Resistor watt rating for high power opamps

[sean87]

Let's say you want to use a power opamp to amplify a small signal into a big one. The opamp would output something like 50V@2A.



In this case, does the resistors of the feedback/input (e.g. R1 and R2 in the above picture) stage need to be of a high watt rating? Or I can get away with normal resistors or even SMD 0806 resistors? which are maybe around 250mW rated or less?

[danadak]

Generally speaking no, The Power OpAmp inputs "look" electrically like
a normal hi-z input, so no significant current is drawn, and input and fdbk
Rs just function as a voltage divider to set loop feedback factor.


Regards, Dana.

[dmills]

R2 power dissipation is set by the maximum output voltage P=E^2/R in the obvious way.
R1 power dissipation is set by the maximum input voltage (same eqn).

This follows from the observation that the inverting input of the opamp is a virtual earth node.

Note that resistors have a maximum voltage as well as a maximum power rating, both must be respected.

Regards. Dan.