HIGH TOLERANCE VARIABLE GAIN AMPLIFIERS

Examples of amplifier circuitry regulate a transconductance value (Gm) of operational transconductance amplifiers (OTAs) in the amplifier to be approximately the same, which value is based on a supply voltage and a reference voltage applied to a reference OTA and the internal resistance of the reference OTA. The reference OTA generates an output current based on Gm and the reference voltage, which current is compared to current generated by the supply voltage and internal resistance of the reference OTA. A tail current transistor of each of the reference OTA and a main OTA that mirrors the Gm of the reference OTA provide a tail current feedback path by which Gm is regulated. Amplifying circuitry is coupled to the main OTA to receive current signals. Based on the received current signals, amplifying circuitry generates a differential output voltage signal. The gain of the amplifying circuitry is proportional to the supply voltage and remains relatively constant across process temperature variations.

FIELD OF DISCLOSURE

This disclosure relates generally to high tolerance variable gain amplifiers, and more particularly to ratiometric variable gain amplifiers with high process and temperature drift tolerance, as well as high tolerance to power supply variations.

BACKGROUND

Amplifiers, and in particular variable gain amplifiers, are used in many applications including sensing applications. For example, an amplifier may be coupled to the output of a sensor that measures an environmental condition or parameter, such as temperature, humidity, pressure, etc., and converts the measurement to an analog electrical signal, e.g., a voltage. That voltage output by the sensor is then amplified before being converted to a digital value by an analog-to-digital converter (ADC) coupled to the output of the amplifier. Typically, the relationship between the measured condition and the analog voltage indicative thereof is linear. In such a system in which a linear output voltage is input to an ADC, the output voltage should be proportional to the power supply of the system to generate a consistent digital output value for a given sensed input across power supply variations.

A previous approach uses a separate ADC to digitally control the gain of the amplifier through a switch and resistor network coupled between the output of the separate ADC and the output of the amplifier. However, in addition to adding an extra component and thus increasing the circuit footprint on a semiconductor substrate, this approach introduces an unwelcome quantization error.

In addition to being able to accommodate power supply variations, sensing/measurement systems should also be able to accommodate process temperature (PT) variations.

In this context, features and aspects of the present disclosure arise.

SUMMARY

In an example, an amplifier includes a first operational transconductance amplification (OTA) circuit including a reference signal input section configured to receive a reference voltage (e.g., Vref) to generate an output current (e.g., Gm*Vref) based on the reference voltage and a transconductance value (Gm) of the first OTA circuit. In a configuration, the first OTA circuit generates a comparison current (e.g., VCC/R1); in another configuration, the first OTA circuit generates a constant voltage. The amplifier also includes a second OTA circuit coupled to the first OTA circuit. The second OTA circuit includes an input signal input section configured to receive an input voltage (e.g., Vin) and an output at which an output signal is output. The second OTA circuit has substantially the same transconductance value (Gm) as the first OTA circuit. The amplifier also includes amplifying circuitry having an input coupled to the output of the second OTA circuit. The amplifying circuitry is configured to amplify the input voltage to generate and output an output signal (e.g., Vout).

In an example, an amplifier includes reference signal circuitry, input signal circuitry, and amplifying circuitry. The reference signal circuitry includes first and second reference signal transistors, each having first and second current terminals and a control terminal, which control terminals are configured to receive a differential reference voltage signal. The first current terminals of the first and second input transistors are coupled to define a first node. The reference signal circuitry also includes a first tail current transistor having current terminals respectively coupled to the first node and a ground terminal. The reference signal circuitry defines an output current path and a comparison current path. The input signal circuitry includes first and second input signal transistors each having first and second current terminals and a control terminal, which control terminals are configured to receive a differential input voltage signal. The first current terminals of the first and second input signal transistors are coupled to define a second node. The input signal circuitry also includes a second tail current transistor having current terminals respectively coupled to the second node and the ground terminal. The amplifying circuitry includes first and second current inputs respectively coupled to the second current terminals of the first and second input signal transistors, and first and second outputs at which a differential output voltage signal is output.

