Why are two transistors often used instead of one?

Question

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?

Answer 1

What you have is basically a two stage amplifier - two consecutive amplifiers. In such a circuit configuration the gain of both amplifiers multiply. Since each stage has negative gain in your example, the overall gain is positive again.

So let's say Q1 and R2 have a voltage gain of -10 and Q2 together with R3 create a gain of -10, too. Then the overall gain is 100 which is positive and much larger than the gain of a single stage.

In your example this means the following: If UART_TXD goes High, TX_TTL will go High, too. If you omit Q1 and directly feed Q2 with UART_TXD, then TX_TTL will go Low when UART_TXD is High.

Answer 2

As noted by others, the main aim here is to achieve a non-inverting level converter.

For "extra points" you could use the circuit below.
The driver needs to be able to provide the output current (but not the voltage.)
As Iload_max =~ 5V/10k = 0.5 mA most input drive sources will be OK.

Vin = high = 3V3 -> Q1 off
Vout pulled high by R2.

Vin = low = ground -> Q1 on.
Vout pulled to Vin via Q1 CE on
I load = 5V/10k must be sunk by input drive.

This circuit is of special value when driving a high voltage load from eg a microcontroller. Vout max is set by the voltage rating of Q1.
The input drive pin must be able to sink the load current.

This is a "common base" amplifier 'drawn funny'.

^{simulate this circuit – Schematic created using CircuitLab}