BiCMOS AC filter circuit

Details BiCMOS AC filter circuit BiCMOS AC filter circuit

2002-06-26

2004-08-10

Nguyen, Patricia (Department: 2817)

Amplifiers

With periodic switching input-output

C330S011000, C330S257000, C327S307000

Reexamination Certificate

active

06774715

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to filter circuits and, more particularly, to alternating current filter circuits.
BACKGROUND
An alternating current (AC) filter circuit is an important component of many integrated circuits among them high-speed optical communications receivers. Conventional implementations of AC filter circuits for complementary metal oxide semiconductor (CMOS) type differential amplifiers typically rely on a simple resistor-capacitor filter fed to a differential amplifier, such as shown in FIG.
1
.
In the circuit of
FIG. 1
, the parallel-connected resistor (R
1
) and capacitor (C
1
) in combination act as a low-pass filter to produce an average value of the input signal to the gate electrode of the metal oxide semiconductor MOS transistor (M
1
). This average value is used as the threshold level for decoding the input signal into a stream of 1s and 0s. To achieve the threshold function, the filtered average value
12
from low-pass filter
10
is fed to the gate electrode of first MOS transistor (M
1
) of differential amplifier
20
, and the original non-filtered signal
14
is fed to the gate electrode of second MOS transistor (M
2
) of differential amplifier
20
. The two input values (i.e., signals
12
,
14
) are compared at the differential amplifier
20
. If the signal at M
2
(i.e., signal
14
) is greater than the signal at M
1
(i.e., signal
12
), then the output from differential amplifier
20
would be a ‘1’ otherwise it is a ‘0’. The output of the differential amplifier may be further amplified for a processing of the input signal.
CMOS type transistors offer the advantages of being relatively inexpensive to process, having low power dissipation, and they can be tightly packed and scaled. Bipolar junction transistors, on the other hand, are a preferred choice for analog applications because of their ability to provide high speed, high drive current, and a good noise margin. Additionally, bipolar junction transistors provide smaller threshold voltage variations and are therefore often used as voltage references.
What is needed is a way to create high-speed circuits having the speed of BiCMOS differential amplifier circuitry but does not have a time fluctuating base current into the amplifier input which may cause decoding errors in the BiCMOS amplifier because it is sensitive to fluctuating inputs.
SUMMARY OF THE INVENTION
An AC filter circuitry for a differential amplifier is disclosed in which the AC filter automatically compensates for the voltage drops caused by the fluctuating base current from the a differential amplifier built with BiCMOS technology.
An exemplary alternating current filter circuit of the present invention for a BiCMOS differential amplifier has a circuit that computes a base current of the BiCMOS differential amplifier, and a circuit that generates an offset current based upon the computing.
In another aspect of the invention, the required current to offset the resistive voltage drops caused by the base current of the differential amplifier is calculated by the computing circuit and the offset current is then generated by the generating circuit. Subsequently, the generated offset current is fed into buffering circuitry which in turn is input to the differential amplifier.
Other and further aspects of the present invention will become apparent during the course of the following detailed description and by reference to the attached drawings.
The advantages and features described herein are a few of the many advantages and features available from representative embodiments and are presented only to assist in understanding the invention. It should be understood that they are not to be considered limitations on the invention as defined by the claims, or limitations on equivalents to the claims. For instance, some of these advantages are mutually contradictory, in that they cannot be simultaneously present in a single embodiment. Similarly, some advantages are applicable to one aspect of the invention, and inapplicable to others. Thus, this summary of features and advantages should not be considered dispositive in determining equivalence. Additional features and advantages of the invention will become apparent in the following description, from the drawings, and from the claims.


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