Multi-segment gain control system

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of priority from a co-pending U.S. Provisional patent application entitled “System for a Multi-segment AGC Circuit” having Ser. No. 60/533,523 and filed on Dec. 29, 2003, the disclosure of which is incorporated by reference herein in its entirety for all purposes.

FIELD

The present invention relates generally to gain control circuits, and more particularly to a system for providing multi-segment gain control.

BACKGROUND

Variable gain amplifiers find widespread use in wireless transceivers. They are used in receivers to compensate for varying input levels and in transmitters to adjust the output power level.

A typical receiver includes a switched-gain low noise amplifier (LNA) and multiple baseband variable gain amplifiers. The switched-gain LNA provides at least two modes—high gain and bypass. It invariably suffers from switching transients that adversely affect the magnitude and phase of the received signal, which can be avoided with an LNA offering continuous gain control.

The receiver is generally characterized by its sensitivity and selectivity. The sensitivity of the radio receiver measures the minimum signal that can be detected and demodulated. This takes into account the noise figure of the radio receiver FT, the bandwidth B of the system, and the performance of the demodulator.

The noise figure of a cascaded radio receiver depends on the gain and noise contributed by each circuit, with the first few stages dominating. The selectivity, or linearity, of the radio receiver indicates the largest interfering signal that can be rejected by the system. These signals create intermodulation distortion products that increase rapidly as the interfering signal level increases and degrade receiver signal quality. As a result, the later stages in the receiver system dictate overall linearity.

The sensitivity and selectivity of the radio receiver create conflicting requirements. It therefore becomes advantageous to adapt the receiver to the operating environment and this requires individual control of the LNA and all the variable gain amplifiers.

A transmitter similarly uses baseband and radio frequency (rf) variable gain amplifiers to set the output power level, which varies dramatically in some systems such as in code division multiple access (CDMA) systems.

The transmitter is generally characterized by its maximum output power, distortion, and efficiency. The maximum output power relates to the range of the radio transmitter. It's limited by distortion, which causes spectral regrowth and interferes with nearby radio channels. In turn, amplifier distortion depends on the input signal amplitude and the operating bias, which ties directly to the efficiency of the system.

The gain of the different variable gain amplifiers directly affects the performance of the transmitter. As such, it becomes advantageous to adjust the transmitter based on its output power level and this requires control of each variable gain amplifier.

An automatic gain control (AGC) network must provide monotonic gain control with some precision; otherwise, problems develop due to feedback. Ideally, the AGC network also displays a linear-dB control response. The resulting linear control signal varies exponentially and greatly magnifies errors plus discontinuities due to segmenting of the gain control response. It would therefore be advantageous to have an AGC network that operates to overcome the above problems.

SUMMARY

In one or more embodiments, a multi-segment gain control system is provided that comprises an AGC system that optimally maps a single control signal to multiple variable gain amplifiers to allow the gain of each amplifier to be adjusted independently resulting in a smooth overall gain response, and thereby achieving greater flexibility and superior performance.

In one embodiment, apparatus is provided for a multi-segment gain control. The apparatus comprises logic to convert a gain control signal to an exponential signal, and logic to map the exponential signal to two or more control signals that are used to control two or more gain stages to produce linear multi-segment gain control.

In one embodiment, apparatus is provided for a multi-segment gain control. The apparatus comprises means for converting a gain control signal to an exponential signal, and means for mapping the exponential signal to two or more control signals that are used to control two or more gain stages to produce linear multi-segment gain control.

In one embodiment, a communication device is provided that comprises apparatus for providing a multi-segment gain control. The apparatus comprises logic to convert a gain control signal to an exponential signal, and logic to map the exponential signal to two or more control signals that are used to control two or more gain stages to produce linear multi-segment gain control.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and the attendant advantages of the embodiments described herein will become more readily apparent by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:

FIG. 1 shows a diagram of a wireless receiver that comprises one embodiment of a multi-segment gain control system;

FIG. 2 shows a diagram of one embodiment of an exponential current generator;

FIG. 3 shows a graph that illustrates the relationship between exponential current and receiver gain;

FIG. 4 shows a diagram a receiver that comprises one embodiment of an AGC system;

