Bandgap reference with input amplifier for noise reduction.

Abstract

The 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 to the base of the second BJT (M2). The second resistor (R2) is coupled to a fourth emitter of the fourth BJT (M4). The third resistor (R3) is coupled to the second resistor (R2) and the emitter of the second BJT (M2). An operational amplifier (OP1) has a first input coupled to the first resistor (R1) and the collector of 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] Bandgap voltage reference supplies are used in a variety of electronic applications. These voltage supplies provide a constant reference voltage regardless of power supply variations, load variations, and temperature changes. Summary of the Invention

[0002] In one example, a bandgap reference circuit The power supply includes first through 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 collector of the first BJT. The base and collector of the fourth BJT are coupled together. A second resistor is coupled to a 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. [Brief explanation of the drawings]

[0003] [Figure 1] FIG. 1 is a circuit schematic illustrating an example bandgap reference circuit in accordance with various examples.

[0004] [Figure 2] 2 is a circuit diagram of an operational amplifier included in the bandgap reference circuit of FIG. 1.

[0005] [Figure 3] 1 is a circuit schematic diagram of an implementation of one stage of an operational amplifier.

[0006] [Figure 4] FIG. 1 is a circuit schematic diagram illustrating an alternative embodiment of a bandgap reference circuit.

[0007] In the drawings, the same reference numbers are used for the same or similar (either by function and / or structure) features. DETAILED DESCRIPTION OF THE INVENTION

[0008] As mentioned above, a bandgap reference voltage supply provides a constant, highly accurate reference voltage regardless of various fluctuating parameters, including ambient temperature. Many bandgap reference voltage supplies operate on the principle of offsetting the negative temperature coefficient of one circuit with the positive temperature coefficient of another. A bandgap reference voltage supply includes a complementary to absolute temperature (CTAT) voltage source coupled in series with a proportional to absolute temperature (PTAT) voltage source. For a CTAT voltage source, the voltage is inversely proportional to temperature (as temperature increases, the voltage decreases, and as temperature decreases, the voltage increases), whereas for a PTAT voltage source, the voltage is directly related to temperature (as temperature increases, the voltage increases, and vice versa). Because the voltage of the CTAT voltage source changes in the opposite direction to the PTAT voltage source with changes in temperature, the resulting output voltage of the bandgap reference voltage remains nearly constant.

[0009] Such bandgap reference voltage supplies suffer from multiple sources of flicker noise. While some of the flicker noise originates from transistors (e.g., bipolar junction transistors, or BJTs) within the voltage supply, this type of flicker noise can be addressed by adding a resistor between the base and collector of each BJT. The remaining majority of the flicker noise originates from the base current of the input transistor pair of the operational amplifier contained within the bandgap reference voltage supply. The input transistor pair of the operational amplifier can generate flicker noise that is low frequency (e.g., 0.1 Hz to 10 Hz). This low-frequency noise can be difficult to remove because one approach to removing low-frequency noise is to use large filter capacitors, which may be impractical in low-power applications. Additional resources (e.g., power) can be expended to reduce the noise, but this may also be impractical in low-power applications. Therefore, there is a need for a bandgap reference voltage supply that can reduce flicker noise at low frequencies without excessive power consumption.

[0010] Described herein is an example of a bandgap reference circuit that includes a bandgap network coupled to an operational amplifier. At least one of the transistors in the bandgap network is configured to also function as part of a gain stage. In one example, the transistor is a BJT configured as a common-emitter amplifier. Including the gain stage in the bandgap network attenuates flicker noise generated in the operational amplifier.

[0011] FIG. 1 is a circuit schematic of an exemplary bandgap reference circuit 100. The bandgap reference circuit of FIG. 1 includes transistors M1, M2, M3, and M4, resistors R1, R2, and R3, and an operational amplifier OP1. The combination of transistors M1 and M2 and resistor R1 functions as gain stage 150 described above. In this illustrative example, transistors M1-M4 are NPN BJTs. Operational amplifier OP1 has a non-inverting (positive) input, an inverting (negative) input, and an output 123. Output 123 of operational amplifier OP1 provides the output bandgap voltage (VBG) from bandgap reference circuit 100.

[0012] The output 123 of operational amplifier OP1 is 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 T 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 thus to resistor R3 and the collector of transistor M2.

[0013] The bases of transistors M2 and M4 are coupled together and to the collector of transistor M4. The emitter of transistor T 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 between ground and the emitter of transistor M2.

[0014] 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 is N times larger than transistor M3, meaning that transistor M1 includes N transistor fingers while transistor M3 has 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." Thus, transistor M1 is N times larger than transistor M3, and similarly, transistor M4 is N times larger than transistor M2.

