Electronic Design

High-Q Bandpass Filter Is Well Suited To Current-Mode Signal Processing

Analog signal processing has conventionally been viewed as a voltage-dominated form of design. But voltage-mode processing can restrict the system's dynamic range. There is also a limitation on the input range of signals for linear operation. An approach to overcoming these problems is the use of voltage-to-current signal transformation. Recent advances in this context have opened a new dimension of analog design referred to as current-mode signal processing (CMP).

A novel current-mode second-order bandpass filter (BPF) is presented that can easily provide quality factors as high as 100 while still using nominal component values. Such high-Q filters are particularly important in applications like communication receivers and graphic equalizer displays. The circuit shown in Figure 1 provides a current-mode bi-quad output (for Y1 = sC1 ,Y2 = sC2 ,Y3 = G3, Y4 = 1/sL). Terminal W is the voltage-buffered output. If needed, the circuit can be programmed as a high-pass or low-pass filter as well (see the table).

Analog Devices' AD844, a monolithic current-feedback op amp, forms the heart of the circuit (Fig. 2). A dual-OTA implementation of a synthetic inductor, the LM13600 from National Semiconductor, is used for Y4. This provides the additional advantage of current-tunable filter characteristics. The current-mode transfer function of the BPF is modeled as:

where L = C4/ gM2 and G3 = 1/R3 with IB1 = IB2.

Upon analysis,

where gM is the OTA's transconductance, which is the function of the biasing current. For the bias configuration shown:

gM = IB/2VT

where IB = biasing current.

From these expressions, we can observe that independent control of Q is possible by varying only R3. Varying R3 from 1 to 5 kÙ results in values of Q that range from 20 to 100, respectively.

Over this range of Q values, the other circuit parameters, with ùo = 50 krad/sec (i.e., fC is approximately 7.96 kHz) and with a simulated value of L = 4 H are:

C1 = C2 = 0.1 nF
gM = 1 mÙ-1 (at IB ~ 50 µA)
C4 = 4 µF

The experimental results for three values of Q are shown in Figure 3. Note that in all the cases as G3 is varied no shift in the natural frequency is observed, confirming the independent Q-factor control.

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