FIG. 5 shows a graph that illustrates how, in one embodiment, an exponential current is limited to a maximum value;

FIG. 6 shows a graph that illustrates how, in one embodiment, an exponential current is limited to a minimum value;

FIG. 7 shows a diagram of one embodiment of an AGC mapping circuit that limits the maximum output level;

FIG. 8 shows a diagram of one embodiment of an AGC mapping circuit that limits the minimum output level;

FIG. 9 shows a diagram of one embodiment of a cube circuit suitable for use in one or more embodiments of the AGC system;

FIG. 10 shows graphs that illustrate the response of one embodiment of the multi-segment gain control system;

FIG. 11 shows a diagram of one embodiment of an improved square circuit that is suitable for use in one or more embodiments of the AGC system;

FIG. 12 shows a diagram of one embodiment of an improved square-root circuit that is suitable for use in one or more embodiments of the AGC system; and

FIG. 13 shows a communication network that includes various communication devices that comprise one or more embodiments of a multi-segment gain control system.

DETAILED DESCRIPTION

In one or more embodiments, a multi-segment gain control system is provided that comprises an AGC system that optimally maps a single control signal to multiple variable gain amplifiers to allow the gain of each amplifier to be adjusted independently resulting in a smooth overall gain response.

FIG. 1 shows a diagram of a wireless receiver that comprises one embodiment of a multi-segment gain control system. The gain control system comprises an AGC circuit that controls the gain of multiple variable-gain amplifiers (rf and baseband “bb”) independently (via control signals 102) and thereby maximizes the receiver's performance. The AGC uses a “received signal strength indicator” (RSSI) signal that represents the power of the received signal in dB to adjust the system gain. Since most variable gain amplifiers require linear control, the RSSI signal must be converted from dB to linear format to form a basis for the control signals 102.

FIG. 2 shows one embodiment of an exponential generator that operates to convert the RSSI signal from dB to linear format. The exponential generator may be included as part of the AGC circuit shown in FIG. 1. The exponential generator receives a signal I_RX that represents the RSSI signal and operates to generate the current I_exp. Current I₁ develops a base-emitter voltage across transistor Q₁ that mirrors to transistor Q₂ through resistor R₁. The resulting loop equation is;
V_be1=I_RxR₁+V_be2
where I_Rx represents the RSSI signal (and is inversely related to the required gain). This equation can be rewritten as;

I exp = I 1 ⁢ exp ⁡ ( - I Rx ⁢ R 1 V T )
where V_T is the thermal voltage. During operation, the transistor loop (comprising devices Q₃, Q₄ and P₁, P₂) biases transistor Q₁, while transistor N₁ provides the base current for transistor Q₂. Resistor R₂ provides a current source for transistor N₁.

FIG. 3 shows a graph that illustrates the above exponential current I_exp as it varies with receiver gain.

FIG. 4 shows a diagram of a receiver that comprises one embodiment of an AGC system. The AGC system comprises the exponential current generator (exp), the rf mapping circuit 406, and the baseband mapping circuits shown generally at 408. The AGC system receives the exponential current I_exp, which forms a basis that maps (via the mapping circuits 406, 408) to form multiple control signals that are input to associated variable gain amplifiers. Since the exponential current I_exp simultaneously controls multiple amplifiers, its required control range shrinks.

For the following description, it will be assumed that the receiver comprises an rf variable gain amplifier 402 and three baseband (bb) variable gain amplifiers, shown generally at 404. During operation, the three baseband variable gain amplifiers are adjusted in tandem, and as a result, the control range is reduced to;

Δ ⁢ ⁢ G exp = G range 3
where ΔG_exp is the range of the exponential current defined as;

Δ ⁢ ⁢ G exp = 20 ⁢ ⁢ log ⁡ ( I sw I min )
and G_range is the overall gain control range of the system. (The currents I_min and I_sw correspond to the exponential currents needed to control the baseband variable gain amplifiers.) This approach advantageously raises the minimum current I_min above the noise floor, while lowering the maximum current I_sw and reducing power consumption. As a result, the exponential current I_exp can be mapped to directly control the gain of the baseband amplifiers 404 using the control signals I_bb, where I_bb=I_exp from I_min to I_sw.

