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/ g_M² and G₃ = 1/R₃ with I_B1 = I_B2.

Upon analysis,

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

g_M = I_B/2V_T

where I_B = 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 ù_o = 50 krad/sec (i.e., f_C is approximately 7.96 kHz) and with a simulated value of L = 4 H are:

C₁ = C₂ = 0.1 nF
g_M = 1 mÙ^-1 (at I_B ~ 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.