Telecommunications – Transmitter – Power control – power supply – or bias voltage supply
Reexamination Certificate
2001-12-21
2004-07-13
Urban, Edward F. (Department: 2682)
Telecommunications
Transmitter
Power control, power supply, or bias voltage supply
C455S234100, C330S278000, C330S291000
Reexamination Certificate
active
06763228
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to gain control of amplifiers, and more particularly to a precision automatic gain control circuit.
DESCRIPTION OF RELATED ART
Automatic Gain Control (AGC) circuits are used in many communication and signal processing applications. For example, in the receiver of a wired or wireless communication link, the intended signal to be processed may be a short distance away and therefore relatively strong while another signal may be a long distance away and much weaker. The receiver must process both the strong signals and the weak signals which implies a variable gain function. High gain is used to detect and amplify weak signals and low gain and/or attenuation is used to process strong signals.
The most simple gain stage that can implement an AGC function is the basic differential pair of transistors. An exemplary differential pair stage includes a pair of bipolar transistors Q
1
, Q
2
having their emitters coupled together and to a bias current sink. A pair of load or bias resistors is each coupled between a respective collector of the differential pair and a voltage supply signal. A differential input is applied across the bases of the transistors, and a differential output is developed across the respective collectors. The gain of this stage is the transconductance of either transistor Q
1
or Q
2
multiplied by the load resistance. By simply varying the transconductance, the gain is changed. The transconductance can be varied by changing the bias current. A fundamental problem with this type of Automatic Gain Control (AGC) circuit is that it has limited input signal swing capability. Input differential voltages of approximately 50 millivolts (mV) peak to peak begin to cause significant nonlinearities, which are unacceptable in many applications. Such nonlinearities, for example, may result in a total harmonic distortion (THD) that is greater than 1%. In high performance systems, AGC functions may need to handle input differential voltages as large as two (2) volts peak to peak, making this AGC stage unacceptable.
The next most common gain stage used for AGC functions is the differential pair with emitter degeneration. This gain stage is similar to the simple gain stage just described and further includes a pair of emitter degeneration resistors to increase the input signal swing capability. In particular, the emitters of the differential pair of transistors Q
1
, Q
2
are not connected to each other. Instead, each emitter is coupled to one end of a respective one of the emitter degeneration resistors. The other ends of the resistors are coupled together and to the bias current sink. The emitter degeneration resistors are ideally linear. The overall transconductance of this stage is decreased by the emitter resistors and their presence allows for more of the input signal to appear across these resistors than across the nonlinear base-emitter junctions of the transistors Q
1
or Q
2
. This results in significantly improved linear handling of large input differential voltages. As the emitter resistors are increased, however, the overall transconductance of the stage becomes less and less dependent on the transistor's transconductance and more dependent on the emitter resistors. A fundamental problem with this arrangement is that the ability to vary the gain by changing the bias current is severely limited as the emitter resistors are increased.
More advanced AGC circuits have been suggested. One idea is to provide an analog attenuator in front of a fixed gain operational amplifier (op-amp). There are several problems with this arrangement for certain applications. First, the analog attenuator circuit requires a stack (cascode) of at least three transistors and resistors, which reduce voltage swing capability. Next, placing an attenuator in front of a large fixed gain amplifier forces the resistors that make up part of the attenuator circuitry to be very low-valued in order to meet reasonable noise performance. These low valued resistors require significant supply current.
SUMMARY OF THE PRESENT INVENTION
An automatic gain control (AGC) amplifier according to an embodiment of the present invention includes a high gain amplifier, a feedback network and multiple amplifier stages coupled in the feedback path of the AGC amplifier. The feedback network has a first end that receives an input signal of the AGC amplifier, a second end coupled to the output of the high gain amplifier and multiple intermediate nodes. Each amplifier stage has an input coupled to a corresponding intermediate node of the feedback network and an output coupled to the input of the high gain amplifier. Each amplifier stage is independently controllable to position a virtual ground within the feedback network to control the closed loop gain of the AGC amplifier.
In respective embodiments, the high gain amplifier may be a transimpedance amplifier. Also, each amplifier stage may be a transconductance stage. In this manner, a voltage at an intermediate node of the feedback network is converted to a current by a transconductance stage, which current is applied to the input of the transimpedance amplifier. The transimpedance amplifier converts currents from all of the transconductance stages into an output voltage, which is applied to an output end of the feedback network. Further, each transconductance stages may include a controllable bias current device, so that the gain of the AGC amplifier is controlled by a transconductance ratio, which is further controlled by the respective currents of the bias current devices. In particular, the bias current devices are controlled to change the transconductance of each transconductance stage, which varies the overall closed loop gain of the AGC amplifier.
The feedback network may include resistors coupled in series between the input signal and the output signal and having at least two intermediate junctions. The amplifier stages are controlled to position a virtual ground at or between the intermediate junctions, so that the gain of the AGC amplifier may be defined by a resistive ratio of resistors in the feedback network.
The AGC amplifier may be configured to operate with differential signals. In particular, the high gain amplifier may be a differential amplifier having a differential input and a differential output. The feedback network may have a differential input for receiving a differential input signal, a differential output coupled to the differential output of the high gain amplifier and two or more intermediate differential nodes. Each amplifier stage may have a differential input coupled to a respective intermediate differential node of the feedback network and a differential output coupled to the differential input of the high gain amplifier.
In a more particular differential embodiment, each amplifier stage includes a differential pair of bipolar transistors (e.g. NPN bipolar junction transistors) having a common-coupled pair of emitters, a pair of bases forming the differential input and a pair of collectors forming a differential output. Each amplifier stage further includes a controllable bias current device coupled to the common-coupled pair of emitters of the differential pair of bipolar transistors. Also, the feedback network may include first and second sets of resistors, each set coupled in series between corresponding polarities of the differential input signal and the differential output of the high gain amplifier. The dual series-coupled resistor sets form two or more intermediate differential nodes. Shunt resistors may be included, each coupled between first and second polarities of an intermediate differential node of the series-coupled resistor sets.
In one embodiment, each amplifier stage has a bias terminal. A control circuit is provided that develops a plurality of bias currents, each provided at a bias terminal of a corresponding amplifier stage. In this manner, the bias currents of the control circuit are used to vary the gain of the AGC amplifier. The control circuit may be implemented
Landy Patrick J.
Prentice John S.
Hunton & Williams
Intersil America's Inc.
Le Nhan T
Urban Edward F.
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