Method and apparatus for amplifying a radio frequency signal

[0002] The present invention relates to the field of signal amplification, and more particularly, to a method and apparatus for amplifying a radio frequency (RF) signal employed, for example, in wireless communication devices.

[0003] Typical RF communication devices (or similar wireless devices such as cellular phones, for example) receive RF signals from a RF transmission source or similar transmission base (such as a cellular phone transmission tower or RF base unit, for example). If the operator is located near the RF transmission source, the RF communication device does not need to amplify or provide gain to the RF signal received because the RF signal received by the communication device is strong enough to be easily processed by the communication processing circuits within the RF communication device. However, if the operator is further away from the RF base as determined by the signal strength from signal strength indicator circuitry (such as, for example, item 61 in FIG. 1, which could include a received signal strength indicator or RSSI), the RF signal received by the RF communication device may be weak. Further, the RF signal may be near other signals having the same or similar frequency. In these cases, the RF signal received is difficult to process. Therefore, in such situations, the RF communication device must be able to amplify or provide gain to the RF signal received which originates from a far away source.

[0004] For example, when an operator is using a RF communication device remotely located from the RF transmitting source, the RF amplifier is engaged to amplify the incoming RF signal (an electromagnetic signal or wave) to allow processing by processing circuitry. When the operator is closer to the RF transmitting source, the incoming RF signal is relatively strong (corresponding to a high electromagnetic signal) which means that the RF amplifier must switch to a lower gain state (or amplification state). If the large RF signal continues to be amplified when near a RF transmitting source, the RF communication device's processing circuitry would be overloaded and will not function. Thus, a lower gain amplification state is required when near a RF transmitting source.

[0005] Linearity is an important characteristic in the design of wireless communications devices because linear amplifiers provide an RF amplified signal without, for example, degradation of the signal due to third order intermodulation. Usually, RF wireless devices employ bipolar transistor technology to amplify the incoming RF input signal. While most prior art RF variable gain bipolar amplifiers can change the amount of amplification or gain applied to the received RF signal by changing the direct current (DC) bias current flowing through the amplifiers, such amplifiers suffer because the amplifiers employed are not linear in the low gain mode because of the low DC bias current. Thus, if the RF signal contains additional unwanted large amplitude signals (such as, for example, interferers, which differs slightly from the desired RF signal in frequency), and these signals experience third order nonlinearity from the amplification process of the amplifier, the resulting effect of the third order nonlinearity between the two interferers can be a false signal created on the same frequency as the desired amplified signal. This resulting signal is known as the third order intermodulation product. The third order intermodulation product is a signal at the same frequency of the desired RF signal which corrupts the desired RF signal. In any event, the resulting intermodulation product signal may be difficult to process correctly by the RF communication device's processing circuitry.

[0006] Maintaining linearity of the RF amplifier is therefore an important consideration in RF communication design because a highly linear RF amplifier will be able to provide the desired RF signal to the RF communication device's processing circuitry at the correct frequency while minimizing the effects of third order intermodulation. Thus, there is a need for a method for variable linear amplification and an apparatus for a variable linear amplifier, particularly suited for RF communication applications, which amplifies in a linear fashion so that a desired RF input signal corresponds to a desired RF output signal transmitted to the RF communication device's processing circuitry without corruption from third order intermodulation. Further, it is desirable that the amplifier in RF communication devices should be able to maintain a constant input and output impedance from the low gain to high gain mode so that the filters before and after the amplifier will operate properly.

[0007] The accompanying Figures, where like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form part of the specification, further illustrate the present invention and, together with the detailed description of the drawings, serve to explain the principles of the present invention.

[0008] FIG. 1 is a schematic block diagram illustrating an embodiment of a configuration according to the present invention;

[0009] FIG. 2 illustrates one embodiment of the block diagram corresponding to FIG. 1; and

[0010] FIG. 3 illustrates another embodiment of the block diagram corresponding to FIG. 1.

[0011] The present invention concerns a method for variably amplifying a RF signal and an apparatus for variable linear amplification. In accordance with one embodiment of the present invention, and referring to FIG. 1, the present method and apparatus 10 includes a controllable high gain circuit 21 in communication with a transconductor 11 that receives an RF input signal, a controllable low gain circuit 31 in communication with the transconductor 11 and a controllable current dissipation circuit 41 in communication with the transconductor 11 and a ground source. The transconductor 11 is preferably a voltage to current or power to current conversion circuit such as, for example, those typically used in RF communication devices. Because the same amount of DC bias current flows through transconductor 11 when switching from the low gain mode to the high gain mode, the input impedance at the RF input does not change.

[0012] In operation, and referring to FIG. 1, a RF input signal is received by the RF communication device and transmitted to the transconductor 11 resulting in a transconducted signal or electrical current signal 11a. The transconducted signal 11ais coupled to the high gain circuit 21, the low gain circuit 31 and the current dissipation circuit 41. If the RF communication device is located remotely or far away from the RF transmitting source as determined by an external RSSI within a control circuit 61, control circuit 61 controls high gain circuit 21 via control signal 61ato receive the transconducted signal or current 11aand amplify or provide gain to the transconducted signal 11awhile preventing the low gain circuit 31 from modifying the transconducted signal 11a. A resulting high gain signal 21ais generated which is transmitted to an impedance matching circuitry 71 to generate a RF output signal. The large DC current bias allows the high gain circuit 21 to be a linear circuit and the RF output signal is now substantially free of the undesired third order intermodulation product which can corrupt the desired signal. The RF output signal is a low noise amplified signal which is transmitted to the communication device's processing circuitry for further communication processing.

