Amplifiers and methods for preventing phase reversal

Abstract

Amplifier, comprising: a bootstrap circuit (210) for improving the linearity of the amplifier; and a feedforward control circuit (216) for modifying a voltage of the bootstrap circuit in response to a change in an input signal, the feedforward control circuit comprising a capacitor (218), wherein modifying the voltage prevents a phase reversal condition of the amplifier by forward biasing a collector-base junction of an input transistor (212), wherein the pre-control circuit (216) comprises a plurality of diodes (224, 226) that cooperate with the capacitor (218) to maintain the base-collector voltage of the input transistor (212).

Description

FIELD OF THE INVENTION

[0001] Embodiments of the present invention relate generally to amplifiers, and more particularly to a feedforward circuit of an amplifier that prevents phase reversal. STATE OF THE ART

[0002] An amplifier is a device that receives an input signal (e.g., a voltage or current) and generates an output signal based on that input. A differential amplifier, for example, is a common type of amplifier. It receives positive and negative versions of an input signal (i.e., a differential input signal) and generates an amplified output signal based on the voltage difference between the two input versions.

[0003] Conventionally, a bootstrap circuit including various electronic components (e.g., resistors or transistors) can be used to improve the linearity of an amplifier by preventing a logarithmic change in the base-emitter voltage caused by a change in the collector-base voltage of an amplifier's input transistor. The unity-gain bootstrap circuit implemented around the input transistor in this example ties its collector voltage to its base, thereby maintaining the base-collector junction at a constant voltage. As a result, the collector voltage and the voltage gain of the emitter circuit (i.e., β) remain constant at all operating input voltages.

[0004] Phase reversal refers to the output voltage changing in the "wrong" direction with respect to a change in the input voltage. An amplifier can experience phase reversal if, for example, the input voltage changes too quickly for the bootstrap to react. The collector-base junction of an NPN bipolar junction transistor (BJT) is reverse-biased in a forward-active region. After receiving a large and / or rapid voltage change, the voltage at the base may be higher than the voltage of the collector, which has not yet reacted. This condition leads to saturation of the NPN BJT, which in turn can cause the output voltage to temporarily change in the wrong direction (i.e., phase reversal occurs). This problem is exacerbated when the input signal varies at a high frequency and / or when the input step has a slow rise time.

[0005] As a result, there is a need for circuits that can economically improve the linearity of an amplifier (e.g., via a bootstrap circuit) but can prevent phase reversal upon receiving a large, rapid change in the input signal.

[0006] US 2008 / 0 061 883 A1 discloses a circuit for thermal compensation. The circuit comprises an amplifier circuit, a control circuit, and an emitter-follower circuit.

[0007] DE 14 87 485 B discloses a transmission quadrupole network consisting of a differential amplifier with high input resistance. In one embodiment, unwanted phase shifts are counteracted by providing a capacitor to at least partially bridge the resistors of the differential amplifier arranged between the emitters of the input transistors.

[0008] JP 2012- 65 376 A discloses a three-phase inverter for operating a motor, for example for an air conditioner.

[0009] US 5 754 072 A discloses a programmable circuit for generating precise clock and bias signals with low noise. SUMMARY

[0010] The claimed subject matter is defined in the independent claims. Advantageous further developments are described in the dependent claims.

[0011] In various embodiments, the present invention relates to systems and methods for effectively improving the linearity of an amplifier using a bootstrap circuit without a phase inversion condition of the amplifier, using a feedforward circuit that accelerates the bootstrap circuit in response to a fast (i.e., faster than the slew rate of the amplifier, such as two to ten times faster than the slew rate of the amplifier) ​​change in an input signal and / or prevents saturation during a high change in an input signal (i.e., higher than the operating range).The bias circuit may include two capacitors: a first capacitor connected between the input signal and an internal node of the bootstrap circuit, and a second capacitor connected between the input signal and a current source that sets the voltage biasing the bootstrap circuit. Upon a rapid change in the input signal, the first capacitor of the bias circuit immediately feeds the input voltage to the base of the collector drive circuit, while the second capacitor amplifies the current biasing the voltage level shifter of the bootstrap circuit during larger transients. The bias circuit includes several diodes that work cooperatively with the first capacitor to maintain the base-to-collector voltage of the input stage transistor and prevent failure conditions of the collector drive transistor.The pre-control circuit prevents the input transistors from entering saturation mode, which can prevent a phase-reversal condition of the amplifier. The pre-control circuit can also reduce overshoot or undershoot of the amplifier's output voltage.

