Patent Grant
7148744
1. Field of the Invention
The present invention relates to electrical circuits, and, in particular, to variable-gain amplifiers.
2. Description of the Related Art
Variable-gain amplifiers (VGAs) are often implemented using multiple amplifier stages connected in series, where each successive amplifier stage further amplifies the output from the previous amplifier stage. As indicated by its name, a VGA can be operated over a range of different gain settings, where each amplifier stage contributes, e.g., proportionately, to the overall amplifier gain.
In such a multi-stage VGA, deviations from ideal operations can result from voltage offsets that can occur at both the input and the output of each amplifier stage, where the input and output offset levels can be independent from each other and also independent from the offsets at different stages. These offsets can result from process variations during fabrication/manufacturing as well as from changes in operating conditions such as age, temperature, humidity, and the like.
One conventional technique for compensating multi-stage VGAs for these input and output offsets relies on AC-coupling and zero-forcing during squelch intervals. One disadvantage of this technique is that a relatively long squelch interval (e.g., about 50–100 nanosec) is typically required, during which time the amplifier is not available for signal processing of user data. As a result, analog storage of the offset compensation is required. Moreover, zero-forcing involves the use of a high-gain, low-offset, high-speed auxiliary amplifier, which typically increases the cost, size, and complexity of the VGA.
Problems in the prior art are addressed in accordance with the principles of the present invention by a technique for calibrating a variable-gain amplifier that does not suffer from all of the disadvantages of conventional techniques that rely on AC-coupling and zero-forcing. According to certain embodiments, the present invention is circuitry having a VGA comprising one or more amplifier stages and two or more offset correction sources connected to apply two or more offset correction signals at two or more different locations within the VGA. According to other embodiments, the present invention is a method for calibrating a VGA comprising one or more amplifier stages and two or more offset correction sources connected to apply two or more offset correction signals at two or more different locations within the VGA, the method comprising controlling the two or more offset correction sources to achieve desired corresponding amplifier stage output signals.
The offset correction achieved using such techniques can be independent of the VGA's gain setting. Moreover, no additional poles need to be added to the signal path, and the offset correction results can be stored in digital storage elements, thereby eliminating the need for sample/hold or similar analog memory elements that require periodic refreshing and which are typically used in conventional VGA calibration techniques.
Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements.
VGAs Having Input and Output Offset Correction Sources
Ignoring the offset correction voltages Vc1–Vc4 applied by D/A converters 104a–d for the time being, under ideal conditions, an input signal Vin is input to and amplified by initial amplifier stage 102a to generate output signal Vo1, which is then input to second amplifier stage 102b, which generates output signal Vo2, which is then input to third and final amplifier stage 102c, which generates output signal Vo3, which is also the output signal for VGA 100.
Unfortunately, due to process variations and/or changes in operating conditions, an offset voltage that occurs in an amplifier stage can be treated as if it were either an offset voltage appearing at the input of the amplifier stage or an offset voltage appearing at the output of the amplifier stage. These are represented in
As shown in
At step 206, the gains of the amplifier stages are initialized, e.g., to unity. For this particular embodiment, all that is needed is that the gains of the currently selected amplifier stage and any preceding amplifier stages be initialized. In some VGA designs, however, the amplifier stages might be controlled together, in which case, all of the gains would be initialized together. Either way, the implementation of this particular embodiment of the present invention should not be affected.
At step 208, the output signal Vo1 of initial amplifier stage 102a is measured, and offset correction signal Vc2 is adjusted (i.e., up or down as appropriate) until Vo1=0V. With Vin and Vc1 both initialized to 0V and amplifier stage 102a set at unity gain, Equation (1) applies as follows:
Vo1=Vofsi1+Vofso1+Vc2. (1)
After adjusting Vc2 in step 208 such that Vo1=0V, Equation (2) applies as follows:
Vc2=−Vofso1−Vofsi1. (2)
At step 210, the gains of the amplifier stages are changed, e.g., to 2. Here, too, for this particular embodiment, all that is needed is that the gains of the currently selected amplifier stage and any preceding amplifier stages be changed. Changing the gains of the amplifier stages will typically result in changes to the output signals of the amplifier stages (e.g., output signal Vo1 of amplifier stage 102a).
