How do you increase Q factor using active device

[iVenky]

I was studying in Lee book on Rf microelectronics that it's possible to increase Quality factor using active deice. You know how it's possible? I am not sure if it's the same thing as restoring the energy dissipated by an RLC tank.

 

[pancho_hideboo]

Impossible.

LC oscillator is equilibrium state when loss and negative resistance are equal.

So Q factor as oscillator can never be improved even if you introduce active circuits which have negative resistance.

 

[Warpspeed]

Positive feedback with an amplifier can do it.
Or a suitable negative resistance in series with the RLC tank will do it too.

Negate resistance can come from a Gunn or tunnel diode, or Lambda circuit, or even some configurations of tetrode electron tubes.

Its really the basis of all LC oscillators, but you can reduce the feedback just below the point of oscillation where the circuit Q is hugely magnified.
Any disturbance will cause it to ring almost forever.

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Positive feedback with an amplifier can do it.
Or a suitable negative resistance in series with the RLC tank will do it too.

Negate resistance can come from a Gunn or tunnel diode, or Lambda circuit, or even some configurations of tetrode electron tubes.

Its really the basis of all LC oscillators, but you can reduce the feedback just below the point of oscillation where the circuit Q is hugely magnified.
Any disturbance will cause it to ring almost forever.

 

[RichardHead]

The best you can achieve without positive feedback is to load the resonant circuit as little as possible. You then approach the unloaded Q of the circuit.
Incidentally, the old regenerative receivers used positive feedback with the amplifier operating on the ragged edge of oscillation. That was mainly done to get more gain out of the amplifiers though.

 

[betwixt]

The OP never mentioned oscillators but the yes/no debate suggests the question isn't adequately explained.

A single component may be regarded as having 'Q' and this is fixed by it's design.
An LC resonant circuit circuit has a bandwidth which is narrower and has a taller peak (or deeper notch) if the 'Q' is higher.
Applying controlled positive feedback will reduce the bandwidth of a resonant circuit, exhibiting the same characteristics as a increased 'Q'.

A "Q multiplier" works on the principle of offsetting the circuit losses using positive feedback but it is only practical to use it for small signal tuned circuits. You can not practically use a Q multiplier to improve an LC output filter in a PWM power application for example.

Brian.

 

[E-design]

Here is an interesting piece: Active Inductors Tune Low-Noise VCOs