Bandgap reference circuit with input amplifier for noise reduction

Abstract

A bandgap reference circuit (100) includes first to fourth bipolar junction transistors (BJTs) (M1 to M4). The base and collector of the first BJT (M1) are shorted together. The second BJT (M2) is coupled to the first BJT (M1) via a first resistor (R1). The base of the third BJT (M3) is coupled to the base of the first BJT (M1). The base and collector of the fourth BJT (M4) are coupled together and also coupled to the base of the second BJT (M2). A second resistor (R2) is coupled to the fourth emitter of the fourth BJT (M4). A third resistor (R3) is coupled to the second resistor (R2) and the emitter of the second BJT (M2). The operational amplifier (OP1) has a first input coupled to the collector of the first resistor (R1) and the second BJT (M2), a second input coupled to the emitter of the third BJT (M3) and the collector of the fourth BJT (M4), and an output coupled to the collectors of the first and third BJTs (M1 and M3).

Description

Technical Field

[0001] A bandgap reference circuit with an input amplifier for noise reduction is disclosed. Background Technology

[0002] Bandgap reference voltage suppliers are used in a variety of electronic applications. Despite power supply variations, load variations, and temperature changes, these voltage suppliers provide a constant reference voltage. Summary of the Invention

[0003] In one example, a bandgap reference circuit includes first to fourth bipolar junction transistors (BJTs). The base and collector of the first BJT are shorted together. The second BJT is coupled to the first BJT via a first resistor. The base of the third BJT is coupled to the base of the first BJT. The base and collector of the fourth BJT are coupled together. A second resistor is coupled to the fourth emitter of the fourth BJT. A third resistor is coupled to the second resistor and the emitter of the second BJT. An operational amplifier has a first input coupled to the first resistor and the collector of the second BJT, a second input coupled to the emitter of the third BJT and the collector of the fourth BJT, and an output coupled to the collectors of the first and third BJTs. Attached Figure Description

[0004] Figure 1 These are circuit diagrams depicting illustrative bandgap reference circuits in various examples.

[0005] Figure 2 yes Figure 1 The bandgap reference circuit includes the operational amplifier circuit.

[0006] Figure 3 This is a circuit diagram of an operational amplifier stage implementation scheme.

[0007] Figure 4 This is a circuit diagram depicting an alternative embodiment of the bandgap reference circuit.

[0008] The same reference numerals are used in the accompanying drawings for the same or similar (in terms of function and / or structure) features. Detailed Implementation

[0009] As mentioned above, bandgap reference voltage suppliers provide a constant, high-precision reference voltage under various fluctuating parameters, including ambient temperature. Many bandgap reference voltage suppliers operate by using the positive temperature coefficient of one circuit to offset the negative temperature coefficient of another. A bandgap reference voltage supplier contains a temperature-proportional-to-absolute-temperature (PTAT) voltage source coupled in series with a temperature-complementary-absolute-temperature (CTAT) voltage source. For a CTAT voltage source, the voltage is negatively correlated with temperature (the voltage decreases as the temperature rises and increases as the temperature falls), while for a PTAT voltage source, the voltage is positively correlated with temperature (the voltage increases as the temperature rises and vice versa). Because the voltage of the CTAT voltage source changes in the opposite direction to that of the PTAT voltage source with varying temperature, the output voltage of the bandgap reference voltage supplier remains approximately constant.

[0010] Such bandgap reference voltage suppliers suffer from multiple sources of flicker noise. Some flicker noise originates from the transistors (e.g., bipolar junction transistors, BJTs) within the voltage supplier, but this type of flicker noise can be mitigated by adding a resistor between the base and collector of each BJT. The majority of the remaining flicker noise originates from the base current of the input transistor pairs of the operational amplifiers contained within the bandgap reference voltage supplier. These input transistor pairs can generate low-frequency (e.g., 0.1 Hz to 10 Hz) flicker noise. This low-frequency noise can be difficult to eliminate because one technique for eliminating low-frequency noise is the use of large filter capacitors, which may be impractical in low-power applications. Additional resources (e.g., power) could be consumed to reduce the noise, but this may also be impractical in low-power applications. Therefore, there is a need for a bandgap reference voltage supplier capable of reducing flicker noise at low frequencies without excessive power consumption.

[0011] This document describes an example of a bandgap reference circuit that includes a bandgap network coupled to an operational amplifier. At least one of the transistors within the bandgap network is configured to also serve as part of a gain stage. In one example, the transistor is a BJT configured as a common-emitter amplifier. By including a gain stage within the bandgap network, flicker noise generated within the operational amplifier is attenuated.

