3v to 5v Level Shifter

The below circuit uses a BJT to shift a 0-3.3v data signal into a 0-5v signal. Specifically, 0-2.4v on the input will result in a logical 0 on the output, while 2.4v to 3.3v on the input will result in a logical high on the output. This happens because the NPN transistor is closed once the base voltage is greater than a diode drop (0.7v) above the input (ie the BJT closes once the input voltage reaches 3.3 - 0.7 = 2.4 volts)

Therefore the modes of operation are:

1. Input is low, therefore the BJT is closed, driving the output low.
2. Input is high, therefore the BJT is open, allowing the pullup resistor to pull the output to 5v.

Behaviour Analysis

To better understand the different operating modes of BJTs I took a closer look at exactly how the output voltage responds to a change in the input voltage. In the below oscilloscope image:

• the top plot is the input linearly increasing from 0v to 3.3v
• the middle plot is the resulting output
• the bottom plot is the calculated difference between the two

In this image we can see the BJT operating in three modes:

1. The first thing to notice is that the calculated difference between the two plots remains at 0 in the input’s 0-2.3v range (the output linearly increases as the input increases). Put another way, the input and output are short circuited in this region, as expected for a BJT in saturation.

2. From 2.3v to 2.6v volts on the input, the output rapidly increases up to 5v. This is the BJT in active mode, acting like an amplifier.

3. Finally, for any voltage on the input above 2.6, the output remains constant at 5v. This is the BJT in cutoff mode, the output is effectively disconnected.