FIG. 5 shows a graph 500 that illustrates the result of a maximum limit applied to the exponential current I_exp to form the control signal I_bb. As illustrated in the graph 500, the current increases from a minimum I_min to a maximum limit shown at I_sw, which occurs at a gain of approximately 75 dB above the minimum gain of the receiver.

Above the current I_sw (i.e., above 75 dB of receiver gain), the rf variable gain amplifier 402 takes over. It is preferable that the gain of the rf variable gain amplifier 402 change at the same rate as the tandem of three baseband variable gain amplifiers 404, meaning its gain control signal (I_rf shown in FIG. 4) should obey a cube function as expressed by;
I_rf=k(I_exp)³
where I_exp ranges from I_sw to I_max, and where k is a scaling constant.

FIG. 6 shows a graph 600 that illustrates the result of a minimum limit applied to the exponential current to form the control signal I_rf. As illustrated in the graph 600, the current increases from a minimum value I_sw to a maximum limit I_max, starting from a point that corresponds to the same gain as where the current I_sw is reached in FIG. 5.

FIG. 7 shows a diagram of one embodiment of a mapping circuit 700 that limits an exponential current to a maximum level. For example, the mapping circuit 700 is suitable for use as the mapping circuits 408 shown in FIG. 4. The mapping circuit 700 operates to limit the exponential current I_exp to a maximum value equal to the current I_sw as shown in the graph 500. A current mirror establishes the exponential current I_exp in devices N₂ and N₃. A similar current mirror (transistors P₁-P₂) sets the current I_sw. The diode D₁ provides a path for the excess current when current I_sw is larger than current I_exp and sets the voltage at the non-inverting input to the operational amplifier 702 (plus the voltage at the drain of transistor N₃). When the current I_exp exceeds the current I_sw, transistor N₂ is pushed into triode region to limit its current to I_sw. The operational amplifier 702 forces the voltage at the drain of transistor N₃ to follow the voltage at the drain of transistor N₂ so that transistor N₃ also limits to the current I_sw. Degeneration resistors (R1 thru R3) are added to the NMOS current mirrors (devices N₁ thru N₃) to increase the sensitivity of the network as the drain-source voltage decreases (in triode region). As a result, the output current I_bb equals the exponential current I_exp to a maximum level of I_sw, where it flattens out.

FIG. 8 shows a diagram of one embodiment of a mapping circuit 800 that prevents an output current (I_rfl), (which is based on the exponential current I_exp) from falling below the current I_sw. A set of three current mirrors establishes current I_exp in transistor N₂ and current I_sw in transistors N₄ and P₂. The diode D₁ remains biased off until the current I_exp exceeds the current I_sw, at which point, the excess current I_exp-I_sw flows through the diode D₁ from the output. As a result, the output current I_rf1 follows the current I_exp at high levels but flattens out (to the level of I_sw) at low levels of current I_exp.

FIG. 9 shows a diagram of one embodiment of a cube circuit 900 that scales the current I_rf1 by the power three to produce a control current I_rf, as required by one or more embodiments of the AGC system. For example, in one embodiment the mapping circuit 800 and the cube circuit 900 are suitable for use at the mapping circuit 406 in FIG. 4. In one embodiment, the cube circuit comprises an analog squarer and multiplier based on translinear principles. The current I_rf1 is mirrored by transistor P₁ to transistor P₂ and diodes D₁-D₂, setting the bias of transistors Q₁-Q₂ such that;
I_D1I_D2=I_Q1I_Q2

The constant current I_k1 flowing through transistor Q₁ results in;

I Q ⁢ ⁢ 2 = I rf ⁢ ⁢ 1 2 I k ⁢ ⁢ 1
which is the square of the input current I_rf1. The input current I_rf1 is also mirrored to transistors P₃ and P₄. This forces the input current I_rf1 through diode D₃ and transistor N₁. The current flowing through diode D₄ is then I_k2-I_rf1, with the current I_k2 constant and greater than the maximum value of I_rf1.