[0012] Figure 1 This is a circuit diagram of an illustrative bandgap reference circuit 100. Figure 1The bandgap reference circuit includes transistors M1, M2, M3, and M4, resistors R1, R2, and R3, and operational amplifier OP1. The combination of transistors M1 and M2 with resistor R1 serves as gain stage 150, as mentioned above. In this illustrative example, transistors M1 through M4 are NPN BJTs. Operational amplifier OP1 has a non-inverting (positive) input, an inverting (negative) input, and an output 123. The output 123 of operational amplifier OP1 provides the output bandgap voltage (VBG) from the bandgap reference circuit 100.

[0013] The outputs 1, 2, and 3 of operational amplifier OP1 are coupled to the collectors of transistors M1 and M3. The bases of transistors M1 and M3 are coupled together and to their collectors. Resistor R1 is coupled between the emitter of transistor M1 and the collector of transistor M2. The connection between resistor R1 and the collector of transistor M2 is labeled node A. The non-inverting input of operational amplifier OP1 is coupled to node A, and therefore to resistor R3 and the collector of transistor M2.

[0014] The bases of transistors M2 and M4 are coupled together and also coupled to the collector of transistor M4. The emitter of transistor M3 is coupled to the collector of transistor M4 at node B. The inverting input of operational amplifier OP1 is coupled to node B, and therefore to the emitter of transistor M3 and the collector of transistor M4. Resistor R2 is coupled to the emitter of transistor M4. Resistor R3 is coupled between ground and resistor R2, and also between ground and the emitter of transistor M2.

[0015] Transistor M1 is larger than transistor M3, as indicated by the size ratio "N:1", where N is an integer greater than 1. Transistor M1 being N times larger than transistor M3 means that transistor M1 includes N transistor fingers, while transistor M3 has only one transistor. In one example, N is 8. In another example, N is 24. Transistor M2 is smaller than transistor M4, as indicated by the size ratio "1:N". Therefore, transistor M1 is N times larger than transistor M3, and similarly, transistor M4 is N times larger than transistor M2.

[0016] When the circuit is in steady state (producing a roughly constant bandgap voltage VBG) and the transistor input pairs are of the same size (1:1 ratio), the voltage difference between the inverting and non-inverting inputs of operational amplifier OP1 is approximately 0V. Therefore, the voltage at node A is approximately equal to the voltage at node B. Applying Kirchhoff's Voltage Law (KVL) around the loop containing node A, operational amplifier OP1, transistors M3 and M1, and resistor R1, the voltage (V1) across resistor R1 is the base-emitter voltage (Vbe) difference between transistors M3 and M1. Specifying the Vbe of transistor M3 as Vbe_M3 and the Vbe of transistor M1 as Vbe_M1, the Vbe difference (ΔVbe) between the two transistors is ΔVbe = Vbe_M3 - Vbe_M1, where Vbe_M3 is the Vbe of transistor M3 and Vbe_M1 is the Vbe of transistor M1. Although the Vbe of an individual BJT is the CTAT voltage, the Vbe difference between transistors M3 and M1 is the PTAT voltage. The fact that ΔVbe is the PTAT voltage stems from the fact that ΔVbe between transistors M3 and M1 is: ΔVbe = VT * ln(N), where N is the size ratio between transistors M1 and M3, and V... T This is the thermal voltage of the BJT, and "ln" is a function of the natural logarithm. Thermal voltage V T It equals kT / q, where T is the temperature (in Kelvin), q is the charge on the electron, and k is the Boltzmann constant. Because V T It is a function of temperature T, so the thermal voltage (V) T The voltage across resistor R1 is the PTAT voltage, and therefore the ΔVbe between transistors M3 and M1 is also the PTAT voltage. The ΔVbe between transistors M3 and M1 is the voltage V1 across resistor R1, meaning the voltage across resistor R1 is the PTAT voltage. The current through resistor R1... Figure 1 The voltage V1 is represented as I1 and is ΔVbe / R1. Since the voltage V1 is the PTAT voltage, the current I1 is the PTAT current.

[0017] Applying a similar analysis to the loop containing transistors M2 and M4, resistor R2, and operational amplifier OP1, the voltage across resistor R2 (shown as V2) is ΔVbe between transistors M2 and M4. For the same reasons described above for voltage V1, voltage V2 is also a PTAT voltage. Because voltage V2 is also a PTAT voltage, the current through resistor R2 (shown as I2) is a PTAT current.