[0015] When the circuit is in steady state (producing a nearly constant bandgap voltage, VBG), and for input pairs of transistors of equal size (1:1 ratio), the voltage difference between the inverting and non-inverting inputs of operational amplifier OP1 is nearly 0V. Therefore, the voltage on node A is nearly equal to the voltage on node B. Applying the Kirchoff Voltage Law (KVL) around the loop containing node A, operational amplifier OP1, transistors M3 and M1, and resistor R1, the voltage across resistor R1 (V1) is the difference in base-emitter voltages (Vbe) between transistors M3 and M1. If the Vbe of transistor M3 is Vbe_M3 and the Vbe of transistor M1 is Vbe_M1, then the difference in Vbe between these two transistors (ΔVbe) is Vbe = Vbe_M3 - Vbe_M1, where Vbe_M3 is the Vbe of transistor M3 and Vbe_M1 is the Vbe of transistor M1. The Vbe of each BJT is the CTAT voltage, but the difference between the Vbe of transistors M3 and M1 is the PTAT voltage. This ΔVbe is the PTAT voltage because the ΔVbe between transistors M3 and M1 is ΔVbe=V T × In(N), where N is the size ratio between transistors M1 and M3, and V T is the thermal voltage of the BJT, and "In" is the natural logarithm function. T is equal to kT / q, where T is the temperature in Kelvin, q is the charge on the electron, and k is Boltzmann's constant.T is a function of temperature T, so the thermal voltage (V T ) is the PTAT voltage, therefore, ΔVbe between transistors M3 and M1 is also the PTAT voltage. ΔVbe between transistors M3 and M1 is the voltage V1 across resistor R1, which means that the voltage across resistor R1 is the PTAT voltage. The current through resistor R1 is shown as I1 in FIG. 1 and is ΔVbe / R1. Since voltage V1 is the PTAT voltage, current I1 is the PTAT current.

[0016] Applying a similar analysis to the loop including transistors M2 and M4, resistor R2, and operational amplifier OP1, the voltage across resistor R2 (denoted as V2) is the ΔVbe between transistors M2 and M4. Voltage V2 is also a PTAT voltage for much the same reasons as described above for voltage V1. Because voltage V2 is also a PTAT voltage, the current through resistor R2 (denoted as I2) is a PTAT current.

[0017] The current through resistor R3 is denoted as I3 and is the sum of currents I1 and I2. Because 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-M4 is a CTAT voltage.

[0018] The bandgap reference circuit 100 starts at the ground terminal and progresses through the circuit to voltage VBG, including a PTAT voltage V3 in series with a CTAT Vbe voltage for transistor M2, a near-zero voltage drop between the non-inverting and inverting inputs of operational amplifier OP1, and the CTAT Vbe voltage for transistor M3. Similarly, the circuit includes a series combination of the PTAT voltage V3 and the CTAT Vbe voltage for transistors M4 and M1.

[0019] Continuing with reference to FIG. 1, the base of transistor M4, coupled to the collector of transistor M4, configures transistor M4 as a diode. The base and collector of transistor M4 are coupled to node B and the inverting input of amplifier OP1. However, the base of transistor M2 is not coupled to the collector of transistor M2; instead, it 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 its collector (node ​​A). The combination of transistor M1, resistor R1, and transistor M2 operates as gain stage 150, as represented by the dashed box. The transconductance of transistor M1 is represented as gm_M1, and the transconductance of transistor M2 is represented as gm_M2. The gain of gain stage 150 is gm_M2 × R, where R = R1 + l / (gm_M1). The configuration of the bandgap circuit 100 to include the gain stage 150 advantageously results in the flicker noise generated in the operational amplifier OP1 being attenuated by a factor of gm_M2×R.

[0020] Figure 2 shows a circuit diagram with additional details of operational amplifier OP1. As shown in the example of Figure 2, operational amplifier OP1 includes a first stage 210 coupled to a second stage 250. First stage 210 includes a transconductance circuit 211 (having a transconductance value of GM0) coupled to a capacitor C0.

[0021] The second stage 250 includes transistors M21-M24 and gain elements AP and AN coupled together to form a super source follower buffer. In this example, transistors M21-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 at the source of transistor M21. The second gain stage 250 has an offset voltage due to the gate-to-source voltage (Vgs) of transistor M21, but is configured for unity gain.

