Single transistor level up shifter

Question

I need a simple, single directionnal level shifter for 3.3V -> 5V conversion.

There are many options on the internet, some using a logic ic and some using 2 NPN transistors (converter and inverter), but I never found an option using only a single transistor (and 2 resistors).

My understanding is that when input is at 3.3V, the transistor is blocking and R2 pulls output up ; while when input is 0V the transistor is passing and pulls output down to transistor VCE(sat).

So, why wouldn't such a converter work? There must be a reason...

Answer 1

The single-BJT level shifter in the question would work: If the input impedance of the device on the 5 Volt side is significantly higher than the 6.8 k shown in the question, the expected ~0.3 to ~5 Volt signal would be received (taking a 2n2222 as an example).

However, for lower impedance inputs, the input would act as a voltage divider with the 6.8 k resistor, attenuating the high part of the signal significantly.

For instance, if the input impedance of the load on the 5 Volt side were, say, 100 k, the signal would top out at around 4.6-4.7 Volts. Still not too bad.

Any lower, and the level becomes problematic. This is when one needs an alternative, such as a two-transistor set-up mentioned in the question, to drive the output rail harder.

Answer 2

I like your solution. Since the question is about simple solutions, I have a few alternatives (some solutions provided by Microchip HERE):

1) Direct connection: If Voh (high-level output voltage) from your 3.3V logic is greater than Vih (high-level input voltage), all you need is a direct connection. (it is also required for this solution that Vol (low-level output voltage) of the 3.3V output is less than the Vil (low-level input voltage) of the 5V input).

2) If the above conditions are close, you can often boost the high-level output voltage slightly with a pull-up resistor (to 3.3V) and direct connect the signals.

3) The pull-up resistor can provide a small amount of high-level voltage increase. For more, you can use diodes and pull-up to 5V. The circuit shown will not pull-up clear to 5V, but it will increase the high-level input voltage to the 5V logic by the amount of one diode voltage drop (appx 0.7v). Care must be taken with this method that you still have a valid low-level as that is also raised by one diode drop. Schottky diodes may be used for a slight increase in high-level voltage while minimizing the undesired increase in low-level voltage. Refer to the above mentioned app note for more on this circuit.:

^{simulate this circuit – Schematic created using CircuitLab}

4) If you can deal with a logic inversion (and don't require active pull-up), a mosfet and pull-up resistor may be used:

^{simulate this circuit}

5) I know you aren't looking for a logic ic solution, but for completeness I will mention one (of probably many). The MC74VHC1GT125 is a "Noninverting Buffer / CMOS Logic Level Shifter with LSTTL−Compatible Inputs" in a SOT23-5 or SOT-353 package. Small simple and cheap.

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Apparently this subject was also discussed the other day: Step up 3.3V to 5V for digital I/O although the solution there is incorrect (thanks Dave Tweed).