[0018] The current through resistor R3 is represented as I3, which is the sum of currents I1 and I2. Since currents I1 and I2 are PTAT currents, current I3 is also a PTAT current. Therefore, the voltage V3 across resistor R3 is a PTAT voltage, and the Vbe of each of transistors M1 to M4 is a CTAT voltage.

[0019] Starting from the ground terminal and raising the circuit to voltage VBG, the bandgap reference circuit 100 includes a PTAT voltage V3 connected in series with the CTAT Vbe voltage of transistor M2, an approximately zero voltage drop between the non-inverting and inverting inputs of operational amplifier OP1, and the CTAT Vbe voltage of transistor M3. Similarly, the circuit includes a series combination of the PTAT voltage V3 and the CTAT Vbe voltage of transistors M4 and M1.

[0020] Still referencing Figure 1 The base of transistor M4 is coupled to the collector of transistor M4, configuring transistor M4 as a diode. The base and collector of transistor M4 are coupled to node B and the inverting input of operational amplifier OP1. However, the base of transistor M2 is not coupled to the collector of transistor M2, but instead is coupled to the base and collector of transistor M4, and therefore also to the inverting input of operational amplifier OP1. In this configuration, transistor M2 also functions as a common-emitter amplifier, with its input being the base of transistor M2 and its output being the collector of transistor M2 (node ​​A). The combined operation of transistor M1, resistor R1, and transistor M2 constitutes gain stage 150, as indicated by the dashed box. The transconductance of transistor M1 is denoted as gm_M1, and the transconductance of transistor M2 is denoted as gm_M2. The gain of gain stage 150 is gm_M2*R, where R = R1 + 1 / (gm_M1). The configuration of the bandgap circuit 100, which includes a gain stage 150, advantageously results in the attenuation of flicker noise generated within the operational amplifier OP1 based on the factor gm_M2*R.

[0021] Figure 2 A circuit diagram showing additional details of operational amplifier OP1. (e.g.) Figure 2 As shown in the example, operational amplifier OP1 includes a first stage 210 coupled to a second stage 250. The first stage 210 includes a transconductance circuit 211 coupled to capacitor C0 (having a transconductance value of GM0).

[0022] The second stage 250 includes transistors M21 to M24 coupled together to form a super source follower buffer, and gain elements AP and AN. In this example, transistors M21 to M23 are P-type metal-oxide-semiconductor field-effect transistors (PMOS transistors) and transistor M24 is an N-type metal-oxide-semiconductor field-effect transistor (NMOS transistor). The output voltage VBG is the voltage across the source of transistor M21. The second gain stage 250 is configured for unity gain, although it has an offset voltage due to the gate-to-source voltage (Vgs) of transistor M21.

[0023] Figure 2 An example of a startup circuit 220 comprising transistors M25 to M28 and resistor R20 is also shown. Startup circuit 220 sets the gate voltage of transistor M22 below a selected threshold voltage value, such as a threshold voltage (Vt) for a transistor Vthresh. During startup of the bandgap reference circuit, transistor M22 will turn off when VBG is less than Vthresh. This causes transistor M23 of the second stage 250 to turn on and quickly turn on the load capacitor C. L And bandgap core charging. Once the voltage VBG becomes greater than Vthresh, M22 turns on and the second stage 250 switches from gyroscopic mode to source follower mode, where the bandgap loop initiates regulation of VBG and brings it to its final steady-state value. The voltage Vthresh is selected such that the final VBG - Vthresh is slightly greater than the drain-to-source saturation voltage of transistor M21 (Vd). DS,SAT This is to keep transistor M21 operating in the saturation region. Setting the gate voltage of transistor M22 to Vthresh-Vt is achieved by passing a pre-generated ΔVbe / R (where R is the sum of resistor R20 and the resistance of any other resistors along the current path through M28 to ground) through a series of diode-connected BJTs (e.g., transistor M28) and resistors, and tapping from the nodes between them (see the dashed line from the collector of transistor M28 to the gate of transistor M22). In addition to setting the gate voltage on transistor M22, startup circuit 220 injects current and attempts to pull up nodes A and B (the sources of transistors M25 and M26) with the help of M25 and M26. Startup circuit 220 is configured such that transistors M25 and M26 are turned off when the voltages at nodes A and B become close to their steady-state values.

