Many circuit designs I see with transistors use two transistors chained together instead of just using one transistor. Case in point:
This circuit is designed to allow a device with a 3.3V UART to communicate with a 5V microcontroller.
I understand that when Q2 is off, TX_TTL will be high, and when Q2 is on, TX_TTL will be low. My question is, why not run UART_TXD directly to the base of Q2 instead of using Q1 to control the base voltage of Q2?