#378 BJT Phase Splitter

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

Notes

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.

Testing

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

Vcc=5V

With VCC=5V, the quiescent point:

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

CH2 (blue) is offset -2V

Vcc=9V

With VCC=9V, the quiescent point:

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

CH2 (blue) is offset -4v

Vcc=15V

With VCC=15V, the quiescent point:

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

CH2 (blue) is offset -4v

Construction

Testing on a breadboard:

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

Credits and References

• The Art of Electronics, 2nd Edition - 2.08 Unity-gain phase splitter, p77.
• 2N3904 datasheet
• ..as mentioned on my blog