[0012] Accordingly, in one aspect, the invention relates to an amplifier. In various embodiments, the amplifier includes a bootstrap circuit for improving a linearity of the amplifier and a feedforward circuit for modifying a voltage of the bootstrap circuit in response to a change in an input signal, wherein the feedforward circuit comprises a capacitor. Modifying the voltage prevents a phase reversal state of the amplifier by forward biasing a collector-base junction of an input transistor (212). The feedforward circuit may include a first capacitor connected between the input signal and a collector drive of an input transistor of the amplifier.The amplifier may additionally include an input stage amplifier including a plurality of transistors, and the bias circuit may include a plurality of diodes; each diode is associated with one of the plurality of transistors of the input stage amplifier. The first capacitor may forward input the input signal when the change in the input signal is between 0 V and a forward-biased voltage of one of the plurality of diodes. The bias circuit may include a second capacitor connected between the input signal and a current source biasing the bootstrap circuit. The second capacitor may forward input the input signal when the change in the input signal is between 1 V and 10 V.

[0013] In various embodiments, the amplifier further includes a first amplifier stage for amplifying the input signal and providing an intermediate signal, and a second amplifier stage for amplifying the intermediate signal and providing an amplified output signal. The first and second amplifier stages may include different amplifiers. The bootstrap circuit may improve the linearity of the first amplifier stage.

[0014] In various embodiments, the amplifier is an instrumentation amplifier. The bootstrap circuit can cause the amplifier to respond to changes in the input signal at a rate faster than its slew rate. For example, the rate of change can be one, five, or ten times faster than the slew rate.

[0015] In another aspect, an amplifier includes an input amplifier stage for receiving an input signal, a bootstrap circuit for improving linearity of the amplifier, and a feedforward circuit for modifying a voltage of the bootstrap circuit in response to a change in the input signal to prevent a phase reversal condition of the amplifier. The feedforward circuit may include a first capacitor connected between the input signal and an output node of the bootstrap circuit, a second capacitor connected between the input signal and an internal node of the bootstrap circuit, and a plurality of diodes each associated with a transistor of the input amplifier stage.

[0016] In yet another aspect, a method for preventing a phase reversal condition of an amplifier includes detecting a change in an input signal and applying a bias signal via a capacitor to a node in a bootstrap circuit in response to the change. Applying the bias signal may prevent a phase reversal condition of the amplifier by forward biasing a collector-base junction of an input transistor. In some embodiments, applying the bias signal to the node of the bootstrap includes modifying a voltage of the bootstrap circuit.

[0017] The terms "about" and "approximately" as used herein mean ±10% (e.g., by amplitude), and in some embodiments ±5%. Reference in this specification to "a (particular) example," "an example," "a (particular) embodiment," or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the prior art. Therefore, when the phrases "in a (particular) example," "in an example," "a (particular) embodiment," or "an embodiment" appear in various places in this specification, they do not necessarily refer to the same example. Moreover, the particular features, structures, acts, steps, or characteristics may be combined in one or more examples of the prior art in any suitable manner.The headings provided herein are for convenience only and are not intended to limit or interpret the scope or meaning of the claimed technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the drawings, like reference characters generally refer to the same parts throughout the several views. Furthermore, the drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: Fig. Figure 1A is a diagram of an existing instrumentation amplifier; Fig. 1B is a graph of an output signal from an existing instrumentation amplifier exhibiting phase inversion; Fig. 2 is a diagram of a bootstrap circuit and a feedforward circuit according to an embodiment of the present invention; Fig. 3 is a graph of voltage levels representing reduced phase reversal, output overshoot, and output undershoot of an amplifier employing a bootstrap circuit and a feedforward circuit according to an embodiment of the present invention; and Fig. 4 is a diagram of an instrumentation amplifier incorporating bootstrap circuits and feedforward circuits according to an embodiment of the present invention. DETAILED DESCRIPTION

[0019] Fig. Figure 1A illustrates an existing instrumentation amplifier 100 including two stages 102, 104: a first stage 102 including preamplifiers 106, 108 for providing differential amplification of a differential input signal +IN, -IN, and a second stage 104 including a differential amplifier 110 for providing a single-ended output signal OUTPUT. The differential amplifier 110 removes all or a portion of the common-mode voltage in the output of the preamplifiers 106, 108, so that only differential-mode signals are output by the second stage 104.