[0024] The first stage 210 can be configured to have relatively high gain. Figure 3An example implementation of the first stage 210 is shown. The first stage 210 includes a bias circuit 310 coupled to an amplifier 350. The bias circuit 310 generates various bias voltages for the amplifier 350, such as PBIAS_PTAT. The amplifier 350 includes an input transistor pair 352, which includes transistors M31 and M32. In this example, transistors M31 and M32 are PNP transistors. The base of transistor M31 is coupled to... Figure 1 The non-inverting input of node A in the transistor. The base of transistor M32 is coupled to... Figure 1 The inverting input of node B in the circuit. In this example, the size ratio between transistors M31 and M32 is... Figure 1 The transistor pairs are identical (e.g., 1:N). Due to the different transistor sizes, the Vbe voltage of each of transistors M31 and M32 is different. The voltage difference between the base of transistor M31 and the base of transistor M32 (i.e., between the non-inverting and inverting inputs of operational amplifier OP1) is the difference between the Vbe voltage of transistor M31 and the Vbe voltage of M32. In the above example where the inverting and non-inverting inputs have the same voltage, the size ratio between the corresponding input transistor pairs is 1:1, but... Figure 3 In the example, the size ratio is 1:N.

[0025] Refer again Figure 1 The voltage difference between the inverting and non-inverting inputs of operational amplifier OP1 is equal to the Vbe difference between transistors M31 and M32 (as opposed to the approximately zero voltage difference described above). The voltage V1 across resistor R1 is ΔVbe_OP1 - Vbe_M3 - Vbe_M1, where ΔVbe_OP1 is the Vbe voltage difference between transistors M31 and M32. The Vbe difference between transistors M31 and M32 adds an extra value ΔVbe to the voltage V1 across resistor R1. Because voltage V1 is larger due to the extra ΔVbe from the first stage 210 of the operational amplifier (compared to the voltage V1 when the input pair M31 and M32 are the same size), Figure 1 As the current I1 increases, the total current I3 also increases. In order to maintain the same PTAT voltage V3 across resistor R3 for the increased current I3, the resistance value of resistor R3 is reduced (e.g., reduced to 1 / 2).

[0026] Will The gain is applied to the noise generated within operational amplifier OP1. The advantage of configuring operational amplifier OP1 to present an additional ΔVbe voltage across resistors R1 and R2 (by manufacturing transistor M32 as N times larger than transistor M31) is that resistor R3 can and should be reduced to maintain the same PTAT V3 voltage and the same VBG level. Advantageously, as resistor R3 decreases, the noise of operational amplifier OP1 is further attenuated.

[0027] Figure 4 It is a description Figure 1 The circuit diagrams of alternative embodiments shown in the figures are shown. Figure 4 This is a schematic diagram of a bandgap reference circuit 400 including transistors M41, M42, M43, and M44, resistors R41, R42, and R43, and operational amplifier OP2. Transistors M41 to M44 in the bandgap reference circuit 400 are PNP BJTs. The operation of the bandgap reference circuit 400 is largely related to… Figure 1 The operation is the same as that of the bandgap reference circuit 100. Transistors M41 and M42 and resistor R41 form a gain stage 450, which is operable to press with... Figure 1 The gain stage 150 attenuates the flicker noise generated inside the operational amplifier OP2 in almost the same way. Figure 4 The transistor M42 in the middle is also in accordance with Figure 1 The transistor M2 operates in almost the same way as an amplifier.

[0028] In this description, the term "coupled" may encompass a connection, communication, or signaling path that enables a functional relationship consistent with this description. For example, if device A generates a signal to control device B to perform an action, then: (a) in a first instance, device A is coupled to device B via a direct connection; or (b) in a second instance, device A is coupled to device B via an intermediary component C, provided that the intermediary component C does not alter the functional relationship between device A and device B such that device B is controlled by control signals generated by device A via device A.

[0029] A device “configured to” perform a task or function may be configured by the manufacturer at manufacturing time (e.g., programmed and / or hardwired) to perform the function and / or may be configured (or reconfigured) by the user after manufacturing to perform the function and / or other additional or alternative functions. The configuration may be achieved through firmware and / or software programming of the device, through the construction and / or layout of the device’s hardware components and interconnects, or a combination thereof.

[0030] As used herein, the terms “terminal,” “node,” “interconnect,” “pin,” and “lead” are used interchangeably. Unless specifically stated otherwise, these terms are generally used to refer to interconnects or terminations between device elements, circuit elements, integrated circuits, devices, or other electronic or semiconductor components.

[0031] A circuit or device described herein as containing certain components may be adapted to couple to those components to form the described circuit system or device. For example, a structure described as containing one or more semiconductor elements (e.g., transistors), one or more passive elements (e.g., resistors, capacitors, and / or inductors), and / or one or more sources (e.g., voltage and / or current sources) may be adapted to contain only semiconductor elements within a single physical device (e.g., semiconductor dies and / or integrated circuit (IC) packages) and may be adapted to couple to at least some passive elements and / or sources during or after manufacturing, for example, by an end user and / or a third party, to form the described structure.

