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Wideband Active Filter

IP.com Disclosure Number: IPCOM000081689D
Original Publication Date: 1974-Jul-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 52K

Publishing Venue

IBM

Related People

Sierra, HM: AUTHOR

Abstract

Active filters make use of operational amplifiers plus external components to synthesize transfer functions. A typical example of a voltage-controlled voltage-source active filter is shown in Fig. 1. This implementation has the transfer function: (Image Omitted) These filters have the disadvantage that they are restricted to low frequencies.

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Wideband Active Filter

Active filters make use of operational amplifiers plus external components to synthesize transfer functions. A typical example of a voltage-controlled voltage- source active filter is shown in Fig. 1. This implementation has the transfer function:

(Image Omitted)

These filters have the disadvantage that they are restricted to low frequencies.

A wide-band active filter is proposed that employs a functional amplifier having a preferred transfer function. The amplifier has three stages, Fig. 2, including an input stage 10 consisting of a differential pair of transistors T1 and T2 to which an input signal is applied: a voltage shifting stage 12 having a differential pair of transistors T3 and T4 and a diode string D1 and D2; and an output stage 14 having a differential emitter-follower pair of transistors TS and T6.

When operating this amplifier in a filter similar to the one in Fig. 1, an input AC signal is applied to transistors T1, T2, and passed to T3, T4, T5 and T6. Diodes D3 and D4 are the references for current sources T7, T8, T9 and T10.

The filter amplifier is highly stable and provides very wide-band-width The combination of the functional amplifier plus the external components R(a), R(b), R and C offers the transfer function:

(Image Omitted)

The functional amplifier of Fig. 2 has a very large-gain bandwidth product Omega Rho.

There are no known commercially available operational amplifiers that achieve these values of gain-bandwidth...