[0020] In one embodiment, the preamplifiers 106, 108 are precision current feedback amplifiers. Because the input transistors 112, 114 are biased with a fixed current, each input signal forces the output voltages of the amplifiers 116, 118 to change accordingly. The differential signal applied to the inputs is applied across the terminals 120 of the resistor R G replicated; the one by R GThe current flows through resistors 120, 122, creating an amplified differential voltage between node 1 and node 2. The amplified differential and common-mode signals from amplifiers 106, 108 are then applied to differential amplifier 110 in second stage 104. Differential amplifier 110 rejects the common-mode voltage but receives the amplified differential voltage. Instrumentation amplifier 100 generally provides low output error (e.g., less than 0.01%) and low output noise (e.g., less than 60 nV / √Hz), but, as described above, is susceptible to phase reversal, as in Fig. 1B. In response to a rapid and / or large increase in an input voltage, an output voltage 150 initially decreases (i.e., moves in a direction opposite the direction of change of the input) in a first region 152. This region 152 corresponds to a phase reversal condition in the amplifier 100. Approximately 0.125 µs thereafter, the output voltage 150 changes direction and begins to increase (i.e., moves in the same direction as the input). However, the output voltage 150 overshoots its upper level 154 in an overshoot region 156, and the output voltage undershoots its lower level 158 in a undershoot region 160 as the input signal decreases.

[0021] Fig. 2 illustrates a preamplifier circuit 200 (analogous in function to, for example, one of the two in Fig. 1) that incorporates a bootstrap circuit 210 to improve the linearity of circuit 200. Bootstrap circuit 210 may be biased with a relatively low power supply (e.g., four times less than a bias voltage of input transistor 212 and, in one embodiment, 30 µA); however, bootstrap circuit 210 may not react quickly enough to respond to a high and / or fast voltage change (e.g., a voltage swing greater than 120 mV that occurs at a faster rate than the slew rate of the circuit, e.g., once, five times, or ten times faster than the slew rate). Upon receiving, for example, a high / fast voltage boost, the current flowing through the collector of input transistor 212 into input amplifier stage 214 increases accordingly.This increase leads to a voltage drop at the collector of transistor 212 because the bootstrap circuit 210 has a low power supply (e.g., 30 µA) and therefore cannot provide a sufficiently high voltage quickly enough to increase the collector voltage of transistor 212 to compensate. The voltage at the collector of the input NPN transistor 212 therefore becomes smaller than the voltage at the base; this results in a phase reversal condition.

[0022] A feedforward circuit 216 is used in various embodiments to modify the voltage of the bootstrap circuit 210 in response to a rapid and / or large change in the input signal, thereby reducing or eliminating the occurrence of phase reversal. The feedforward circuit 216 includes, for example, two capacitors 218, 220 for transferring a voltage change in the input IN to nodes A and B, thereby enabling nodes A and B of the bootstrap circuit 210 to respond more quickly to a large input voltage change. A first capacitor 218, connected between the input signal IN and the base of transistor (or a collector drive of the input transistor) 222, labeled as node A of the feedforward circuit 216, supplies the entire available input current (i.e., a current available from a drive source) upon receiving a high and / or rapid input voltage increase (e.g.,higher than 120 mV but lower than 400 mV), thereby preventing the part of the phase reversal that occurs immediately (or immediately after) the rising edge of the input signal. A second capacitor 220, connected between the input signal and the current source (i.e., node B) preceding the voltage level shifter of the bootstrap circuit 210, upon receiving a higher input voltage increase of, for example, 0.5 V to 1-2 V, can immediately provide a short path for the input current to boost the available voltage preceding the bootstrap circuit 210 at node B, thereby causing node A to respond quickly to the input change, faster than the slew rate of the circuit.The voltage across the first capacitor 218 is further defined as a voltage drop across series-connected diodes 224, 226; this prevents a high voltage buildup during a high electrical current and failure of the base-emitter voltage of the transistor 222. All components of the pre-control circuit 216, including the two capacitors 218 and 220 and the two diodes 224, 226, work together to accelerate the bootstrap circuit 210 to keep the input transistor 212 and its collector drivers 222 in a forward-active region, thereby preventing phase reversal.

