As a follow-up to my previous blog, Integration Choices: Analog Filters vs. Digital Filters, the next step would be to discuss how to integrate filters into the IC itself.
What techniques and architectures have evolved over the years that make integrating a filter onto an IC easier, smaller, better matched, variable, and lower power than ever before? Let's take a look at some of the most innovative ones, and I invite you to comment and add your own favorites too.
The MEMS resonator
In wireless communications circuitry, band-pass filters (BPFs) are widely used and need pretty high performance in order to minimize signal loss and attenuation of out-of-band signals and interferers. Let's take a look at MEMS resonators (See Figure 1) as an alternative to the traditional quartz crystal and surface acoustical wave (SAW) filters.
The MEMS resonator may be a good, low-cost alternative to conventional filter components. The square in the center of the device is suspended and held in place by the four arms. The device mechanically resonates at particular, well-separated frequencies.
(Source: Reference 1)
The BPF usually resides in the RF and IF stages of the RF analog front end (AFE). A major drawback in the MEMS resonator has been high series motion resistance to the signal that will lead to poor insertion loss. There are a number of techniques to help improve the attenuation of the signal due to this technique, but one effective approach involves reducing the gap between the electrodes and the resonating body. This technique reduces the motional impedance with the fourth power of the transduction gap. See Reference 1 for more details.
Using post-processing techniques, the basic CMOS process remains untouched. New devices are fabricated on top of the initially processed die, leading to a miniaturized and low-cost, mass production technique.
The active inductor
CMOS in RF has become the go-to process for low-power and mixed-signal integration in the industry. Let's look at an example of an RF band-pass filter (BPF) for a receiver front-end and see how an active inductor can play a critical role by implementing it in the CMOS process.
It has been demonstrated that an on-chip and tunable BPF can be constructed with an active inductor architecture in CMOS. The traditional architecture of an RF receiver front end has been to have the band-pass filter external from the CMOS receiver using discrete components such as in a lumped LC or a SAW filter design. With the advent of the software defined radio (SDR), an on-chip filter that can tune across a wide frequency range and also has adequate selectivity, is imperative to the high integration goal and ultimately single-chip solution of the SDR.
Until recently, passive spiral inductors have been integrated into the CMOS IC for this function. Their disadvantage is ohmic and eddy current losses and sensitivity to EMI. Shields can be added to help this, but are added cost in assembly and bill of materials (BOM) costs in the system. Spiral inductors can only achieve low Q-factors and low inductance values, plus they take up a great deal of real-estate on the chip when multiple inductors are needed. Enter the active inductor architecture.
The active inductor is architected using the gyrator principle shown in Figure 2(a). Figure 2(b) shows how the current-voltage characteristic of the intrinsic capacitance in the transistors is inverted to make the load appear inductive in nature. See Reference 2 for more details.
(a) The gyrator topology creates an inductive load Zin.
(b) The equivalent passive circuit is shown for the realized circuit.
(Source: Reference 2)
At the IC level, the inductor is realized by combining a common-source transductor (-gm) with either common-gate or common-drain transconductors (+gm). See Figure 3 for some examples of various popular configurations.
There are a few good active inductor configurations shown in literature, as seen here.
(Source: Reference 2)
This architecture has the benefits of small area on the chip as well as large inductance values. The center frequency (ω) and Q of this active inductor tunable filter has the added advantage of being tuned by varying the transconductance (or capacitance) of the transistor.
Reference 2 has more details regarding improvement of the Q for this type of filter.
There are still a number of issues with this technology, but Reference 2 outlines some ways to help solve these.
Moore's law implies that as RFIC semiconductors continue to downscale, the complexity of the ICs will increase.
In this architecture, we will look at a third order voltage mode active-C band pass filter as an example of the Active-C technique. This architecture only consists of three equal value capacitors, a single dual output current controlled current conveyor (DOCCCII), Figure 4, and two current controlled current conveyors (CCCIIs). See Reference 3 for more details.
The block diagram of the DOCCCII.
(Source: Reference 3)
This architecture is well suited for IC design since it uses a minimum number of active analog building blocks as well as a minimum of passive components. All of the used capacitors are grounded and resistors are not required. Nice!
The transistor structure of the DOCCCII.
(Source: Reference 3)
Current conveyors have the unique properties of wide dynamic range, high signal bandwidth, small chip area and low power consumption making for an ideal architecture for today's CMOS integrated RFICs.
The parasitic resistance at Port X in Figure 4, is adjustable via the bias current, thereby allowing tunability. Biquadratic filters have been built using this architecture and even higher-order filters can be built by such techniques as cascading lower order filters, signal flow graphs, and state variable techniques. See Reference 3 for more details.
These are only a few of the elements and architectures that are enabling the growth highly integrated analog ICs. Please share with us any of the favorite designs and technolgies that you have used.
- Mechanical resonators on CMOS for integrated passive band pass filters, S. N. R. Kazmi, C. Salm and J. Schmitz, MESA+ Institute for Nanotechnology, University of Twente, Enschede, Netherlands
- Development of Active Inductor in CMOS Tunable RF Bandpass Filter, Nur Diyana Mohd Asri, Norhayati Soin, Department of Electrical Engineering, University Malaya (UM), Kuala Lumpur, Malaysia.
- Realization of Active-C Voltage Mode Third Order Band Pass Filter with Current Controlled Current Conveyor (CCCII), Ashish Ranjan and Sajal K. Paul, Department of Electronics Engineering,Indian School of Mines, Dhanbad, India