What are the advantages of op amp circuits over transistor ones?
Opamps are made of transistors. The advantages of using opamps as gain blocks instead of simpler transistor circuits is usually simpler design made with fewer mounted parts (even though the total device count is higher, considering the complexity inside the opamps) and more ideal and predictable performance, often at lower supply current. There are exceptions, where individual transistors are more suited to the desired functions, when higher frequencies, larger currents, voltages or higher power are involved. Also, if cost is more important than ideal performance, transistor designs can often beat opamp ones, even though there are some very low cost opamps available.
