Operational Amplifiers (opamps) have the tendency to drop their Large Signal Voltage Gain when the load is too low. This behaviour is usually described in the datasheet. Loads of 500 ohm and lower usually result in very low gain.
How can I prevent this loss of gain?
I've tried using another opamp as a unity buffer, but the top of the signal gets clipped. I've tried a transistor, but the signal got distorted. My target load is 50 ohm and the signal is a triangle in 50 – 10000 Hz range, 0 – 10 V. The opamps in use are LT1077.
EDIT: I'm aware of the trouble the LT1077 has at low loads. I'd hoped there would be a buffer solution like I tried with the transistor (as far as I know though, Class A are used when the load gets too heavy, not when too low).
When trying the transistor, the top of the signal got clipped. I made sure there was enough voltage available, so I'm not sure how that happened.
You need an opamp with more output current. Search for power opamps. (LM675 and OPA544 are two I've used.) At 200mA you might get away with a TCA0372. Or use some audio driver chips if you don't care about DC.
Edit:(more thoughts, comments) Warning the LM675 has a minimum gain of 10. And all the power opamps I've looked at have some cross-over distortion. (Still looking for the perfect power opamp.) If your output is single sided, then you could use a transistor (Class A) with the transistor in the opamp feedback loop.. but that will waste a lot of power. Or make you own push-pull output stage. (again as part of the opamp feed back loop)