In an example, an amplifier comprises first and second operational transconductance amplification (OTA) circuits and amplifying circuitry. The first OTA circuit is configured to receive a reference voltage to generate an output current based on the reference voltage and a transconductance value of the first OTA circuit. The first OTA circuit is further configured to generate a comparison current and a first tail current. The second OTA circuit is coupled to the first OTA circuit and configured to receive an input voltage and to generate a second tail current; the second OTA circuit also has first and second current outputs. The amplifying circuitry has first and second current inputs that are respectively coupled to the first and second current outputs of the second OTA circuit; the amplifying circuitry also has first and second outputs at which a differential output voltage signal is output.

In an example, a method comprises applying a reference voltage to a first operational transconductance amplifier (OTA) circuit to generate an output current; comparing a current generated in the first OTA circuit from a voltage supply and an internal resistance of the first OTA circuit to the output current; obtaining a transfer gain value of the first OTA circuit and a second OTA circuit coupled to the first OTA circuit; generating tail currents through a first tail current element of the first OTA circuit and through a second tail current element in the second OTA circuit to regulate a transconductance value of the first and second OTA circuits, the second tail current element being coupled to the first tail current element to form a tail current feedback path; inputting an input voltage to the second OTA circuit; and generating an output voltage at an output of amplifying circuitry coupled to the second OTA circuit.

These and other features will be better understood from the following detailed description with reference to the accompanying drawings.

DETAILED DESCRIPTION

Specific examples are described below in detail with reference to the accompanying figures. These examples are not intended to be limiting. In the drawings, corresponding numerals and symbols generally refer to corresponding parts unless otherwise indicated. The objects depicted in the drawings are not necessarily drawn to scale.

In example arrangements, systems, circuits and methods are provided that utilize a tail current feedback path to regulate transconductance of a ratiometric amplifier to minimize the variance of the gain of the amplifier across a range of process and temperature variations. In an example, the transconductance value is regulated by the supply voltage, a reference voltage, and internal resistance of a portion of the ratiometric amplifier. In an example, the gain is proportional to the supply voltage.

FIG.1is a diagram of an example sensing system100includes a linear output device102and a discrete analog-to-digital converter (ADC)104that includes ADC circuitry for digitizing an analog electrical signal. Linear output device102includes a sensor or signal source106and an amplifier108. Both the ADC circuitry of ADC104and amplifier108are coupled to a supply terminal or rail110that supplies a voltage (VCC). Sensor/signal source106may also be coupled to VCCor may be powered by another power supply.

Sensor/signal source106may be any of various sensors, such as a temperature sensor, humidity sensor, pressure sensor, magnetic sensor, etc., or other type of signal source, e.g., a converter. Sensor/signal source106may be configured to measure an environmental parameter and convert the measurement to an electrical signal, e.g., a voltage signal. Sensor/signal source106(simply sensor106hereafter) has an output112at which an analog electrical signal is output. In an example, the outputted analog electrical signal is a voltage signal (Vin).

Amplifier108has an input114coupled to output112of sensor106to receive the voltage signal (Vin), which may be a differential voltage signal. Amplifier108amplifies yin by an amplification factor (A) and generates an amplified voltage given by the expression A*Vin. Amplifier108has an output116at which it outputs the amplified voltage, which is denoted Vout. In a ratiometric system, Vouthas to be proportional to VCC.

The ADC circuitry of discrete ADC104has an analog input118coupled to output116of amplifier108to receive Vout. The internal ADC circuitry samples the analog signal Voutand converts each sample to a digital value. ADC has an output120at which the digital values, each of which may be represented as a code, is output. ADC104may be configured such that 1 least significant bit (LSB) is proportional to VCC.

