The circuit in Fig 1a synthesizes a negative resistance. Putting this network in series with the inductor in Fig 1b increases the inductor's Q factor. Fig 1b's filter was designed to attenuate a noise-power-ratio test signal's adjacent carriers of 200 kHz ±4 kHz by 20 dB. The single parallel LC circuit provided the necessary low distortion and high stability, but a higher inductor Q was necessary.

Fig 1a is a modified version of the Howland current source, and is rearranged and designed to be open-circuit stable. If V_OV₂, which negative feedback ensures, then V₁2V₂. Then

and

You can adjust the negative resistance by using R_T to achieve 20 dB of attenuation at 200 kHz ±4 kHz. (DI#1678)