BANDGAP VOLTAGE REFERENCE CIRCUIT WITH CURRENT MIRROR LOOP

Abstract

A bandgap reference circuit includes a first diode-coupled transistor having a first control terminal and first and second current terminals and a second transistor having a second control terminal and third and fourth current terminals. The second control terminal is coupled to the first control terminal. A third transistor have a third control terminal and fifth and sixth current terminals. A fourth diode-coupled transistor has a fourth control terminal and seventh and eighth current terminals. The fourth control terminal is coupled to the third control terminal. An operational amplifier has a first input, a second input, and an output. The output is coupled to the first current terminal. A first resistor is coupled between the output and the third current terminal. A second resistor is coupled between the sixth and eighth current terminals.

Claims

1. A bandgap reference circuit, comprising: a first diode-coupled transistor having a first control terminal and first and second current terminals; a second transistor having a second control terminal and third and fourth current terminals, the second control terminal coupled to the first control terminal; a third transistor having a third control terminal and fifth and sixth current terminals; a fourth diode-coupled transistor having a fourth control terminal and seventh and eighth current terminals, the fourth control terminal coupled to the third control terminal; an operational amplifier (OP AMP) having a first input, a second input, and an output, the output coupled to the first current terminal; a first resistor coupled between the output and the third current terminal; and a second resistor coupled between the sixth and eighth current terminals.

2. The bandgap reference circuit of claim 1, further comprising a third resistor coupled between the eighth current terminal and a reference terminal.

3. The bandgap reference circuit of claim 1, wherein the second input is coupled to the fourth and seventh current terminals.

4. The bandgap reference circuit of claim 1, further comprising: a fifth transistor having a fifth control terminal and ninth and tenth current terminals, the fifth control terminal coupled to the first control terminal; a third resistor coupled between the output and the ninth current terminal; and a sixth diode-coupled transistor having eleventh and twelfth current terminals, the eleventh current terminal coupled to the tenth current terminal and to the second input.

5. The bandgap reference circuit of claim 4, further comprising: a seventh transistor having a seventh control terminal and thirteenth and fourteenth current terminals, the seventh control terminal coupled to the fourth control terminal; a fifth resistor coupled between the eighth current terminal and the fourteenth current terminal; and an eighth diode-coupled transistor having fifteenth and sixteenth current terminals, the sixteenth current terminal coupled to the thirteenth current terminal and to the first input.

6. The bandgap reference circuit of claim 4, wherein a size of the fifth transistor is smaller than a size of the first diode-coupled transistor.

7. The bandgap reference circuit of claim 1, further comprising: a fifth transistor having a fifth control terminal and ninth and tenth current terminals, the fifth control terminal coupled to the fourth control terminal; a third resistor coupled between the eighth current terminal and the tenth current terminal; and a sixth transistor having a sixth control terminal and eleventh and twelfth current terminals, the twelfth current terminal coupled to the ninth current terminal and to the first input.

8. The bandgap reference circuit of claim 1, wherein: the first diode-coupled transistor and the second transistor are PNP bipolar junction transistors; and the third transistor and the fourth diode-coupled transistor are NPN bipolar junction transistors.

9. A bandgap reference circuit, comprising: an operation amplifier having a first input, a second input, and an output; a current mirror loop having a first current mirror loop terminal, a second current mirror loop terminal, a first voltage terminal, and a second voltage terminal, the first current mirror loop terminal coupled to the first input, and the second current mirror loop terminal coupled to the second input, the first voltage terminal coupled to the output; and a first resistor coupled between the second voltage terminal and a reference terminal.

10. The bandgap reference circuit of claim 9, wherein the current mirror loop comprises: a first diode-coupled transistor having a first control terminal and first and second current terminals; a second transistor having a second control terminal and third and fourth current terminals, the second control terminal coupled to the first control terminal; a third transistor having a third control terminal and fifth and sixth current terminals; a fourth diode-coupled transistor having a fourth control terminal and seventh and eighth current terminals, the fourth control terminal coupled to the third control terminal; a second resistor coupled between the output and the third current terminal; and a third resistor coupled between the sixth current terminal and the first resistor.