[0023] In some embodiments, the diode 224 connected between the base and collector of the input transistor 212 is a Schottky diode having a lower turn-on voltage or forward voltage drop (e.g., 0.15-0.45 V) than the collector-base voltage of the input transistor 212 (e.g., 0.8 mV). The Schottky diode 224 therefore serves as a clamp for the input transistor 212, thereby preventing voltage saturation of the input transistor 212. For example, when the base of the input transistor 212 receives a high, high-frequency input signal and the bootstrap circuit 210 near the input transistor 212 does not respond quickly enough, the Schottky diode 224, which has a lower turn-on voltage than the collector-base voltage of the input transistor 212, is activated first.The Schottky diode 224 can therefore increase the collector voltage of the input transistor 212 upon receiving an increased base voltage, thereby preventing the input transistor 212 from entering a deep saturation mode. The diode 226, in some embodiments, connects the base and emitter of the second transistor 222 in the input amplifier stage 214 to protect the transistor 222. Upon receiving a high / fast voltage, for example, the collector voltage of the input transistor 212 increases the subsequent voltage change at the input IN via the Schottky diode 224; this can lead to failure of the transistor 222 if its base has not yet moved. The diode 226, connected between the emitter and base of the transistor 222, conducts the voltage to the base of the transistor 222 and raises the voltage, maintaining a low reverse voltage (e.g.0.15-0.4 V) across the base and emitter of transistor 222; diode 226 thus prevents failure of transistor 222.

[0024] Fig. Figure 3 illustrates an output 300 of an amplifier incorporating a feedforward control circuit 216 according to an embodiment of the invention. As shown in the figure, the output 300 experiences no (or very little, e.g., less than 10 mV) phase reversal (at, e.g., a point 302) in response to the same high and / or rapid input voltage increase. The output 150 and the phase reversal region 152 of the existing amplifier circuit 100 of Fig. 1 are shown for comparison. The phase reversal is suppressed using the feedforward circuit 216 to allow instantaneous response to a rapid (e.g., 5 MHz) high voltage change without using a large current source (as shown in the output voltage profile 300).

[0025] In addition to suppressing or eliminating phase reversal, the feedforward control circuit 216 included in the input stage of the differential amplifier 200 can also reduce or eliminate the overshoot and / or undershoot of the amplifier output voltage. Implementing the feedforward control circuit 216 in the amplifier reduces the output voltage overshoot and undershoot by approximately 5% and 10%, respectively. Fig. 3, the output 150 of the existing amplifier 100 overshoots Fig. 1, for example, overshoots the 1.6 V high voltage level by approximately 0.625 V, whereas the output 300 of the embodiment of the present invention only overshoots the 1.6 V high voltage level by approximately 0.5 V. Likewise, the undershoot improves by approximately 0.8 V to 0.6 V.

[0026] Fig.Figure 4 illustrates an instrumentation amplifier 420 that includes bias circuits 410, 412 in both preamplifiers 414, 416 to prevent phase reversal conditions of the preamplifiers caused by low-power bootstrapping circuitry implemented around the input transistors. The bias circuits 410, 412 provide a robust, low-cost approach for the instrumentation amplifier 420 to sustain the voltage transient (e.g., faster than the amplifier's slew rate) input signal, whereas the additional bootstrap circuitry is driven by a much lower drive condition (e.g., the bootstrap level shift is biased at an input current, such as 30 µA, less than one-quarter of the input current, such as 120 µA).

