Information
-
Patent Grant
-
6587018
-
Patent Number
6,587,018
-
Date Filed
Friday, May 25, 200123 years ago
-
Date Issued
Tuesday, July 1, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
-
CPC
-
US Classifications
Field of Search
US
- 333 175
- 333 176
- 333 184
- 333 185
- 333 174
- 330 286
- 330 302
- 330 306
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International Classifications
-
Abstract
An apparatus and method are provided for attenuating an undesired frequency (f1) relative to a desired frequency (f0) in a signal transmitted from an out-put (220) of an active element (210). Generally, the apparatus includes a parallel-resonant trap (145) coupled to the output of the active element (210), the trap including a network of reactive elements (235).
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to amplifier circuits, and more particularly to an apparatus and method for attenuating an undesired frequency in a signal output from an RF amplifier circuit in a wireless communications system, for example.
2. State of the Art
Wireless communications systems may typically include a chain of amplifier circuits in stages, each stage comprising an amplifier an filter circuit through which a received or modulated signal is passed in series. At each stage the filter circuit filters out unwanted (out of band) frequencies while the amplifiers amplify the remaining signals. Typically, each stage in the chain of amplifier circuits is a Radio Frequency (“RF”) amplifier circuit.
RF amplifier circuits are well known and widely used in, for example, receivers, transmitters and transceivers including devices such as cellular telephone handsets, base stations, pagers and wireless modems.
An example of an RF amplifier circuit suitable for use in a cellular telephone handset is shown in FIG.
1
. Referring to
FIG. 1
, a conventional RF amplifier circuit
10
typically includes an amplifier
12
having at least one active element or device
14
for amplifying a desired frequency or frequencies in a signal received on an input
16
thereto, and a network
18
performing the dual role of impedance transformation and suppressing or attenuating an undesired frequency or frequencies in the signal from an output
20
thereof. In the example shown in
FIG. 1
, the filter
18
is a particular type of low-pass filter (LPF), known as a three-section LPF, having a shunt capacitor
22
, a series inductor
26
and another shunt capacitor
24
, connected in that order. Values of the capacitors
22
,
24
, and the inductor
26
are selected to pass substantially unimpeded all frequencies below a predetermined first frequency (f
0
) while attenuating all frequencies above f
0
. For simplicity the amplifier circuit
10
is shown as including a single amplifier
12
with a single active element
14
and a single filter
18
, however it will be appreciated that the amplifier circuit can include additional active elements and filters.
A graph of the output versus frequency of the amplifier circuit
10
of
FIG. 1
is illustrated in FIG.
2
.
FIG. 2
is a graph of the gain, that is the change in strength of the signal between the input
18
and the output
20
, versus frequency. As shown by line
28
in
FIG. 2
, the amplifier
12
is biased and the filter designed such that all frequencies below f
0
have a generally constant gain and are passed through the filter
18
substantially unimpeded, while all frequencies greater than f
0
are attenuated by an amount or factor that increases in proportion to the frequency. Generally, it is desirable to suppress the undesired frequencies above f
0
to avoid distortion of the desired output waveform.
A particular problem with convention amplifier circuits is the suppression of harmonics of the desired frequency, and more particularly the suppression of a second harmonic of a desired or fundamental frequency. Because of the proximity of the second harmonic, 2f
0
, to the fundamental frequency, f
0
, conventional amplifier circuits using simply a low-pass filter have generally been unable to sufficiently suppress the second harmonic to avoid signal distortion. For example, as shown in
FIG. 2
, for an amplifier circuit using a conventional LPF
18
as shown in
FIG. 1
, the signal out will include in addition to the fundamental frequency a second harmonic that is attenuated by a factor of less than about 30 dB relative to the fundamental.
Several approaches have attempted to provide an amplifier circuit having a filter or apparatus for sufficiently suppressing the second harmonic while pass the fundamental frequency substantially unattenuated. One approach, also shown in
FIG. 1
, is the addition of a series-resonant trap
30
in shunt with the output
20
of the amplifier circuit
10
. The series-resonant trap
30
is designed to have a low impedance to any frequencies occurring at the second harmonic, thereby shunting a portion of this component of the signal to ground. The result, as shown by line
32
in
FIG. 2
, is a dip, or notch, in the output from the amplifier circuit
10
at the second harmonic. However, while this approach is effective to a degree, it is not wholly satisfactory. For example, to generate 1 W of power from a 3 V battery requires the active device to have load impedance below 4 ohms. For the shunt trap to successfully remove signals at the second harmonic, its impedance must be significantly lower, such as 0.5 ohms. Such low impedances are difficult to attain.
