Emitter-Coupled LC Oscillator

An emitter-coupled pair supplies regenerative feedback to an LC tank.

Circuit operation

Q1 collector drives an LC tank and couples an attenuated AC voltage to Q2 base through CF, RF, and RB. The divider keeps the feedback within a practical base-voltage range. The two transistors share a fixed tail current. A rising tank voltage steers current toward Q2, reducing Q1 loading and reinforcing the rise.

Expected behavior

$$f_0\approx\frac{1}{2\pi\sqrt{LC}}=50.3\,\mathrm{kHz}$$ $$I_{E1}+I_{E2}=2\,\mathrm{mA}$$

The initial tank-capacitor voltage starts regenerative current steering. The tank voltage can exceed the DC supply. RLOSS models finite tank loss. Device loading and nonlinear current steering set the amplitude. The chart opens on the last 200 µs after startup.

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 2.0 ms with a maximum step of 0.10 µs. Compare settled cycles after startup and use time-axis cursors to measure delays or periods.

Try a change

Set CT to 20 nF; the resonance estimate falls by a factor of √2. 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.