[0022] 2 also shows an example of a start-up circuit 220 including transistors M25-M28 and resistor R20. The start-up circuit 220 sets the gate voltage of transistor M22 to one of the transistor threshold voltages (Vt) below a selected threshold voltage value, e.g., Vthresh. During start-up of the bandgap reference circuit, when VBG is less than Vthresh, transistor M22 is turned off. This causes transistor M23 of the second stage 250 to be connected to the load capacitor C, as well as the bandgap core. L The voltage V BG When V becomes greater than Vthresh, M22 turns on, the second stage 250 shifts from the through mode to the source follower mode, and the bandgap loop switches to V BG The voltage Vthresh begins to regulate to its final steady-state value. The voltage Vthresh is determined by the final VBG-Vthresh being the drain-source saturation voltage (V DS,SAT) 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 the resistances of resistors R20 and any other resistors along the current path through M28) current through a series of diode-connected BJTs (e.g., transistor M28) and resistors, and tapping the node 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, start-up circuit 220 also injects current with the help of M25 and M26 to attempt to pull up nodes A and B (the sources of transistors M25 and M26). Start-up circuit 220 is configured so that transistors M25 and M26 turn off when the voltages at nodes A and B approach steady state.

[0023] The first stage 210 can be configured to have a relatively high gain. FIG. 3 shows an example implementation of the first stage 210. The first stage 210 includes a bias circuit 310 coupled to an amplifier 350. The bias circuit 310 generates various bias voltages, such as PBIAS_PTAT, for use with the amplifier 350. The amplifier 350 includes an input transistor pair 352 including transistors M31 and M32. In this example, transistors M31 and M32 are PNP transistors. The base of transistor M31 is the non-inverting input connected to node A in FIG. 1. The base of transistor M32 is the inverting input coupled to node B in FIG. 1. In this example, the size ratio between transistors M31 and M32 is the same as in the transistor pair of FIG. 1 (e.g., 1:N). The Vbe voltages of each of transistors M31 and M32 are different due to the different transistor sizes. 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 voltages of transistors M31 and M32. Whereas in the above example the inverting and non-inverting inputs have the same voltage, but the size ratio between corresponding pairs of input transistors was 1:1, in the example of Figure 3 the size ratio is 1:N.

[0024] Referring again to FIG. 1, when the voltage difference between the inverting and non-inverting operational amplifiers of operational amplifier OP1 is equal to the difference in Vbe between transistors M31 and M32 (as opposed to a near-zero voltage difference as described above), the voltage V1 across resistor R1 is ΔVbe_OP1-Vbe_M3-Vbe_M1, where ΔVbe_OP1 is the difference in Vbe voltage between transistors M31 and M32. The Vbe difference between transistors M31 and M32 adds an additional value of ΔVbe to the voltage V1 across resistor R1. Because voltage V1 is larger due to the additional ΔVbe from the first stage 210 of the operational amplifier (compared to voltage V1 when input pairs M31 and M32 are the same size), current I1 in FIG. 1 increases, and as a result, summed current I3 also increases. To maintain the same PTAT voltage V3 across resistor R3 for the increased current I3, the resistance of resistor R3 is decreased (e.g., doubled).

[0025] Noise generated within operational amplifier OP1 is given a gain of (1 + (R3 / R1)). The advantage of configuring operational amplifier OP1 to provide an additional ΔVbe voltage across resistors R1 and R2 (by fabricating transistor M32 to be N times larger than transistor M31) is that resistor R3 can be reduced to provide the same PTAT V3 voltage and the same level of V BG Advantageously, as resistor R3 is reduced, the noise of operational amplifier OP1 is further attenuated.

[0026] FIG. 4 is a schematic diagram illustrating an alternative embodiment to that shown in FIG. 1. FIG. 4 is a schematic diagram of a bandgap reference circuit 400 including transistors M41, M42, M43, and M44, resistors R41, R42, and R43, and an operational amplifier OP2. Transistors M41-M44 in bandgap reference circuit 400 are PNP BJTs. The operation of bandgap reference circuit 400 is substantially the same as that of bandgap reference circuit 100 in FIG. 1. Transistors M41 and M42 and resistor R41 form gain stage 450, which operates to attenuate flicker noise generated within operational amplifier OP2 in substantially the same manner as gain stage 150 in FIG. 1. Transistor M42 in FIG. 4 also functions as an amplifier in substantially the same manner as transistor M2 in FIG. 1 operates as an amplifier.

[0027] As used herein, the term "couple" may encompass a connection, communication, or signal path that enables a functional relationship consistent with this specification. For example, if device A generates a signal to control device B to perform an action, then (A) in a first example, device A is coupled to device B by a direct connection, or (b) in a second example, device A is coupled to device B through an intervening component C, and device B is controlled by device A through a control signal generated by device A, where intervening component C does not change the functional relationship between device A and device B.