11. The bandgap reference circuit of claim 10, wherein the first diode-coupled transistor, the second transistor, the third transistor, and the fourth diode-coupled transistor are each bipolar junction transistors.

12. The bandgap reference circuit of claim 10, wherein the first diode-coupled transistor and the second transistor are one of PNP or NPN bipolar junction transistors, and the third transistor and the fourth diode-coupled transistor are the other of the PNP or NPN bipolar junction transistors.

13. The bandgap reference circuit of claim 10, wherein the first diode-coupled transistor and the fourth diode-coupled transistor are PNP bipolar junction transistors.

14. A bandgap reference circuit, comprising: an operation amplifier having a first input, a second input, and an output; a current mirror loop having a first voltage terminal and a second voltage terminal, the first voltage terminal coupled to the output, the current mirror loop including a first transistor having a control input; a first resistor coupled between the second voltage terminal and a reference terminal; a second transistor having a control input coupled to the control input of the first transistor such that a current through the first transistor is mirrored through the second transistor, the second transistor having a current terminal coupled to the second input; and a third diode-coupled transistor coupled between the second transistor and the reference terminal.

15. The bandgap reference circuit of claim 14, wherein the current mirror loop includes a fourth transistor having a control input, and the bandgap reference circuit further comprises: a fifth transistor having a control input coupled to the control input of the fourth transistor such that a current through the fourth transistor is mirrored through the fifth transistor, the fifth transistor having a current terminal coupled to the first input; and a sixth diode-coupled transistor coupled between the output and the fifth transistor.

16. The bandgap reference circuit of claim 15, wherein a size of the second transistor is smaller than a size of the first transistor, and a size of the fifth transistor is smaller than a size of the fourth transistor.

17. The bandgap reference circuit of claim 15, wherein the sixth diode-coupled transistor and the third diode-coupled transistor are PNP bipolar junction transistors.

18. The bandgap reference circuit of claim 14, wherein the first transistor is a diode-coupled transistor having first and second current terminals, and the current mirror loop comprises: a fourth transistor having a second control terminal and third and fourth current terminals, the second control terminal coupled to the control input of the first transistor; a fifth transistor having a third control terminal and fifth and sixth current terminals; a sixth diode-coupled transistor having a fourth control terminal and seventh and eighth current terminals, the fourth control terminal coupled to the third control terminal; a second resistor coupled between the output and the third current terminal; and a third resistor coupled between the sixth current terminal and the first resistor.

19. The bandgap reference circuit of claim 14, wherein the first transistor and the third diode-coupled transistor are PNP bipolar junction transistors.

20. The bandgap reference circuit of claim 14, wherein the first transistor is a diode-coupled transistor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] FIG. 1 is a schematic of an example bandgap voltage reference circuit.

[0005] FIG. 2 is a schematic of another example of a bandgap voltage reference circuit.

[0006] FIG. 3 is a schematic of yet another example of a bandgap voltage reference circuit.

DETAILED DESCRIPTION

[0007] The same reference numbers or other reference designators are used in the drawings to designate the same or similar (either by function and/or structure) features.

[0008] As mentioned above, bandgap voltage reference circuits provide generally constant, high-precision reference voltages in the face of various fluctuating parameters including ambient temperature, power supply voltage, and load current. Many bandgap voltage reference circuits include one or more bipolar junction transistors (BJTs). The base-to-emitter voltage (Vbe) of a BJT typically is complementary to absolute temperature (CTAT). At least some bandgap voltage reference circuits operate on the principle of offsetting the BJTs' CTAT dependence with a current that is proportional to absolute temperature (PTAT), thereby resulting in the bandgap voltage reference circuit's output voltage having very little if any dependence on temperature.

[0009] Several sources of noise may be present in a bandgap voltage reference circuit. One type of noise is a relatively low frequency noise (e.g., 0.1 to 10 Hz) associated with an operational amplifier (OP AMP) present in the circuit. The OP AMP's noise may cause changes in the bandgap voltage produced by the bandgap voltage reference circuit. The OP AMP's noise may be modeled as an input-referred noise associate with one of the OP AMP's input terminals. Unfortunately, for some bandgap voltage reference circuits, the ratio (also referred to as gain) of the change in the bandgap voltage to the change in the OP AMP's input referred noise may be substantial (e.g., a gain of 2, 3, 4, etc.). For example, a gain of 4 means that for a 1 mV input-referred noise offset, the output bandgap voltage may change by 4 mV. The examples described herein are directed to a bandgap voltage reference circuit that addresses this issue, and advantageously has a lower gain (e.g., approximately unity gain).

