More Characterization of the Multivibrator

The direct coupled multivibrator has proven to be a most interesting circuit. It also illustrates transistor action around Vbe rather directly. Some additional characterization was recently done with an updated version of the circuit. The updates were simple changes to improve measurement accuracy, and to act as stand-ins for an eventual circuit more enhanced for sythesizer module use. 

The following updates were applied:

1. The 1kΩ, 10kΩ, and 100kΩ ±5% resistors were replaced with ±1% tolerance resistors.
2. C1 and C2 were increased to 1.0μF, with ±5%, 100V Poly capacitors. This was to bring the astable frequency down toward more musically useful ranges, as versus the original test circuit.
3. The Vcc from the power supply was pre-calibrated, and measured (to 15.020V).

A lot more was learned about the transistor switching action occurs, and how this circuit uses that to develop the waveforms. A key insight is that the Vce can be very close to 0V, indicating that the transistor is in saturation -- but interestingly, this can also happen while both the collector and emitter voltages are rising simultaneously (along with the base signal 1Vbe about the emitter). In most multivibrators, the emitters are either directly or resistivevly coupled to ground or a negative Vee power rail, and are not free to move that much. In this circuit, the emitters move up and down a full Vbe, tracing out complementary triangle waves on each side.

The text for the scope fotos below will expand on these findings.

The basic waveform tableau, noting the 2.08V baseline for one of the triangle waves.

Examining the 1kΩ, 10kΩ, and 100kΩ junctions, where DC offset of the the triangle waveforms is more readily apparent. This is due to the ΔVbe of the transistors. The junctions also clearly act as a 2:1 attenuators. 

Lining up the Q1 collector/emitter (top traces) and the Q2 collector/emitter (bottom traces) with the same scale at 1V/div ...

... so we can use the math feature on the digital scope in order to compute Q1 Vce. The transistor switching action can now be seen, and how it phases triangle wave generation relative to the sawtooth waveforms.

The complementary action seen for the Q2 transistor Vce.