Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
1999-12-17
2001-07-10
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S279000, C330S296000, C330S297000
Reexamination Certificate
active
06259323
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to amplifier gain controls and, more particularly, to active gain controls for low noise amplifiers.
BACKGROUND OF THE INVENTION
Various amplifier applications are known in which low noise and high linearity performance are required. Examples of such applications include low noise amplifiers (LNAs), variable gain amplifiers (VGAs), transmit/receive (T/R) modules, power amplifiers, transimpedance amplifiers, and optical receivers used in various products. Such products can include wireless communications, cellular telephony, fiber optic data links, portable electronics.
Many circuits, particularly communication system circuits, require devices for controlling signal level. Variable gain and automatic gain control amplifiers and attenuators are typically used for this purpose.
A problem in reception in mobile communication applications is that the strength of received electromagnetic waves varies over a broad range. In light of this problem, the receiver usually includes an AGC circuit. For the received signal strength indicator (RSSI) to work properly the gain levels must be accurately defined.
A typical conventional variable gain low noise amplifier is implemented utilizing a dual gate MOS FET (Metal Oxide Semiconductor Field Effect Transistor) as a transconductance amplifier. An input signal is applied to a first gate of the FET via an impedance matching circuit. The amplified signal is fed out from the FET via a matching circuit. A DC control voltage is applied to a second gate of the FET in order to determine the gain of the FET. However, in this type of amplifier, the gain control characteristics are non-linear. Additionally the signal gain linearity increase due to reduced gain is often insufficient.
In wireless products there is a need for multiband communications. That is, where the same wireless protocol can be used in varying parts of the RF spectrum. Presently, multi-band enabled wireless products require multiple LNAs (typically 1 per band). The typical LNA for a wireless product is implemented on Gallium-Arsenide (GaAs) integrated circuits (ICs). Multiple LNAs implemented in this manner require a relatively large amount of area on a radio frequency (RF) IC. The IC real estate required for implementation tends to exceed the specification for the implementation.
SUMMARY OF THE INVENTION
A method and system of gain control in an amplifier. Three modes of gain control are provided: high gain, low gain, and no gain. The gain controller is an active amplifier device which provides high and low gain by switching DC biasing at the gain control terminals and also switching device terminal designations. In high gain mode, the DC voltage source is connected to a first bias terminal of the gain controller. The device terminal connected to said first bias terminal through a load is designated as the drain terminal. A constant current source is connected to a second bias terminal of the gain controller which is designated as the source terminal. In high gain mode, the gain controller acts as a common-source, or transconductance, amplifier. In low gain mode, the constant current source is connected to the first bias terminal. Now the device terminal connected to the said first bias terminal through the load is redesignated as the source terminal. The DC voltage source is connected to the second bias terminal of the gain controller and the terminal is redesignated as the drain terminal. In low gain mode, the gain controller acts as a source-follower, or buffer, with no voltage gain. For automatic gain control, only DC level signals are switched, no switches are needed in the signal path. The method and apparatus can be implemented using discrete components, an integrated chip solution, or a combination of the two. In no gain mode the signal transistor is biased off and high attenuation is achieved.
The described gain controller provides several advantages. For example, a low noise amplifier with multiple modes of gain control can be designed wherein the gain controller utilizes only one high performance signal transistor. For another example, control of the gain does not require switches in the signal path. The absence of switches in the signal path can result in low noise operation of the amplifier. Moreover, the absence of switches in the signal path can also result in high linearity of amplification. For another example, the gain control can be designed using fewer components resulting in a less complex controller with no loss in gain control. For another example, the cost of the gain controller can be reduced because cheap DC-only switches can be used for the bias terminals. For another example, the lower component count that can be achieved in the presently preferred embodiment leads to a lower cost solution. For another example, the configuration of the presently disclosed innovations yields a well-controlled low gain mode with a high degree of linearity.
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Lineberry Allen Scott
Nokia Mobile Phones Ltd.
Rivers Brian T.
Shingleton Michael B
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