At step 212, output signal Vo1 of initial amplifier stage 102a is measured, and offset correction signals Vc1 and Vc2 are adjusted until the sign of Vo1 just changes. For example, if, after changing the gains of the amplifier stages, Vo1>0V, then Vc1 and Vc2 are incrementally adjusted according to Equations (3) and (4) as follows:
Vc1=Vc1−Δν (3)
Vc2=Vc1+Δν, (4)
where Δν is an appropriate, selected voltage increment (e.g., 0.5 mV). Otherwise, if, after changing the gains of the amplifier stages, Vo1<0V, then Vc1 and Vc2 are incrementally adjusted according to Equations (5) and (6) as follows:
Vc1=Vc1+Δν (5)
Vc2=Vc1−Δν. (6)
The incremental adjustments of Equations (3) and (4) or of Equations (5) and (6) are continued until the sign of Vo1 just changes.
With Vin=0V and the gain amplifier stage 102a set at 2, Equation (7) applies as follows:
Vo1=2*(Vofsi1+Vc1)+Vofso1+Vc2. (7)
Just before the incremental adjustments of step 212, Vc1=0V (from the earlier amplifier initialization) and Vc2 is given by Equation (2). Substituting these equations into Equation (7) yields Equation (8) as follows:
Vo1=Vofsi1. (8)
Using the incremental adjustments of Equations (3)–(4) or Equations (5)–(6) ensures that the relationship between the overall (i.e., accumulated) change ΔVc1 to offset correction voltage Vc1 and the overall change ΔVc2 to offset correction voltage Vc2 is given by Equation (9) as follows:
ΔVc2=−Vc1. (9)
Based on Vc1 having been initialized to 0V and Equation (2) resulting from step 208, the overall changes to Vc1 and Vc2 result in Equations (10) and (11) as follows:
Vc1=ΔVc1 (10)
and
Vc2=−Vofso1−Vofsi1−ΔVc1. (11)
Substituting Equations (10) and (11) into Equation (7) yields Equation (12) as follows:
Vo1=2*(Vofsi1+ΔVc1)+Vofso1−Vofso1−Vofsi1−ΔVc1, (12)
which reduces to Equation (13) as follows:
Vo1=Vofsi1+ΔVc1. (13)
At the point where Vo1 just changes sign (i.e., Vo1≈0), Equation (13) implies Equation (14) as follows:
ΔVc1=−Vofsi1, (14)
where Vc1=ΔVc1, since Vc1 was previously initialized to 0V. Substituting Equation (14) into Equation (11) yields Equation (15) as follows:
Vc2=−Vofso1. (15)
Thus, at the completion of step 212, offset correction Vc1 substantially—if not exactly—compensates for the input offset voltage Vofsi1 of amplifier stage 102a, and offset correction Vc2 substantially—if not exactly—compensates for the output offset voltage Vofso1 of amplifier stage 102a.
In the context of VGA 100 of
After the processing of
Those skilled in the art will appreciate that some or all of the settings previously described for the method of
ΔVci=ΔVc(i−1)*(G−1), (16)
where G is the gain of the amplifier stage, where G>1. This relationship may be useful for VGAs whose amplifier stages cannot produce gains of 2 for steps 210 and 212.
Similarly, in theory, the method of
Referring again to
VGAs Having Input Offset Correction Sources
VGA 100 of
At step 306, the gains of the amplifier stages are initialized, e.g., to a high-gain setting, such as 2. At step 308, output signal Vo1 of initial amplifier stage 102a is measured, and offset correction signal Vc1 is adjusted until the sign of Vo1 just changes. At the completion of step 308, offset correction Vc1 substantially—if not exactly—compensates for both the input offset voltage Vofsi1 and the output offset voltage Vofso1 of amplifier stage 102a.
In the context of the modified version of VGA 100 of
Although the present invention has been described in the context of a three-stage VGA, in general, the present invention can be implemented for any VGA having one or more amplifier stages. Moreover, although the present invention has been described in the context of multi-stage VGAs in which one or two voltage correction signals are applied at each amplifier stage, in theory, the invention can be implemented for a multi-stage VGA in which one or more of the amplifier stages do not receive any voltage correction signals.
Digital controller 106 can be implemented using any suitable circuitry, including possible implementation as a single integrated circuit (such as an ASIC or an FPGA), a multi-chip module, a single card, or a multi-card circuit pack. For example, the digital controller can be implemented as a relatively small macro in an integrated circuit that also implements the rest of VGA 100. As would be apparent to one skilled in the art, various functions of circuit elements may also be implemented as processing steps in a software program. Such software may be employed in, for example, a digital signal processor, micro-controller, or general-purpose computer.
The present invention can be embodied in the form of methods and apparatuses for practicing those methods. The present invention can also be embodied in the form of program code embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. The present invention can also be embodied in the form of program code, for example, whether stored in a storage medium, loaded into and/or executed by a machine, or transmitted over some transmission medium or carrier, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. When implemented on a general-purpose processor, the program code segments combine with the processor to provide a unique device that operates analogously to specific logic circuits.
Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range.
It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.
Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.
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