Amplifiers – With semiconductor amplifying device – Including signal feedback means
Reexamination Certificate
1998-01-30
2001-04-03
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including signal feedback means
C330S294000, C330S302000
Reexamination Certificate
active
06211738
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to minimizing feedback in transistor amplifiers. More specifically, the invention uses inductive coupling to cancel a portion of the feedback inherent in these amplifiers.
BACKGROUND
Amplifiers are subject to reduced gain when feedback between the input and output signals occurs. Conventional transistor amplifiers are especially subject to instability from feedback. Such feedback is inherent in transistors due to the presence of parasitic and junction capacitance between the collector-base or the drain-gate terminals.
For example, in a cordless telephone system, a handset generates, amplifies and then transmits the signals to a base station. Any instability or decrease in gain of the amplifier, however, may decrease the quality of the transmitted signal, and as a result, can decrease the performance of the telephone system.
The inclusion of an inductive element between the output and the input terminals can reduce the amount of amplifier feedback at specific frequencies. The presence of the inductive element creates a parallel resonant circuit with the feedback capacitance which presents a high impedance feedback path at the resonant frequency. This technique is known as neutralization. A neutralized amplifier will demonstrate improved stability and increased gain because of the reduction of capacitive feedback.
The technique of neutralization in discrete circuits is not widespread as the feedback is typically minimized at a single frequency. Also, the presence of an inductive element can increase the feedback at lower frequencies and result in instability.
SUMMARY OF THE INVENTION
The present invention provides a unique apparatus and method which improves stability and enhances gain of an amplifier by using mutual inductance to resonate with the feedback capacitance. For example, in cordless or cellular telephone systems, an amplifier amplifies signals prior to transmission to a base station. These amplifiers, however, are subject to reduced performance due to capacitive feedback. The present invention minimizes the capacitive feedback through the use of mutually coupled inductors. Previously, neutralization was accomplished with a single physical inductor. Using mutual inductance to resonate with the feedback capacitance allows the physical inductor element to be eliminated.
Many amplifiers have a first inductor at the input and a second inductor at the output of the amplifier. These inductors act as tuning elements and may provide a direct current path for biasing the amplifier. In common design, it is desired to position these inductors away from each other so the inductors do not inadvertently interact. With discrete designs, shielding is often used to prevent interaction. One embodiment of the invention positions these inductors near each other so to create a mutual inductance between the two inductors. This mutual inductance couples the output terminal to the input terminal of the amplifier. The mutual inductance resonates with the feedback capacitance and thus minimizes the feedback of the amplifier at the resonant frequency.
The use of coupled inductors to neutralize the amplifier also reduces the problem of low-frequency instability. Coupling between inductors decreases at low frequencies. If a mutual inductance is used in the feedback path, this feedback will decrease as the frequency is reduced. Stability of the amplifier is a concern at lower frequencies because the transistor exhibits more gain at lower frequencies. Therefore, the use of a mutual inductance tends to reduce the feedback in the range of frequencies where the gain is greatest, and thus the operation of the amplifier is more stable. By contrast, a discrete inductor between the output and input terminals of the amplifier results in more feedback at lower frequencies, and the operation of the amplifier in this region is less stable.
Another advantage of using coupled inductors is that large values of effective inductance may be realized by choosing an appropriately small value of mutual inductance. In many cases, the value of the inductive element needed to neutralize the amplifier is relatively large. The large value required often makes the use of a discrete inductor unfeasible. This is especially true in integrated circuit amplifiers where the size of each component is critical.
Another advantage of the invention is the ability to use relatively small inductors. Using a small amount of mutual inductance causes a large value of inductance to be sensed across the feedback path. Because the physical proximity of the coupled inductors can control the amount of mutual inductance, inductors with small values can maintain a large value of inductance sensed across the feedback path.
Another advantage is the ability to incorporate the invention within Monolithic Microwave Integrated Circuit (MMIC) amplifiers. MMIC amplifiers are a type of semiconductor amplifier used in the telephone industry. In integrated circuits, printed elements such as spiral inductors are generally tolerant to process and temperature changes. This allows the amount of coupling to be precisely controlled once the layout of the circuit is fixed.
One embodiment of the invention exploits components which may already exist in an amplifier circuit to minimize the capacitive feedback. This allows the amplifier to be neutralized without any increase in cost of manufacturing.
One embodiment of the present invention is a telephone handset for communicating with a base station. The handset generates and amplifies signals to be transmitted to the base station. The handset comprises an amplifier for boosting the gain of the signals prior to transmission, the amplifier having capacitive feedback between an input and an output of the transistor wherein a mutual inductance neutralizes the amplifier. The amplifier comprises a transistor having an input terminal and an output terminal, a first inductor coupled to the input terminal of the transistor, and a second inductor coupled to the output terminal of the transistor. The first inductor and the second inductor are positioned relative to each other to create the mutual inductance between the output terminal of the transistor and the input terminal of the transistor, the mutual inductance resonating with the feedback capacitance in the transistor. The amplifier, the first inductor, and the second inductor are implemented on an integrated circuit without the use of discrete components.
Another embodiment of the present invention is an apparatus comprising an amplifier having an input terminal and an output terminal. A first inductor is coupled to the input terminal of the amplifier and a second inductor is coupled to the output terminal of the amplifier. The first inductor and the second inductor are positioned relative to each other to create a mutual inductance which cancels at least a portion of feedback in the amplifier. The mutual inductance is created between the output terminal of the amplifier and the input terminal of the amplifier. The mutual inductance resonates with the feedback capacitance in the amplifier. The amplifier, the first inductor, and the second inductor may be implemented on a semiconductor material without the use of discrete components.
A further embodiment of the present invention is a method of reducing feedback in an amplifier. The method comprises the acts of coupling a first inductor to an input terminal of the amplifier and coupling a second inductor to an output terminal of the amplifier. The first inductor and the second inductor are then positioned relative to each other to create a mutual inductance which cancels at least a portion of the feedback in the amplifier.
An additional embodiment of the present invention is an apparatus comprising means for amplifying a signal having an input and an output. A first inductive means is coupled to the input of the amplifying means. A second inductive means is coupled to the output of the amplifying means. The first inductive means and the second inductive means are positioned rela
Conexant Systems Inc.
Knobbe Martens Olson & Bear LLP
Shingleton Michael B
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