[0028] A device that is "configured" to perform a task or function may be configurable (e.g., programmed and / or hardwired) to perform that task or function by a manufacturer at the time of manufacture, or may be configurable (or reconfigurable) by a user after manufacture to perform those functions and / or other additional or alternative functions. Such configuration may be via firmware and / or software programming of the device, via the configuration and / or layout of hardware components, via the device's interconnections, or via a combination thereof.

[0029] As used herein, the terms "terminal," "node," "interconnect," "pin," and "lead" are used interchangeably. Unless otherwise noted, these terms are used generally to refer to an interconnection between, or termination of, a device element, circuit element, integrated circuit, device, or other electronic or semiconductor component.

[0030] A circuit or device described herein as including particular components may instead be coupled to those components and adapted to form the described circuit or device. For example, a structure described as including one or more semiconductor elements (such as transistors), one or more passive elements (such as resistors, capacitors, and / or inductors), and / or one or more sources (such as voltage and / or current sources) may instead include only the semiconductor elements within a single physical device (e.g., a semiconductor die and / or integrated circuit (IC) package) and may be adapted to be coupled to at least some of the passive elements and / or sources during or after manufacture, e.g., by an end user and / or third party, to form the described structure.

[0031] Although the use of particular transistors is described herein, other transistors (or equivalent devices) may be substituted. For example, a p-type metal-oxide-silicon FET ("MOSFET") may be substituted for an n-type MOSFET with little or no change to the circuit. Also, other types of transistors (such as bipolar junction transistors (BJTs)) may be used.

[0032] The circuits described herein are reconfigurable to include replaced components to provide functionality at least partially similar to that available prior to the component replacement. A component depicted as a resistor, unless otherwise noted, generally represents any one or more elements coupled in series and / or parallel to provide the amount of impedance represented by the depicted resistor. For example, a resistor or capacitor depicted and described herein as a single component may instead be multiple resistors or capacitors, respectively, coupled in parallel between the same nodes. For example, a resistor or capacitor depicted and described herein as a single component may instead be multiple resistors or capacitors, respectively, coupled in series between the same two nodes as the single resistor or capacitor.

[0033] Use of the term "ground" in the foregoing description includes chassis ground, earth ground, floating ground, virtual ground, digital ground, common ground, and / or any other form of ground connection applicable to or suitable for the teachings of this description. Unless otherwise specified, "about," "approximately," or "substantially" preceding a value means + / - 10% of the stated value. Modifications may be made to the exemplary embodiments described, and other embodiments are possible, within the scope of the claims of the present invention.

[0034] Modifications may be made to the exemplary embodiments described, and other embodiments are possible, within the scope of the claims of the invention.

Claims

1. 1. A bandgap reference circuit comprising: a first bipolar junction transistor (BJT) having a first emitter, a first base, and a first collector shorted to the first base; a first resistor; a second BJT coupled to the first BJT through the first resistor, the second BJT having a second emitter, a second base, and a second collector; a third BJT coupled to the first BJT, the third BJT having a third emitter, a third base coupled to the first base, and a third collector; a fourth BJT coupled to the third BJT, the fourth BJT having a fourth emitter, a fourth base coupled to the second base, and a fourth collector shorted to the fourth base; a second resistor coupled to the fourth emitter; a third resistor coupled to the second resistor and the second emitter; 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; a bandgap reference circuit comprising:

2. 2. The bandgap reference circuit of claim 1, the operational amplifier includes a first stage including a transistor pair having a first size ratio; The bandgap reference circuit, wherein the first and third transistors have a second size ratio that is approximately the same as the first size ratio.

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

4. 2. The bandgap reference circuit of claim 1, A bandgap reference circuit, wherein the first, second, third and fourth BJTs are NPN BJTs.

5. 1. A bandgap reference circuit comprising: a first bipolar junction transistor (BJT) having a first emitter, a first base, and a first collector shorted to the first base; a first resistor; a second BJT coupled to the first BJT through the first resistor, the second BJT having a second emitter, a second base, and a second collector; a third BJT coupled to the first BJT, the third BJT having a third emitter, a third base coupled to the first base, and a third collector coupled to the first collector; a fourth BJT coupled to the third BJT, the fourth BJT having a fourth emitter, a fourth base coupled to the second base, and a fourth collector shorted to the fourth base; a second resistor coupled to the fourth emitter; a third resistor coupled to the first and third collectors; 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; a bandgap reference circuit comprising:

6. 6. The bandgap reference circuit of claim 5, the operational amplifier includes a first stage including a transistor pair having a first size ratio; The bandgap reference circuit, wherein the first and third transistors have a second size ratio that is approximately the same as the first size ratio.

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

8. 6. The bandgap reference circuit of claim 5, A bandgap reference circuit, wherein the first, second, third and fourth BJTs are PNP BJTs.

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