I lost a stone because of morning sickness in the first trimester of each of my pregnancies, which I'd put back on by the time I was 20 weeks.
With DS1 I put on another stone from 20 to 30 weeks, and then 2 stone from 30 to 34 weeks because of pre-eclampsia. With DS2 I put on one stone from 20 to 40 weeks so I finished up two stone heavier than I started.
Once morning sickness had cleared up, though, I was hungry all the time, but I just ate every hour or so: a yogurt, half a sandwich, fruit, crackers with humus. Frankly when you're that hungry salad won't do it for you, you need to eat protein and carbohydrates to keep you going.
Don't worry, though, about your weight gain, though. A stone in 20 weeks sounds perfectly normal. And don't forget, everyone's different.