Application Note - AN-03 - Active Filter Design with IC Op-Amps

INTRODUCTION

This application note is intended to familiarize the filter designer with the fundamentals of active filter design, using monolithic IC op amps. It presents a table of transfer functions and network equations for high-pass, low-pass, band-pass and band-reject  filters. Several design examples are given to illustrate the respective merits and limitations of various filter configurations. Particular emphasis is given to applications of programmable quad operational amplifiers, such as the XR-4202, as an active filter element in FSK Modems.

FUNDAMENTALS OF ACTIVE FILTERS

The availability of low cost dual or quad operational amplifier IC's have made the operational amplifier based active filter techniques cost effective over conventional passive filters. The recent availability of programmable quad operational amplifiers such as the XR-4202 or the XR-346 have provided the active filter designer with the flexibility to externally program gain-bandwidth product, supply current, input bias current, input offset current, input noise and the slew rate. The user, therefore, can trade off bandwidth for supply current or optimize the noise figure. Likewise, other amplifier characteristics can be programmed for a specific
need.

Since the operational amplifier plays such a key role in the active filter, its characteristics are of prime importance. By using operational amplifiers as the basic gain stage of the active filter, problems previously encountered due to low input impedance, high output impedance and low gain are virtually eliminated. Operational amplifiers provide the required response for various filter types. Some of the more popular filters are multiple feedback, state variable, bi-quad and Sallen Key which can be used to obtain high pass, band pass and low pass filter functions (and which are capable of giving the designer all of the standard filter responses, i.e., Butterworth, Chebychev, Bessel, etc.)



Figure 9 above shows a circuit schematic for a complete "Originate or Answer" modem. The values for the XR-2206 are given in Table 6. For an originate modem the transmitting frequencies are 1070 Hz and 1270 Hz, the receiving frequencies are 2025 Hz and 2225 Hz, for a space and mark respectively.

The first op amp in Figure 9 is connected as an active hybrid which should supply a minimum of 10 dB isolation from transmit to receive, while adding 3 dB gain from the line to the receiver.

Note - The text above is a ocr of scan.

This is from an Old Application Note of Exar from - "Applications Data Book" June 1981.