The current I_Q2 flows through transistors Q₃ and Q₄ with;
I_Q2=I_Q3+I_Q4
These currents also satisfy the relationship;

I Q ⁢ ⁢ 3 I Q ⁢ ⁢ 4 = I D ⁢ ⁢ 3 I D ⁢ ⁢ 4
which allows current I_Q4 to be rewritten as;

I Q ⁢ ⁢ 4 = I Q ⁢ ⁢ 3 ⁡ ( I k ⁢ ⁢ 2 - I rf ⁢ ⁢ 1 I rf ⁢ ⁢ 1 ) = I Q ⁢ ⁢ 3 ⁡ ( I k ⁢ ⁢ 2 I rf ⁢ ⁢ 1 - 1 )

Substituting into the expression for I_Q2 yields;

I Q ⁢ ⁢ 2 = I rf ⁢ ⁢ 1 2 I k ⁢ ⁢ 1 = I Q ⁢ ⁢ 3 ⁡ ( I k ⁢ ⁢ 2 I rf ⁢ ⁢ 1 )
which simplifies to the desired cube function to produce the control current I_Q3 (I_rf);

I Q ⁢ ⁢ 3 = I rf ⁢ ⁢ 1 3 I k ⁢ ⁢ 1 ⁢ I k ⁢ ⁢ 2
A fixed current I_k2 flows through diode D₅ to set a constant operating point for transistors Q₃-Q₄.

FIG. 10 shows graphs 1002 and 1004 that illustrate a smooth multi-segment gain control response that results from piecing together the operation of the different mapping circuits provided in one or more embodiments of an AGC system. Referring to graph 1002, the current I_max is set to 100 μA by design, which means the current I_min is 2.6 μA and the current I_sw is 46.4 μA. The resulting control currents I_bb and I_rf are shown. Referring to graph 1004, the resulting gain response of the AGC system is shown to be smooth and linear.

Other gain control responses (for use with different radio systems and any number of variable gain amplifiers) are possible with similar results provided the basis current I_exp and switching current(s) are shared. In one or more embodiments, the AGC system incorporates a variety of limiting and scaling circuits including, but not limited to, linear, cube, square and square root circuits.

FIG. 11 shows a diagram of one embodiment of an improved square circuit that is suitable for use in one or more embodiments of the AGC system. Diodes D₁-D₂ plus transistors Q₁-Q₂ form a translinear loop described by;
I_D1I_D2=I_Q1I_Q2

Ideally, the current I_k2 is constant; unfortunately, this is not possible with a standard current mirror. The potential at the emitter of transistor Q₁ decreases and nearly vanishes at low current levels (I_in), pushing the current source transistor into saturation (bipolar device) or triode region (MOS device). This differs from the input side of the mirror and results in mismatches. To avoid this, an operational amplifier feedback loop is added. It uses resistor R₂ to monitor the current flow through transistor Q₁ and adjusts the gate voltage applied to current source transistor N₃ accordingly. Current source I_k2 and resistor R₁ provide a reference to the operational amplifier 1102.

FIG. 12 shows a diagram of one embodiment of an improved square root circuit that is suitable for use in one or more embodiments of the AGC system. The square root circuit uses operational amplifier feedback to create a constant current source. Transistors Q₁-Q₄ form a translinear loop where;
I_Q1I_Q2=(I_Q3)²

The input current I_in biases transistor Q₁, while a constant current I_k3 is established in transistor N₁. As a result;
I_Q3=√{square root over (I_inI_k3)}
which is the desired square-root operation.

FIG. 13 shows a communication network 1300 that includes various communication devices that comprise one or more embodiments of a multi-segment gain control (GC) system. The network 1300 includes multiple network servers, a tablet computer, a personal digital assistant (PDA), a cellular telephone, and an email/pager device all communicating over a wireless data network. Each of the devices includes one or more embodiments of a GC system as described herein. The network 1300 illustrates only some of the devices that may comprise one or more embodiments of a GC system. However, it should be noted that one or more embodiments of a GC system are suitable for use in virtually any type of communication device.

Accordingly, while one or more embodiments of a multi-segment gain control system have been illustrated and described herein, it will be appreciated that various changes can be made to the embodiments without departing from their spirit or essential characteristics. Therefore, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.