To push the transient ring into continuous oscillation, the gain of
the FET must be increased. By raising the VIN from 12V to 18V, both
the load current and gm increase. This amplifies the positive
feedback enough to maintain oscillation, as depicted in the scope
capture shown in Figure 9.
Now that the issue has been replicated, let's implement the
well-known fix: a 10Ω gate resistor is placed in series with the
inductor (Figure 10). This addition effectively halts the oscillations,
resulting in a clean startup (Figure 11).
Returning to the basic NFET Colpitts oscillator, the damping effect
of R
GATE
can be observed by introducing a switchable gate resistor
(Figure 12). When stepped from 0Ω to 10Ω, the oscillations are
damped (Figure 13).
Figure 9: Scope capture of continuous oscillations from increasing V
IN
.
(Source: Analog Devices)
Figure 10: Demo board test circuit with gate resistor added.
(Source: Analog Devices)
Figure 11: Scope capture of oscillation-free startup after adding a gate
resistor. (Source: Analog Devices)
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