Another problem with the use of the series-resonant trap of
FIG. 1
is the impact of the trap on other characteristics of the circuit.
BRIEF DESCRIPTION OF THE DRAWING
The present invention may be further understood from the following description in conjunction with the appended drawing. In the drawing:
FIG. 1
is a schematic diagram of a conventional amplifier circuit;
FIG. 2
is a graph of the gain characteristics of the amplifier circuit of
FIG. 1
;
FIG. 3
is a block diagram of an amplifier circuit in accordance with the invention;
FIG. 4
is a graph of the gain characteristics of the amplifier circuit of
FIG. 3
;
FIG. 5
is a more detailed diagram of one embodiment of the amplifier circuit of
FIG. 3
;
FIG. 6
is a circuit diagram of a low-pass filter prior to modification;
FIG. 7
is a circuit diagram of a low-pass filter obtained by modifying the low-pass filter of
FIG. 6
;
FIG. 8
is a plot of the response of the filter of
FIG. 6
;
FIG. 9
is a plot of the response of the filter of
FIG. 7
;
FIG. 10
is a plot of the response of a similar filter circuit designed without the benefit of the methodology presented;
FIG. 11
is a circuit diagram of a high-pass filter prior to modification;
FIG. 12
is a circuit diagram of a high-pass filter obtained by modifying the low-pass filter of
FIG. 11
;
FIG. 13
is a plot of the response of the filter of
FIG. 11
;
FIG. 14
is a plot of the response of the filter of
FIG. 12
; and
FIG. 15
is a plot of the response of a similar filter circuit designed without the benefit of the methodology presented.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to an apparatus and method capable of attenuating an undesired frequency in a signal output from an amplifier, especially, for example, in a portable telephone handset or base station such as in a wireless communications system, while minimizing or eliminating any effect on the main signal.
An embodiment of an amplifier circuit
140
for use in, for example, an RF transmitter will now be described with reference to FIG.
3
. Generally, the amplifier circuit
140
includes a high frequency power amplifier
180
having an input
185
for receiving a signal thereon and an amplifier output
190
for coupling the amplifier signal to a parallel-resonant trap
145
. A circuit output
195
couples the signal from the parallel-resonant trap
145
to subsequent stages or devices such as an antenna or additional amplifying or filtering circuits. Optionally, as shown in
FIG. 3
, the amplifier circuit
140
may include additional filtering elements such as a filter
200
. The filter
200
may include elements configured to form a low-pass, high-pass or band-pass filter.
A graph of the gain versus frequency of the output
195
of the amplifier circuit
140
of
FIG. 3
is illustrated in FIG.
4
. As shown by line
205
in
FIG. 4
, the amplifier
140
is designed and biased such that all frequencies below the desired frequency (f
0
) have a generally constant gain. The filter
200
may be a single or multiple pole low-pass filter configured such that all frequencies above the desired frequency (f
0
) are attenuated by an amount or factor that increases in proportion to the frequency. In accordance with the present invention, the parallel-resonant trap
145
is tuned to further attenuate or suppress an undesired frequency. In the embodiment shown, the undesired frequency is a second harmonic of the fundamental desired frequency, and the parallel-resonant trap
145
is tuned to attenuate this frequency while leaving the desired frequency substantially unchanged.
A schematic diagram of an amplifier circuit
140
having a parallel-resonant trap
145
according to the present invention will now be described with reference to FIG.
5
. Generally, the amplifier circuit
140
includes an amplifying or active element
210
, such as a bipolar or field effect transistor, having an amplifier input
215
and an output
220
coupled through an inductor
225
to a voltage source
230
, shown here as V
DD
, and to the parallel-resonant trap
145
. While shown here as a single active element
210
, it will be appreciated that the amplifying element can include any number of active elements, formed either as discrete devices or as an integrated circuit (IC), and cascaded to further increase gain.
In one embodiment shown in
FIG. 5
, the parallel-resonant trap
145
is a network of reactive elements
235
including capacitive and inductive elements having fixed or variable reactance. Each of the active elements may include one or more discrete components or can be distributed capacitance or inductance due to other circuit elements such as wires, vias, bonding pads and the like. In one version of this embodiment, the network of reactive elements
235
functions as both a low-pass filter and a parallel-resonant trap by proper selection of values of inductance (L) and capacitance (C) for L
1
, CP, C
1
and C
2
, respectively. Specific values of inductance and capacitance for these elements will depend on the application in which the amplifier circuit is to be used, further in view of the desired frequency and the undesired frequency to be suppressed.
