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Project Notes

#378 BJT Phase Splitter

Simple unity-gain phase splitter in a little breadboard-compatible package.



Section 2.08 in the The Art of Electronics describes a unity-gain phase splitter which generates an output signal and its inverse (180˚ out of phase). This is achieved with a common emitter amplifier with emitter degeneration for a gain of -1 (~ 4.7kΩ/4.7kΩ)

With an input signal of 100kHz, the input coupling capacitor should be greater than 39pF - I’m using 100nF here.


In the following tests, the scope is connected as follows

  • CH1 (yellow): SIGNAL input, DC coupled - 100kHz 1v peak-peak sine wave
  • CH2 (blue): SIGNAL- inverted output, DC coupled, offset varies according to Vcc
  • CH3 (red): SIGNAL+ non-inverted output, DC coupled



With VCC=5V, the quiescent point:

  • non-inverting output centered around 0.8V
  • inverting output centered around 4.3V

CH2 (blue) is offset -2V



With VCC=9V, the quiescent point:

  • non-inverting output centered around 1.9V
  • inverting output centered around 7.2V

CH2 (blue) is offset -4v



With VCC=15V, the quiescent point:

  • non-inverting output centered around 3.0V
  • inverting output centered around 7V

CH2 (blue) is offset -4v




Testing on a breadboard:



I put on a piece of protoboard in a breadboard-compatible layout:



Credits and References

About LEAP#378 BJTSignals
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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

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Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.