Astable Multivibrator (Oscillator)

Two capacitor-coupled transistor stages switch alternately to make a free-running square wave.

Circuit operation

Two 47 nF capacitors cross-couple the collectors to the opposite bases. Each 47 kΩ base resistor charges a timing capacitor until its transistor switches on; the resulting collector step turns the other transistor off.

Expected behavior

$$T\approx\ln2\,(R_{B1}C_1+R_{B2}C_2)$$ $$\begin{gathered}T\approx3.06\,\mathrm{ms}\\f\approx327\,\mathrm{Hz}\end{gathered}$$

The collectors alternate between high and low. The RC expression is an estimate; base clamping, finite saturation voltage, and startup initial conditions affect the actual period. Compare settled cycles rather than the first transition.

Reading the results

The first voltage probe is green and the second is red. Additional voltage traces use distinct colors. Current probes, when present, start with blue and then purple. The chart opens with separate traces on a fixed time window; the table retains the same probe order. Zoom to extents includes the complete schematic.

The saved transient runs for 24.0 ms with a maximum step of 2.00 µs. Compare settled cycles after startup and use time-axis cursors to measure delays or periods.

Try a change

Set C1 and C2 to 100 nF; the oscillation period increases. Restore the saved values after comparing the result.

Model limits

These are generic transistor models, not a specific manufactured part. The bipolar models use nominal forward current gain 100 with illustrative junction capacitances and transit times. Device mismatch, temperature drift, layout parasitics, breakdown and detailed noise are not represented. The schematic and component properties expose the values used.