To provide example amplifier108with high tolerance against power supply, e.g., VCC, variation, as well as against process temperature (PT) variation, amplifier108may be configured as shown inFIG.2. Example amplifier108ofFIG.2includes a reference operational transconductance amplifier (OTA) circuit202, a main OTA circuit204, and amplifying circuitry206. Reference OTA circuit202and main OTA circuit204may sometimes be referred to simply as reference OTA202and main OTA circuit204.

Reference OTA202includes two input transistors208and212that form a reference signal input section as indicated by the dashed enclosure around these transistors. Each of transistors208and212may be an n-type metal-oxide-silicon field-effect transistor (n-MOSFET). Transistor208has a gate214, and transistor212has a gate216. Gates214and216are coupled to negative (−) and positive (+) terminals of a voltage source218, which delivers a differential reference voltage signal (Vref) to gates214and216. The sources of transistors208and212are commonly coupled to a drain of an n-MOSFET transistor222through which a first tail current flows during operation. The source of transistor222is coupled to ground or negative supply terminal or rail234.

Reference OTA202also includes a pair of transistors224and226, each of which in an example, is a p-type metal-oxide-silicon FET (p-MOSFET). Transistors224and226are coupled in a current mirror configuration. The drains of transistors224and226are coupled to the drains of input transistors208and212, respectively. The gates of transistors224and226are commonly coupled and also coupled to the drain-drain coupling of transistors208and224. The sources of transistors224and226are coupled to supply terminal110, which delivers supply voltage VCC.

Reference OTA202has a first current path228in which a current (I) flows that is generated by VCCand the internal resistance (R1) of reference OTA202. That is, I=VCC/R1. An output current is generated and flows in a second current path232that extends from first current path228to the drain-drain coupling of transistors212and226. The output current is approximately equal to Gm*Vref, where Gmis the transconductance of reference OTA202.

Main OTA204of amplifier108has a pair of input transistors242and244that form an input signal input section as indicated by the dashed enclosure around these transistors. Each of transistors242and244may be an n-MOSFET. Gates246and248are respectively coupled to negative (−) and positive (+) terminals of a voltage source252, which delivers a differential input voltage signal (Vin) to gates246and248. The sources of transistors242and244are commonly coupled to a drain of n-MOSFET transistor254through which a second tail current flows during operation. The source of transistor254is coupled to ground234. The drains of transistors242and244respectively form current outputs256and258of main OTA204.

Reference OTA202and main OTA204are coupled via the gates of transistors222and254and through the sources of transistors222and254. Thus, transistors222and254may function as a sink-source pair for the tail currents.

Amplifying circuitry206has two inputs262and264, which are respectively coupled to the current outputs256and258of main OTA204. One of the inputs of amplifying circuitry206is a positive (+) input and the other input is a negative (−) input. In an example, positive input262is coupled in an inverting feedback loop via a resistor266to a negative (−) output terminal268of amplifying circuitry206, and negative (−) input264is coupled in an inverting feedback loop via resistor272to positive (+) output terminal274of amplifying circuitry206. Each of resistors266and272is also identified as R2, which indicates that each resistor has approximately the same resistance. R2may be any suitable resistance. A differential voltage (Vout) is output at outputs268and274.

At reference OTA202, a subtract current, which is the difference between I and the output current of OTA202(i.e., subtract current=I−(Gm*Vref)) flows into a gate of transistor222and into a gate of transistor254, which controls tail currents of those transistors and a transconductance, Gm, of reference OTA202and main OTA204. Thus, reference OTA202and a reference current source I generates a feedback system to adjust the transconductance of reference OTA202. With this configuration of amplifier108, first and second tail currents form a tail current feedback path by which the transconductance value (Gm) of reference OTA202, and in particular of input transistors208and212, is mirrored in main OTA204, and in particular in input transistors242and244, so both OTAs have the same Gm. By the tail current feedback path, which significantly reduces gain variation across a range of process temperature (PT) variations, Gmis regulated to be approximately (VCC/Vref)/R1. In the example amplifier108shown inFIG.2: Vout=(VCC/Vref)(R2/R1)Vin, and the gain of amplifier108(A=Vout/Vin) is thus proportional to the supply voltage VCC. Gain accuracy is primarily determined by variation in the internal resistance (R1) of reference OTA202and variation of reference voltage (Vref) variation.

