I have built a circuit that converts a 0-5 V signal to a +/- 9 volt signal using OPA1678 Op-Amps (original design from Here). The 0-5V signal is received from a 10k potentiometer across the +5V and GNDA Rails.
The circuit operates exacly as desired, save one thing: the circuit even without a load attached runs hot, too hot to touch after about 1 minute.
I chose the OPA1678 for its rail-to-rail operation as we desired max swing output from my limited supply. (my Vin is limited to +/- 9V) I'm wondering if I made a poor component choice or if something else is wrong.
Best Answer
The large capacitor on the output of U3B is more than likely causing your amplifier to oscillate, dissipating lots of excess power.
Looking at the datasheet, you can see the following graphs:
With just 600pF the phase margin is only about 15 degrees. With 0.1uF you're in big trouble.
The datasheet has this to say also about capacitive loads:
8.1.1 Capacitive Loads The dynamic characteristics of the OPA167x series are optimized for commonly encountered gains, loads, and operating conditions. The combination of low closed-loop gain and high capacitive loads decreases the phase margin of the amplifier and can lead to gain peaking or oscillations. As a result, heavier capacitive loads must be isolated from the output. The simplest way to achieve this isolation is to add a small resistor (RS equal to 50 Ω, for example) in series with the output. This small series resistor also prevents excess power dissipation if the output of the device becomes shorted.
So if you need a filter on the output of U3B, make it an RC filter, decoupling the capacitance from the amplifier.