How Can One Chip Generate Filters and Oscillators Simultaneously?

A research team has designed an ultra‑compact analog circuit that merges two classic electronic functions: a multi‑mode biquadratic filter and a dual‑mode quadrature oscillator. The trick? A single differential gain amplifier (DDGA) and all‑grounded passive components.

With this architecture, the same layout can work in four different modes – voltage, current, trans‑impedance and trans‑admittance – without any structural changes. As a filter, it delivers low‑pass, high‑pass, band‑pass, band‑stop and all‑pass responses, with electronically tunable pole frequency and quality factor. As an oscillator, it produces simultaneous voltage and current quadrature outputs (90° phase shift) with independent frequency and start‑up condition control.

The grounded‑component design simplifies chip integration, reducing area and fitting well with 0.18 µm CMOS technology. Simulations report total harmonic distortion below 7 %, modest sensitivity to 5 % component variations, and stable operation from –40 °C to 120 °C.

These traits make the circuit perfect for low‑power IoT gadgets, adaptive communication front‑ends, and integrated biomedical systems, where space, energy and flexibility are key.

The core of the project is the Differential Differencing Gain Amplifier (DDGA), a differential amplifier that provides controllable gain and the ability to simultaneously differentiate voltage and current signals. The topology uses only ground‑referenced passive components (resistors and capacitors), eliminating the need for active elements tied to different potentials, which reduces offset noise and improves thermal stability.

The biquadratic filter is implemented via two second‑order sections, each defined by a pair of complex poles with angular frequency ω₀ = 1/√(RC) and quality factor Q = ω₀R/C·G, where G is the DDGA gain. Adjusting G shifts the poles without changing physical components, enabling electronic tuning of the frequency response.

In oscillator mode, the circuit exploits the Barkhausen condition: the feedback loop must have unity gain and a total phase shift of 0° (or 360°). The DDGA supplies the necessary gain, while the capacitors and resistors set the oscillation frequency f₀ = 1/(2π√(RC)). The quadrature configuration yields two outputs 90° apart, useful for I/Q modulation in wireless communications.

Simulations in a 0.18 µm CMOS process with ±0.9 V supply show total harmonic distortion (THD) below 7 % even at signal levels near the DDGA linearity limit. Sensitivity analysis indicates that a 5 % variation in R and C values causes less than a 2 % deviation in oscillation frequency, demonstrating robustness against fabrication tolerances. Moreover, the circuit maintains specifications over a wide temperature range (‑40 °C to 120 °C), thanks to the grounded‑component design that minimizes thermal drift on active parameters.

This architecture is especially suited for integration in System‑on‑Chip (SoC) for IoT devices, where reducing the number of active components and operating in multiple modes cuts overall power consumption and simplifies PCB layout. Additionally, the ability to generate simultaneous voltage and current quadrature signals opens the door to more flexible RF front‑ends capable of dynamically adapting to different frequency bands and channel conditions.