[0010] FIG. 1 is a schematic of a bandgap voltage reference circuit 100, in accordance with an example. In the example of FIG. 1, the bandgap voltage reference circuit 100 includes a current mirror loop 110 and an OP AMP 120. The current mirror loop includes transistors Q.sub.1, Q.sub.2, Q.sub.3, and Q.sub.4 and resistors R.sub.1a, R.sub.1b, and R.sub.2 (resistor R2 may be outside or inside the current mirror loop 110). OP AMP 120 includes a negative (?) input, a positive (+) input, and an output. Transistors Q.sub.1 and Q.sub.2 are PNP BJTs, and transistors Q.sub.3 and Q.sub.4 are NPN BJTs. The collectors of transistor Q.sub.1 and Q.sub.3 are coupled together and to the OP AMP's negative input. The collectors of transistors Q.sub.2 and Q.sub.4 are coupled together and to the positive input of OP AMP 120. Resistor R.sub.2 is coupled between the emitter of transistor Q.sub.4 at node 104 and the power supply reference terminal 102 (e.g., ground). Resistor R.sub.1b is coupled between the emitter of transistor Q.sub.3 and resistor R.sub.2 at node 104. The output of OP AMP 120 is coupled to the emitter of transistor Q.sub.1. Resistor R.sub.1a is coupled between the OP AMP's output and the emitter of transistor Q.sub.2. The output of the OP AMP 120 provides the output bandgap voltage V.sub.BG at an output terminal 101.

[0011] The bases of transistors Q.sub.1 and Q.sub.2 are coupled together. The ratio of the size of transistor Q.sub.1 to transistor Q.sub.2 is 1:N. The combination of transistors Q.sub.1 and Q.sub.2 and resistor R.sub.1a forms a current mirror in which current I.sub.1 through transistor Q.sub.1 is approximately equal to current I.sub.2 through transistor Q.sub.2. Similarly, the bases of transistors Q.sub.3 and Q.sub.4 are coupled together and to the collector of transistor Q.sub.4 and to the positive input of OP AMP 120. The ratio of the sizes of transistor Q.sub.3 to transistor Q.sub.4 is M:1. Transistors Q.sub.3 and Q.sub.4 are coupled together to form a current mirror in which, for the case in which M equals N, current I.sub.1 through transistor Q.sub.3 is approximately equal to current I.sub.2 through transistor Q.sub.4. Currents I.sub.1 and 12 add together to form current I.sub.PTAT through resistor R.sub.2.

[0012] For an ideal OP AMP, the voltages on the positive and negative inputs are equal. Accordingly, the output bandgap voltage V.sub.BG is the sum of the Vbe_Q1 (Vbe of transistor Q.sub.1), the Vbe_Q2 (Vbe of transistor Q.sub.2), and the voltage drop across resistor R.sub.2. The voltage drop across transistor R.sub.2 is the product of the resistance of resistor R.sub.2 and current I.sub.PTAT. Thus, the output bandgap voltage V.sub.BG equals Vbe_Q1+Vbe_Q2+(R.sub.2*I.sub.PTAT). As described above, the Vbe of a BJT has a CTAT temperature dependence. Assuming the resistance of resistors R.sub.1a and R.sub.1b are equal to each other and denoting that equal resistance as R.sub.1, then current I.sub.1, which equals current I.sub.2, equals V.sub.T*ln(N), where V.sub.T is the transistor's thermal voltage, ln is the natural logarithm operator, and N is the ratio of the sizes of transistors Q.sub.1 and Q.sub.2 as mentioned above. The transistor's thermal voltage, V.sub.T, equals kT/q, k is the Boltzmann constant, T is temperature, q is electric charge of an electron. Because currents I.sub.1 and I.sub.2 are proportional to V.sub.T, currents I.sub.1 and I.sub.2 are PTAT currents. Because the currents I.sub.1 and I.sub.2, which combine to flow through resistor R.sub.2, have a PTAT temperature dependence (and thus the voltage drop across resistor R.sub.2 is PTAT) and the Vbe's of transistors Q.sub.1 and Q.sub.4 have CTAT temperature dependencies, the output bandgap voltage V.sub.BG generally has very little dependence on temperature.