In another example, a constant voltage reference may be used instead of a VCC-generated current I. With this configuration, amplifier108has a substantially constant gain value A across process, voltage, temperature (PVT) variations.

FIG.3is a diagram of a feedback loop300in operation of a ratiometric variable gain amplifier, e.g., amplifier108configured as shown inFIG.2. The current (I) generated by the supply voltage (VCC) and the internal resistance (R1) of reference OTA202is compared to the output current (Gm*Vref) generated by the input of Vrefto the reference signal input section of reference OTA202to yield a value (AI) indicative of the difference between the two currents. In each of reference OTA202and main OTA204, AI is transferred to the OTA's Gmvia a transfer function (K). The loop within reference OTA202is repeated to reduce AI to approximately zero to make the Gmof both OTAs202and204approximately the same. With the OTAs having approximately the same Gm, Vinis applied to the input signal input section of main OTA204to generate output currents used to generate Voutvia resistors266and272, each having a resistance of R2.

FIG.4is a flow diagram of an example method400of operating a high tolerance ratiometric variable gain amplifier, e.g., amplifier108shown inFIG.2.

In operation402, a reference voltage (Vref) is input to input transistors of a reference OTA. This generates an output current (Gm*Vref), which, in operation404, is compared with a current generated by a supply voltage (VCC) applied to the reference OTA and its internal resistance (R1). In operation406, a transfer gain value (K) is obtained that transfers the difference between the two currents compared in operation404to the Gm's of both the reference OTA and a main OTA coupled to the reference OTA. In operation408, tail currents generated in the OTAs are used to regulate Gmof the OTAs to be approximately the same, i.e., both approximately equal to (VCC/Vref) In operation410, an input voltage (Vin) is input to input transistors of the main OTA to generate output currents for amplifying circuitry. An output voltage (Vout) is generated in operation412in which Voutis proportional to VCC. In an example, Vout=(VCC/Vref)(R1/R2)Vin. Also, the gain (Vout/Vin) remains relatively constant for a given VCCover a range of PT variations.

FIG.4depicts one possible set and order of operations. Not all operations need necessarily be performed in the order described. Some operations may be combined into a single operation. Additional operations and/or alternative operations may be performed.

FIG.5shows a graph500of Vout, i.e., the output voltage of ratiometric variable gain amplifier, with respect to its input voltage Vin. One plot502of graph500shows Vout(mV) with respect to Vinat a supply voltage VCCof 3.63 V, and another plot504of graph500shows Voutwith respect to yin (mV) at a VCCof 1.65 V. Each plot502and504is over a range of PT combinations, i.e., five process corners and three temperatures: −40° C., 25° C. and 150° C., giving 15 combinations. The gain (A=Vout/Vin) remained relatively constant (less than 1% variation) at each supply voltage across a range (15 combinations) of PT variations.

As used herein, the term “terminal” means “node”, “interconnection”, “pin” and/or “lead”. Unless specifically stated to the contrary, these terms generally mean an interconnection between or a terminus of a device element, a circuit element, an integrated circuit, a device or other electronic or semiconductor component.

While the use of MOSFETs is described herein, other types of transistors (or equivalent devices) may be used instead. For example, instead of using n- and p-type MOSFETs, n- and p-type bipolar junction transistors (BJTs) may be used instead or in addition to MOSFETs in the various circuits described. Circuits described herein are reconfigurable to include the replaced components to provide functionality at least partially similar to functionality available prior to the component replacement.

Modifications of the described examples are possible, as are other examples, within the scope of the claims. Moreover, features described herein may be applied in other environments and applications consistent with the teachings provided.