 50C, that yields a junction temperature of 87.5C, well within the capability of the device, although it can shorten the life of the device. If that is a concern find a device in a chunkier package.

The series resistor I have used yields acceptable performance, but it could be reduced a bit at the expense of more power in the op-amp output stage.

This approach permits the use of general purpose op-amps rather than specialised parts, which should reduce cost as a bonus.

So, I was finally able to solve the motorboat problem by placing an LRC filter with a 50kHz cutoff frequency directly at the output of the transformer. The transformer has a self-resonance frequency at around 280kHz but amplitude starts growing around 90kHz, not sure if this is the cause specifically but killing that resonance and whatever else was making its way into the opamp seems to have eliminated the noise. Thanks everyone for their input.