[0013] Noise generated by OP AMP 120 can be modeled as an input-referred voltage offset (V.sub.offset) on one or the other of the OP AMP's inputs. In the schematic of FIG. 1, the input-referred voltage offset V.sub.offset is modeled on the OP AMP's positive input. As mentioned above, for some bandgap voltage reference circuits, the gain of V.sub.BG to V.sub.offset is substantially greater than 1 (e.g., 2, 3, etc.), which means a small value of V.sub.offset (OP AMP noise) has a dramatic impact on the level of V.sub.BG. As described below, the current mirror loop 110 has a significantly lower V.sub.BG gain relative to V.sub.offset of approximately 1, and thus, due to the operation of the current mirror loop 110, the bandgap output voltage V.sub.BG is impacted to a much smaller degree by any OP AMP noise.

[0014] In the steady state, the current mirror loop 110 forces the currents I.sub.1 and 12 to remain approximately equal to each other and thus unaffected by V.sub.offset. The current mirror loop 110 achieves this result by maintaining the Vbe voltages of each of the four transistors Q.sub.1-Q.sub.4 at approximately the same voltage value. The current through a BJT is largely determined by the value of its Vbe, and thus if the Vbe's of the transistors do not change, then neither do the currents I.sub.1 and I.sub.2, and thus current I.sub.PTAT also remains unchanged. What does change is the voltage on node 104, which is the voltage across resistor R.sub.2. For example, assuming an increase of V.sub.offset of 1 mV, then the voltage on the bases of transistors Q.sub.3 and Q.sub.4 also increases by 1 mV. For currents I.sub.1 and I.sub.2 to remain unchanged, then the voltages on the emitters of transistors Q.sub.3 and Q.sub.4 will increase by 1 mV, and thus voltage on node 104 increases by 1 mV.

[0015] As described above, V.sub.BG is the sum of the Vbe's of transistors Q.sub.1 and Q.sub.4 plus the voltage across resistor R.sub.2. The current mirror loop 110 prevents the Vbe's of transistors Q.sub.1 and Q.sub.4 from changing due to input-referred noise of OP AMP 120, but the voltage across resistor R.sub.2 (voltage on node 104) changes by the same magnitude of the change in V.sub.offset. Continuing the example above, if V.sub.offset increases by 1 mV, then the voltage across resistor R.sub.2 increases by 1 mV, and the net effect on the magnitude of the output bandgap voltage V.sub.BG is to increase the output bandgap voltage V.sub.BG by 1 mV. Accordingly, a 1 mV change in V.sub.offset results in a 1 mV change in V.sub.BG, which is a gain (V.sub.BG/V.sub.offset) of 1.

[0016] Mechanical stress on a semiconductor device (e.g., a die on which multiple transistors are formed) can affect the Vbe of bipolar junction transistors. The Vbe of an NPN transistor may be affected to a larger degree than the Vbe of a PNP transistor. See e.g., F. Fruett et al., Minimization of the Mechanical-Stress-Induced Inaccuracy in a Bandgap Voltage Reference., IEEE J. Solid-State Circuits, vol. 38, no. 7, July 2003, pp. 1288-1291. As described above, the output bandgap voltage V.sub.BG is a function of the Vbe's of NPN transistors Q.sub.1 and Q.sub.4, as well as the voltage across resistor R.sub.2. Because V.sub.BG is a function of the Vbe's of two NPN transistors, any mechanical stress on the die containing the bandgap voltage reference circuit 100 may affect the magnitude of the output bandgap voltage V.sub.BG more than if the output bandgap voltage V.sub.BG was instead a function of PNP transistors.

