Browse Prior Art Database

Tuneable Bandpass Filter

IP.com Disclosure Number: IPCOM000120230D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 2 page(s) / 63K

Publishing Venue

IBM

Related People

Schulz, RA: AUTHOR

Abstract

Varactor diodes and inductors have been used in recent years to implement the voltage-controlled tuning needed for RF amplifiers, but practical inductors cannot be built in semiconductor technologies. This article shows a circuit method which can implement a tuned bandpass filter using bipolar integrated circuit technology.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 54% of the total text.

Tuneable Bandpass Filter

      Varactor diodes and inductors have been used in recent
years to implement the voltage-controlled tuning needed for RF
amplifiers, but practical inductors cannot be built in semiconductor
technologies.  This article shows a circuit method which can
implement a tuned bandpass filter using bipolar integrated circuit
technology.

      The small signal output resistance of an emitter-follower
circuit (and also a diode) is approximately re=(kT/q)/IE, where k is
Boltzman's constant, q is the charge of an electron, T is absolute
temperature, and IE is the DC emitter bias current.  This resistance
is well-defined by physical constants and the DC bias current, so it
is predictable and can be controlled by altering the DC bias current.

      The tuned bandpass filter shown in the figure is based on a
differential amplifier rather than a single-ended amplifier to
simplify biasing, make the circuit insensitive to power supply
changes, and to take advantage of the very good component tracking
which is obtained in integrated circuit fabrication.

      The circuit uses semiconductor diodes in place of collector
load resistors, which does not create significant signal distortion
for small signals.  The gain of a transistor amplifying stage is
given by the ratio of the impedance seen by the collector circuit Zc
to the impedance seen by the emitter circuit Ze.  In this case, the
collector impedance is approximately given by
        ...