[0032] While the use of specific transistors is described herein, other transistors (or equivalent devices) may be used instead. For example, a p-type metal-oxide-semiconductor-silicon FET (“MOSFET”) may be used instead of an n-type MOSFET with little or no change to the circuitry. Furthermore, other types of transistors (such as bipolar junction transistors (BJTs)) may be used.

[0033] The circuits described herein can be reconfigured to include the replaced components to provide functionality at least partially similar to that available before the component replacement. Unless otherwise stated, components shown as resistors generally represent any one or more elements coupled in series and / or parallel to provide the impedance represented by the shown resistor. For example, a resistor or capacitor shown and described herein as a single component can be replaced by multiple resistors or capacitors coupled in parallel between the same nodes. For example, a resistor or capacitor shown and described herein as a single component can be replaced by multiple resistors or capacitors coupled in series between the same two nodes as the single resistor or capacitor.

[0034] In the foregoing description, the use of the phrase “grounding” includes chassis grounding, earth grounding, floating grounding, virtual grounding, digital grounding, common grounding, and / or any other form of grounding connection applicable to or suited to the teachings of this description. Unless otherwise stated, “about,” “approximately,” or “substantially” preceding a value indicates + / - 10% of the stated value. Within the scope of the claims, modifications are possible in the described examples, and other examples are also possible.

[0035] Within the scope of the claims, modifications are possible in the described embodiments, and other embodiments are also possible.

Claims

1. A bandgap reference circuit, comprising: A first bipolar junction transistor (BJT) has a first emitter, a first base, and a first collector, wherein the first base is shorted to the first collector. The second BJT is coupled to the first BJT via a first resistor, and the second BJT has a second emitter, a second base and a second collector. A third BJT is coupled to the first BJT, and the third BJT has a third emitter, a third base, and... The third base electrode is coupled to the first base electrode; A fourth BJT, coupled to the third BJT, the fourth BJT having a fourth emitter, a fourth base, and... The fourth collector electrode, wherein the fourth base electrode is shorted to the fourth collector electrode and the fourth base electrode is coupled to the second base electrode; A second resistor is coupled to the fourth emitter; A third resistor is coupled to the fourth emitter via the second resistor; as well as An operational amplifier having a first input coupled to the first resistor and the second collector, a second input coupled to the third emitter and the fourth collector, and an output coupled to the first and third collectors.

2. The bandgap reference circuit according to claim 1, wherein: The operational amplifier includes a first stage coupled to a second stage, the first stage including transistor pairs having a first size ratio; and The first BJT and the third BJT have a second size ratio that is approximately the same as the first size ratio.

3. The bandgap reference circuit of claim 1, wherein the operational amplifier includes a first stage coupled to a second stage, the first stage including a pair of input transistors having a 1:1 size ratio.

4. The bandgap reference circuit according to claim 1, wherein the first BJT, the second BJT, the third BJT and the fourth BJT are NPN BJTs.

5. A bandgap reference circuit, comprising: A first bipolar junction transistor (BJT) has a first emitter, a first base, and a first collector, wherein the first base is shorted to the first collector. The second BJT is coupled to the first BJT via a first resistor, and the second BJT has a second emitter, a second base and a second collector. A third BJT is coupled to the first BJT, and the third BJT has a third emitter, a third base, and... The third collector electrode is coupled to the first base electrode, and the third collector electrode is coupled to the first collector electrode; A fourth BJT, coupled to the third BJT, the fourth BJT having a fourth emitter, a fourth base, and... The fourth collector electrode, wherein the fourth base electrode is shorted to the fourth collector electrode and the fourth base electrode is coupled to the second base electrode; A second resistor is coupled to the fourth emitter; A third resistor is coupled to the first and third collectors; as well as An operational amplifier having a first input coupled to the first resistor and the second collector, a second input coupled to the third emitter and the fourth collector, and an output coupled to the second emitter and the second resistor.

6. The bandgap reference circuit according to claim 5, wherein: The operational amplifier includes a first stage coupled to a second stage, the first stage including transistor pairs having a first size ratio; and The first BJT and the third BJT have a second size ratio that is approximately the same as the first size ratio.

7. The bandgap reference circuit of claim 5, wherein the operational amplifier includes a first stage coupled to a second stage, the first stage including a pair of input transistors having a 1:1 size ratio.

8. The bandgap reference circuit according to claim 5, wherein the first BJT, the second BJT, the third BJT and the fourth BJT are PNP BJTs.

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