[0013] If the RF communication device is located near the RF transmitting source, control circuit 61 controls low gain circuit 31 to receive the transconducted signal 11aand provides low amplification or insubstantial gain to a portion of the transconducted signal 11awhile preventing the high gain circuit 21 from modifying the transconducted signal 11a. Because the low gain circuit 31 is selected to pass the same amount of electrical current in transconducted signal 11a, it receives most or all of the current corresponding to the transconducted signal 11a. As those of skill in the art will now come to realize, maintaining the current high in the low gain path preserves the high linearity of the amplifier.

[0014] In the case where there is a low supply voltage Vcc, simultaneously a dissipation control circuitry means 51 controls current dissipation circuit 41 to receive a portion of the transconducted signal 11awhich is not transmitted through the low gain circuit 31 and transmit such remaining current to a ground source. As those of skill in the art will now come to realize, dissipation control circuitry means 51 diverts the desired current away from low gain circuit 31 to prevent the gate to source voltage of low gain circuit 31 from being too large whenever the low gain mode is set as determined by the control circuitry 61. A resulting low gain or insubstantial current signal 31ais generated which is transmitted to the impedance matching circuitry 71 to generate a RF output signal. Because the low gain circuit 31 is a linear circuit, the RF output signal is now substantially free of undesired frequency signals (such as, for example, third order intermodulation products) when insubstantial amplification of the RF input signal is provided. The RF output signal or power output is a low noise signal which is transmitted to the communication device's processing circuitry for further processing.

[0015] In one preferred embodiment, the high gain circuit 21 is a common gate amplifier controlled by a control circuit 61 by methods known to those of skill in the art. Additionally, the high gain circuit 21, in one embodiment is a high gain metal oxide semiconductor field effect transistor (MOSFET) amplifier. Preferably, the high gain MOSFET amplifier 21 is a n-channel metal-oxide semiconductor (NMOS) device with a width to length ratio characteristically larger than low gain circuit 31. Thus, as seen in FIG. 2, an exemplary high gain circuit 21 may include a device having 16 gates (number of gates or NG) where each gate has a 0.4 micron length (LG) and the width of each gate is 16 microns (WG) such as manufactured by Motorola for example, part numbers MBC13706 or MBC13707.

[0016] Again referring to FIG. 1, the low gain circuit 31 is also preferably a common gate amplifier controlled by control circuit 61 by methods known to those of skill in the art, and in one embodiment is electrically structured in parallel with high gain circuit 21. Low gain circuit 31 is preferably a low gain metal oxide semiconductor field effect transistor (MOSFET) amplifier. Preferably, the low gain MOSFET amplifier 31 is a n-channel metal-oxide semiconductor (NMOS) device with a width to length ratio characteristically smaller than high gain circuit 21. Thus, as seen in FIG. 2, an exemplary low gain circuit 31 may be a device having 3 gates (number of gates or NG) where each gate has a 16 micron width (WG) and the length of each gate is 0.4 micron (LG) such as manufactured by Motorola for example, part numbers MBC13706 or MBC13707.

[0017] The current dissipation circuit 41, as seen in FIG. 1, is in communication with and controlled by a dissipation control circuitry means 51. In one embodiment, the current dissipation circuit 41 is a metal oxide semiconductor field effect transistor (MOSFET) amplifier. Preferably, as seen in FIG. 2, the current dissipation MOSFET circuit 41 is a n-channel metal-oxide semiconductor (NMOS) device characteristically including 64 gates (number of gates or NG) where each gate has a 16 micron width (WG) and the length of each gate is 0.4 micron (LG) such as manufactured by Motorola for example, part numbers MBC13706 or MBC13707.

[0018] FIG. 2 illustrates one preferred embodiment according to the present invention. As those of skill in the art will know, the particular design and values illustrated in FIG. 2 are exemplary and are not intended to limit the scope of the present invention.

[0019] FIG. 3 illustrates an alternate embodiment 10 in accordance with the present invention. In this embodiment, one or more gain circuits 81 are controlled by control circuit 61 and electrically communicate with and are electrically structured in parallel with low gain circuit 31 and high gain circuit 21. While high gain circuit 21 provides high gain to transconducted signal 11aand low gain circuit 31 provides low gain to transconducted signal 11a, control circuit 61 can be used to control each gain circuit 81 to provide selective or variable gain to the transconducted signal 11aresulting in a variable gain signal 81a. A resulting variable gain signal 81ais generated which is transmitted to the impedance matching circuitry 71 to generate a RF output signal. Because the variable gain circuit 81 is a linear circuit, the RF output signal is now substantially free of undesired frequency signals (such as for example third order intermodulation products) when insubstantial amplification of the RF input signal is provided. The RF output signal or power output is a low noise signal which is transmitted to the communication device's processing circuitry for further communication processing.

[0020] As those of skill in the art will realize, the present invention can be implemented by individual discrete components, on an integrated circuit or like device (such as, for example, a microchip) or by a combination of both The exact implementation is a choice which remains within the spirit and the scope of the present invention.

[0021] Other variations and modifications of the present invention will be apparent to those of ordinary skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. The particular values and configurations discussed above can be varied, are cited to illustrate particular embodiments of the present invention and are not intended to limit the scope of the invention. It is contemplated that the use of the present invention can involve components having different characteristics as long as the principle, the presentation of a method and apparatus for amplifying a radio frequency signal, is followed.