# Use BJT transistor as a switch without inverting the signal

I know I can use a transistor as a switch like this:

simulate this circuit – Schematic created using CircuitLab

Can I use only one transistor that will output the same signal (not inverted)? I want to use it to shift voltage level. The input is digital at 3.3V and should output 5V TTL levels.

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This circuit tends to be used quite often: -

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Absolutely.

This circuit takes advantages of the properties of a MOSFET to bidirectionally switch a signal between two different voltage levels.

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I was interested in doing this with a bipolar PNP or NPN. –  Cornelius Apr 21 at 17:15
@Cornelius This answer is the better one dude because it is bidirectional whereas my circuit is not. –  Andy aka Apr 21 at 17:16
I know it's bidirectional. I have seen this circuit before in a schematic for an I2C level shifter. But for what I need unidirectional is enough. –  Cornelius Apr 21 at 17:19
@Andyaka: You're a good guy. Ignacio: useful circuit :) –  bitsmack Apr 21 at 19:26
@bitsmack yeah I'm all-heart muhuhahaha –  Andy aka Apr 21 at 20:05

The input is digital at 3.3V and should output 5V TTL levels.

For this situation you very likely do not need any conversion circuit at all.

Both 3.3 V and 5 V TTL logic switch with a threshold of about 0.8 V. Therefore no conversion circuit is needed to drive a 5 V TTL input with a 3.3 V logic signal. To be absolutely sure, check the minimum Vih of your 5 V TTL device and make sure this number is less than the minimum Voh of your 3.3 V device.

This figure, from the TI app note, "Selecting the Right Level-Translation Solution", shows that the specified input, output, and threshold levels are the same for 5 V and 3.3 V TTL circuits:

If your devices are actually not TTL but CMOS, you may still be able to connect them without a conversion circuit, but noise margin will be reduced because the CMOS switching threshold is Vcc/2 rather than a fixed voltage level.

As you see from the chart, you cannot reliably use a 3.3 V TTL signal (or even 5 V TTL) signal to switch a 5 V CMOS input, so for either of those situations you would need a converter circuit as outlined by the other answers here.

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