Active low-pass filter

Details Active low-pass filter Active low-pass filter

1999-05-26

2001-01-23

Mottola, Steven J. (Department: 2817)

Amplifiers

Signal feedback

In cascade amplifiers

C330S303000, C330S311000

Reexamination Certificate

active

06177837

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active low-pass filter which consumes less current and assures excellent distortion characteristic.
2. Description of the Related Art
FIG. 3
is a circuit diagram illustrating an active low-pass filter of the related art. As illustrated in
FIG. 3
, the active low-pass filter of the related art comprises of a common-emitter amplifying circuit
31
of the first stage and a common-base amplifying circuits
32
,
33
of the second and third stages, where a feedback resistor
34
is connected, as a negative feedback circuit, between an output end of the common-base amplifying circuit
33
of the third stage as the common-base amplifying circuit of the final stage and an input end of the common-emitter amplifying circuit
31
of the first stage, a first capacitor
35
for setting the cutoff frequency is connected between the connecting point of the common-emitter amplifying circuit
31
and common-base amplifying circuit
32
and the ground, and a second capacitor
36
for setting the cut-off frequency is also connected between the connecting point of the common-base amplifying circuit
32
of the next stage and the common-base amplifying circuit
33
of the third stage and the ground.
The collector of transistor Q
1
and emitter of transistor Q
2
are DC-connected via a resistor
37
so that a common collector current flows into the transistors Q
1
, Q
2
, Q
3
of each amplifying circuit
31
,
32
,
33
and the collector of transistor Q
2
and the emitter of transistor Q
3
are also DC-connected via a resistor
38
. Resistors
37
,
38
are provided to reduce distortion generated in the common-base amplifying circuits
32
,
33
having a low input impedance.
The power supply voltage is applied, via a load resistor
39
, to the collector of transistor Q
3
as the output end of the common-base amplifying circuit
33
, while the emitter of transistor Q
1
of the common-emitter amplifying circuit
31
is grounded through a resistor
40
.
The feedback resistor
34
sets amount of feedback current and also gives abias voltage to the base of transistor Q
1
of the common-emitter amplifying circuit
31
of the initial stage. Therefore, this feedback resistor
34
is connected between the base of transistor Q
1
as the input end of the common-emitter amplifying circuit
31
of the initial stage and the collector of transistor Q
3
of the common-base amplifying circuit
33
of the final stage and a bias resistor
41
is also provided between the base of transistor Q
1
and the ground. Thereby, a bias voltage is applied to the base of transistor Q
1
. Moreover, the bias voltage is also applied to the bases of transistors Q
2
, Q
3
through the bias resistors
42
,
43
,
44
.
In the structure explained above, a high frequency signal input from a signal input terminal
45
is amplified by the common-emitter amplifying circuit
31
through a coupling capacitor
46
and the high frequency element of this signal is cut by the first capacitor
35
. The high frequency signal is further amplified by the common-base amplifying circuit
32
through a resistor
37
and the high frequency element of this signal is then cut by the second capacitor
36
and is then input to the common-base amplifying circuit
33
of the final stage. The high frequency signal amplified by the common-base amplifying circuit
33
of the final stage is output from a signal output terminal
49
through a capacitor
47
, for impedance matching, and a coupling capacitor
48
.
In the active low-pass filter of the related art explained above, since a bias voltage is supplied to the base of the transistor in the common-emitter amplifying circuit of the initial stage through a feedback resistor connected to the collector of the transistor in the common-base amplifying circuit of the final stage and to the base of the transistor in the common-emitter amplifying circuit of the initial stage, a current flowing through the feedback resistor has been wasted. Moreover, since abias voltage is applied to the base of the transistor in the common-emitter amplifying circuit of the initial stage, a resistor is provided between the base and the ground, thereby resulting in loss of input signal.
SUMMARY OF THE INVENTION
It is therefore an object of the active low-pass filter of the present invention to reduce the amount of current dissipation and loss of signal.
In order to solve the problem explained above, the active low-pass filter of the present invention is provided with cascade-connected amplifying circuits of a plurality of stages respectively having the amplifying elements and capacitor for setting cut-off frequency connected between the connecting point of amplifying circuits explained above and the ground. Each amplifying element has an input electrode and an output electrode, and a feedback circuit is connected between the input electrode of the amplifying element in the amplifying circuit of the initial stage and an output electrode of the amplifying element in the amplifying circuit in the final stage. The feedback comprises of a serial circuit of a voltage dropping means which results in almost constant voltage drop regardless of an increase or decrease of current, and a feedback resistor. A bias voltage is supplied to the input electrode of the amplifying element in the amplifying circuit of the initial stage from an output electrode of the amplifying element in the amplifying circuit of the final stage via the feedback circuit.
Moreover, the active low-pass filter of the present invention uses a diode as the voltage dropping means and connects the anode of the diode to the amplifying circuit side in the final stage and the cathode of diode to the amplifying circuit side in the initial stage.
In addition, the active low-pass filter of the present invention uses a transistor as the voltage dropping means and connects the base of the transistor to an output electrode of the amplifying element of the amplifying circuit in the final stage and the emitter of the transistor to the feedback resistor and supplies the power supply voltage to the collector of transistor.
Moreover, the active low-pass filter of the present invention is provided with a constant current source between the input electrode of the amplifying element of the amplifying circuit in the initial stage and the ground.


REFERENCES:
patent: 3707685 (1972-12-01), Geffe
patent: 4363004 (1982-12-01), Englund et al.
patent: 5764100 (1998-06-01), Callicotte et al.
patent: 8-172340 (1996-07-01), None
Rodwell et al., “33 Ghz Monolithic Cascode A1InA3/GaInAs Heterojunction Bipolar Feedback Amplifier”, IEEE J.S.S. Circuits vol. 26, issue 10, Oct. 1991 pp. 1378-1382.

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