[0027] In some embodiments, the feedforward circuitry is implemented in a differential amplifier (e.g., preamplifier 414 or 416) of instrumentation amplifier 420 to provide high-performance linearity and speed while reducing system power consumption. The implementation of the feedforward circuitry may not be limited to instrumentation amplifiers. One of ordinary skill in the art will understand that feedforward circuitry implemented in other types of amplifiers is within the scope of the present invention.

[0028] The terms and expressions employed herein are used as terms and expressions of description and not of limitation. The use of these terms and expressions is not intended to exclude equivalents of the features shown and described, or portions thereof. Furthermore, having described particular embodiments of the invention, it will be apparent to those skilled in the art that other embodiments incorporating the presently disclosed concepts may be utilized without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects only as illustrative and not restrictive.

Claims

[1] Amplifier, comprising: a bootstrap circuit (210) for improving the linearity of the amplifier; and a feedforward control circuit (216) for modifying a voltage of the bootstrap circuit in response to a change in an input signal, the feedforward control circuit comprising a capacitor (218), wherein modifying the voltage prevents a phase reversal condition of the amplifier by forward biasing a collector-base junction of an input transistor (212), wherein the pre-control circuit (216) comprises a plurality of diodes (224, 226) that cooperate with the capacitor (218) to maintain the base-collector voltage of the input transistor (212). [2] The amplifier of claim 1, wherein the feedforward circuit (216) comprises a first capacitor (218) connected between the input signal and a collector drive (222) of an input transistor of the amplifier. [3] The amplifier of claim 2, wherein the amplifier further comprises an input amplifier stage (214) comprising a plurality of transistors, and the pre-control circuit further comprises a plurality of diodes (224, 226), each associated with one of the plurality of transistors of the input amplifier stage. [4] The amplifier of claim 3, wherein the first capacitor (218) forward feeds the input signal when the change in the input signal is between 0 V and a forward bias of one of the plurality of diodes (224, 226). [5] An amplifier according to any preceding claim, wherein the feedforward circuit comprises a second capacitor (220) connected between the input signal and a current source upstream of the bootstrap circuit. [6] An amplifier according to claim 5, wherein the second capacitor feeds the input signal forward when the change in the input signal is between 1 V and 10 V. [7] The amplifier of claim 1 or 2, further comprising a first amplifier stage for amplifying the input signal and providing an intermediate signal and a second amplifier stage for amplifying the intermediate signal and providing an amplified output signal. [8] An amplifier according to claim 7, wherein the bootstrap circuit improves the linearity of the first amplifier stage. [9] An amplifier according to claim 7 or 8, wherein the first and second amplifier stages comprise differential amplifiers. [10] An amplifier according to any one of the preceding claims, wherein the amplifier is an instrumentation amplifier. [11] An amplifier according to any preceding claim, wherein the bootstrap circuit causes the amplifier to respond to the change in the input signal at a faster rate than the slew rate thereof. [12] The amplifier of claim 11, wherein the speed is one, five or ten times faster than the slew rate. [13] Amplifier, comprising: an input amplifier stage for receiving an input signal; a bootstrap circuit for improving the linearity of the amplifier; and a feedforward control circuit for modifying a voltage of the bootstrap circuit in response to a change in the input signal to prevent a phase reversal condition of the amplifier, wherein the feedforward circuit comprises a first capacitor connected between the input signal and an output node of the bootstrap circuit, a second capacitor connected between the input signal and an internal node of the bootstrap circuit, and a plurality of diodes each associated with a transistor of the input amplifier stage. [14] A method for preventing a phase reversal condition of an amplifier, comprising: Detecting a change in an input signal; and Applying a feedforward signal via a capacitor (218) to a node in a bootstrap circuit in response to the change; wherein the application of the bias signal prevents a phase reversal condition of the amplifier by forward biasing a collector-base junction of an input transistor (212); wherein a feedforward control circuit (216) comprises a plurality of diodes (224, 226) cooperatively operating with the capacitor (218) to maintain the base-collector voltage of the input transistor (212). [15] The method of claim 14, wherein applying the feedforward signal to the node of the bootstrap comprises modifying a voltage across the bootstrap circuit.

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