It is to be noted that any of the capacitive and inductive elements in the above embodiments may include a variable reactance element, such as a variable inductor or a variable capacitor. The ability to adjust reactance of one or more elements enables the parallel-resonant trap
145
to be tuned to closely match the undesired frequency and to compensate for parasitic capacitances and inductances that arise from other elements in the amplifier circuit
140
.
An advantageous design procedure allows the network of reactive elements
235
to be readily designed based on well-understood filter design techniques. Using this procedure, first, a filter is designed without regard to a desired notch characteristic. Then, one or more additional reactive elements are added to provide a notch in the filter response, and the values of existing reactive elements are adjusted by formula to preserve substantially unchanged the corner frequency and other characteristics of the original filter. The design procedure is general and may be applied to low-pass, high-pass, or band-pass filters.
The approach may be summarized in accordance with the following table:
|
Initial
Impedance at
Added
Frequency
|
element
f
1
Element
New base element
Added element
restriction
|
|
Inductor L
1
X
L
1
= ω
1
L
1
Series Cs
f
2
< f
1
|
|
Shunt Cp
f
2
> f
1
|
|
Capacitor C
1
Series Ls
f
2
> f
1
|
|
Shunt Lp
f
2
< f
1
|
|
Use of the table may best be illustrated by way of example. Take first the example of a low-pass filter as illustrated in
FIG. 6
, where the values indicated are expressed in picofarads and nanohenries, respectively. In general, a low-pass filter has shunt capacitors and series inductors. Although a three-section filter is shown, the filter may have any desired number of sections. The response of the low-pass filter of
FIG. 6
has a corner frequency of f
1
=1 GHz as shown in FIG.
8
.
Assume now that a notch is to be added to the filter response of
FIG. 8
at a frequency f
2
>f
1
. Such a notch characteristic may be achieved by adding a capacitor C
P
in parallel with the inductor L
1
of
FIG. 6
to arrive at the circuit of FIG.
7
. The values of the capacitor C
P
and the new inductor L
2
may be determined from the foregoing table. Since a shunt capacitor is being added to an inductor (the initial element), the second row of the table applies and may be used to calculate the values of C
P
and L
2
, which are indicated in FIG.
7
. As seen in
FIG. 9
, using the values from the table, the desired notch characteristic is achieved with negligible effect on the original comer frequency f
1
. If instead the value of the initial element were kept unchanged and the new element added having a value calculated to achieved the desired notch characteristic, a loss of about −1 dB would be experienced at the original comer frequency, as illustrated in FIG.
10
.
Instead of adding a shunt capacitor C
P
to the inductor L
1
of
FIG. 6
, a series inductor L
S
could be added to one or both of the capacitors C
1
and C
2
. Row
3
of the table would be used for this purpose.
FIG.
11
through
FIG. 15
illustrate in similar fashion modification of a high-pass filter to add a notch to the filter response. Typically, the form of a high-pass filter is the dual of that of a low-pass filter—that is, inductors and capacitors are interchanged such that, in the high-pass case, capacitors are connected in series and inductors are connected in shunt. A notch is created in this instance by adding a shunt inductor L
P
to the capacitor C
2
, using row
4
of the table. Instead of adding a shunt inductor L
P
to the capacitor C
2
of
FIG. 11
, a series capacitor C
S
could be added to the inductor L
1
. Row
1
of the table would be used for this purpose.
It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.
Claims
- 1. A method of modifying a filter design including a reactive element of a first type and having a given filter response having a given comer frequency to add to a response of the filter a notch characteristic occurring at a desired notch frequency, comprising:adding a reactive element of a second type to the reactive element of the first type to form one of a series-resonant combination and a parallel-resonant combination; determining by formula a value of the reactive element of the second type as a function of the value of the reactive element of the first type, the comer frequency, and the notch frequency; and determining by formula a new value of the reactive element of the first type as a function of an old value of the reactive element of the first type, the comer frequency, and the notch frequency.
- 2. The method of claim 1, wherein a desired notch characteristic is achieved with negligible effect on the original corner frequency.
US Referenced Citations (9)
Foreign Referenced Citations (2)
Number |
Date |
Country |
0213009 |
Sep 1991 |
JP |
0114505 |
Apr 1992 |
JP |