Your plant is indeed in the beginning stage of Early Blight, Alternaria solani. It looks like your plants will pull through fine if you start treating now. The earlier you treat, the better the control, as a strong infection will build up resistance to the fungicide.
Here's what to do:
Remove all leaves showing sings of early blight (yellowing, dry margin, large round dead spot,)
Do not touch the other leaves with the removed portions, or you hands until they are thoroughly washed.
Spray with a copper fungicide (like BONIDE© Liquid Copper Fungicide). Apply once every 5 days, and after any rain. Continue for 3-4 weeks, or until the plant stops developing the symptoms.
There are some preventative measures you can take. To lessen the chances of future attacks, you can try to:
Minimize soil-foliage contact
Make sure the plants get good air circulation (including proper spacing)
When you water, try not to wet the foliage, or splash soil onto the leaf undersides. The fungus spreads faster in wet conditions.
To go along with that, an organic mulch will help keep soil from splashing, and is also useful for many other reasons.
Make sure the plants have a support to climb on.
Rotate the plantings each year, to stop pathogens from inhabiting the soil. Remember that potatoes are also vulnerable to early blight, so plan accordingly.
Here are some comparison pics:
