Astable Multivibrators - 1

I built an astable multivibrator from discrete components:

astable multivibrator circuit schematic

I used 2N2222 transistors instead of BCS547.

Here’s what that schematic looks like on a breadboard:

astable multivibrator breadboarded

This is the circuit’s outputs, powered at 5.0V, 2.16kHz

wave form of the 2 outputs

Just like the schematic says, one output is the mirror-image of the other.

Frequency - calculated vs measured

I tried to figure out how fast the circuit should oscillate.

Here’s the most sensible reference I could find.

According to that web page’s formula, values of 47K Ohm and 10 nF should give a 1541 Hz frequency output.

RC circuits aren’t supposed to be affected by the input voltages, but the frequency of my breadboarded circuit, as measured by oscilloscope, varied a lot with voltage.

V f kHz
< 1.3 <10 to 85 Hz, unstable
1.3 1.67
2.0 1.81
2.5 1.86
3.0 1.89
4.0 1.95
5.0 2.04
6.0 2.15
7.0 2.29

The capacitors measured 6 nF and 7.5 nF, which the resistors measured 46.5K Ohms. The difference in capacitance shows up in the slightly different pulse length in the oscilloscope trace above.

The as-measured frequency is:

f = 1/(0.69*6e-9*46500 + 0.69*7.5e-9*46500) = 2.3 kHz

That’s a better match to the measured frequency, but why the frequency variation with voltage?