Q&A - Changing the gain or the current limit settings of power op-amps - Apex
February 1, 2021
Q?
Can I use relay contacts to change the gain or the current limit settings of my power op-amp?
A!
Yes, you can, but you must be careful.
Because all output current flows through the current limit sense resistor, sudden interruptions in current flow in combination with the effect of parasitic inductance can yield high voltage transients, which have the potential to damage an amplifier's output stage.
Furthermore, because it is within the feedback loop, disconnecting the current limit sense resistor could cause the amplifier to run open loop, thereby producing unexpected output behavior. Similar behavior can also occur if the feedback resistor value is changed.
Therefore, one should always keep the feedback loop closed and add or remove additional resistors through relay contacts. Since electromechanical relay contacts tend to exhibit relay bounce, solid-state relays are preferred. In the schematics below, diode protection is implemented in the configurations labeled 'GOOD..' and 'BEST'.
In these configurations, D1 and D2 clamp the output to the supply rails, providing output protection against overvoltage transients while D3 and D4 protect the inputs against excessive differential voltages.
Back
