Amplifiers – With semiconductor amplifying device – Including distributed parameter-type coupling
Reexamination Certificate
1998-10-23
2001-11-20
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including distributed parameter-type coupling
C330S277000, C330S310000, C333S202000, C333S219000
Reexamination Certificate
active
06320468
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to amplifiers and more particularly to a method and system for suppressing crosstalk in a multi-stage amplifier.
BACKGROUND OF THE INVENTION
Amplifiers are used in the electronic industry for a variety of purposes. One common type of amplifier is a multi-stage amplifier in which a plurality of amplifier stages are connected together to provide desired levels of amplification. In one type of multi-stage amplifier, a field-effect transistor is provided in each stage. The field-effect transistor provides the amplifying effect of an input signal, and the amplified signal is provided as an input signal to the next stage. An example of such an amplifier is a monolithic microwave integrated circuit, which is particularly designed to amplify radio and microwave signals.
Such amplifiers often use a single bias pad connecting all gates or all drains of the field-effect transistors of each stage of the amplifier. The use of a single bias pad allows biasing of the gates or drains in each amplifier stage with a single applied voltage. In such amplifiers, both the gates and the drains are connected to ground through a bypass capacitor. The bypass capacitor is provided as a path for high frequency signals from ground to flow to ground in order to suppress transmission of the high frequency signals along the bias pad to adjacent amplifier stages. The bypass capacitor, however, blocks lower frequency signals from ground and allows lower frequency signals to be transmitted along the bias pad, such as a bias voltage. The bias pad is often implemented by connecting each stage's bypass capacitors together with microstrip lines to form a bias bus.
A problem with the use of such microstrip lines is that they form a resonant structure with the bypass capacitors. At resonance, this structure provides a low loss feedback path for signals into the gate or drain of the field-effect transistors, allowing crosstalk between the respective transistor terminals. This resonance creates a potential gain loop which often causes oscillation problems for the entire amplifier. The resonance also causes the impedance at the bypass capacitors to resonate from a short circuit to an open circuit (or large impedance). High impedances at low frequencies typically cause oscillations in microwave transistors. Prior methods for suppressing such effects of the gain loop are either space prohibitive or adversely affect the amplifier's output power and efficiency.
SUMMARY OF THE INVENTION
Accordingly, a need has arisen for an improved method and system for suppressing feedback in a multi-stage amplifier. The present invention provides a method and system for suppressing feedback in a multi-stage amplifier that addresses shortcomings of prior systems and methods.
According to one embodiment of the invention, a multi-stage amplifier for amplifying radio frequency signals includes a first amplifier stage having a first transistor and a second amplifier stage having a second transistor. The first transistor includes a first drain connected to a reference voltage through a first connection circuit, and the second transistor includes a second drain connected to the reference voltage through a second connection circuit. The amplifier also includes a first transmission line connecting a portion of the first connection circuit to a portion of the second connection circuit and a resistive element connected in parallel with the first transmission line between the portion of the first connection circuit and the portion of the second connection circuit. The resistive element suppresses the radio frequency signals from being transmitted between the first and second drains.
According to another embodiment of the invention, a method for suppressing radio frequency transmission between a first stage and a second stage of an amplifier includes connecting a first node of a first transistor in the first stage to a corresponding second node of a second transistor in the second stage with a transmission line. The method also includes placing a resistive element in parallel with the transmission line.
Embodiments of the invention provide numerous technical advantages. For example, in one embodiment of the invention, an amplifier is provided that suppresses a gain loop in gate and drain biased lines. This is accomplished without the need for radio frequency traps, series resistors, or other methods that adversely affect the amplifier's output power and efficiency. Embodiments of the invention are also inherently broad-band and do not contribute to lower drain efficiencies due to resistors placed serially in the biased lines. Furthermore, according to one embodiment of the invention, gain loop suppression may be effected by modifying standard bias bus microstrip lines. Thus, the present invention can be fabricated at little or no additional expense.
Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.
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Carroll James M.
Heston John G.
Baker & Botts L.L.P.
Nguyen Patricia
Pascal Robert
Raytheon Company
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