[0017] FIG. 2 is a schematic of a bandgap reference circuit 200, which includes the current mirror loop 110, OP AMP 120, additional transistors Q.sub.5 and Q.sub.6 and resistor R.sub.3. In this example, transistors Q.sub.5 and Q.sub.6 are PNP BJTs. Resistor R.sub.3 is coupled between output terminal 101 and the emitter of transistor Q.sub.5. The collector of transistor Q.sub.5 is coupled to the emitter of transistor Q.sub.6 and to the positive input of OP AMP 120. Accordingly, the positive input of OP AMP 120 is not coupled to the collectors of transistors Q.sub.2 and Q.sub.4, as was the case for the bandgap reference circuit 100 in FIG. 1. The negative input of OP AMP 120 is coupled to the collectors of transistors Q.sub.1 and Q.sub.3 as was the case in FIG. 1. Transistor Q.sub.6 is a diode-coupled transistor (its base is coupled to its collector) coupled between transistor Q.sub.5 and resistor R.sub.2 at node 104.

[0018] The base of transistor Q.sub.5 is coupled to the bases of transistors Q.sub.1 and Q.sub.2. In this configuration, transistors Q.sub.5 and Q.sub.1 form a current mirror. In one example, transistor Q.sub.5 is smaller than transistor Q.sub.1, and thus the current I.sub.3 through transistor Q.sub.3 is a function of, but smaller than, current I.sub.1 through transistor Q.sub.1. Transistor Q.sub.5 provides current I.sub.3 as a bias current through diode-coupled transistor Q.sub.6.

[0019] In the configuration of FIG. 2, the output bandgap voltage V.sub.BG is the sum of the transistor Q.sub.1's Vbe, transistor Q.sub.6's Vbe, and the voltage across resistor R.sub.2. Because the output bandgap voltage V.sub.BG is now, in part, a function of PNP transistor Q.sub.6, instead of NPN transistor Q.sub.4, V.sub.BG from the bandgap reference voltage circuit 200 of FIG. 2 is advantageously influenced to a smaller degree by mechanical stress than the bandgap voltage reference circuit 100 of FIG. 1.

[0020] FIG. 3 is a schematic of a bandgap reference circuit 300, which includes the same current mirror loop 110, OP AMP 120, transistors Q.sub.5 and Q.sub.6, and resistor R.sub.3 of FIG. 2, and additional transistors Q.sub.7 and Q.sub.8 and resistor R.sub.4. In the example of FIG. 3, transistor Q.sub.7 is an NPN transistor, and transistor Q.sub.8 is a PNP BJT. Resistor R.sub.4 is coupled between resistor R.sub.2 and the emitter of transistor Q.sub.7. The collector of transistor Q.sub.7 is coupled to the collector and base of transistor Q.sub.8 and to the negative input of OP AMP 120. Accordingly, the negative input of OP AMP 120 is not coupled to the collectors of transistor Q.sub.1 and Q.sub.3, as was the case for the bandgap reference circuits 100 and 200 in FIGS. 1 and 2, respectively. Transistor Q.sub.8 is a diode-coupled transistor (its base is coupled to its collector) coupled between transistor Q.sub.7 and output terminal 101.

[0021] The base of transistor Q.sub.7 is coupled to the bases of transistors Q.sub.3 and Q.sub.4. In this configuration, transistors Q.sub.7 and Q.sub.4 form a current mirror. In one example, transistor Q.sub.7 is smaller than transistor Q.sub.4, and thus the current I.sub.4 through transistor Q.sub.7 is a function of, but smaller than, current I.sub.2 through transistor Q.sub.4. Transistor Q.sub.7 provides current I.sub.4 as a bias current through diode-coupled transistor Q.sub.8.

[0022] In the configuration of FIG. 3, the output bandgap voltage V.sub.BG is the sum of Vbe's of transistors Q.sub.6 and Q.sub.8 and the voltage across resistor R.sub.2. Because V.sub.BG in FIG. 3 is a function of PNP transistors Q.sub.6 and Q.sub.8, instead of, in part, NPN transistor Q.sub.4 as was the case for FIG. 1, the output bandgap voltage V.sub.BG from the bandgap reference voltage circuit 300 is advantageously influenced to a smaller degree by mechanical stress than the bandgap voltage reference circuits 100 of FIG. 1.

[0023] In this description, the term couple may cover connections, communications, or signal paths that enable a functional relationship consistent with this description. For example, if device A generates a signal to control device B to perform an action: (a) in a first example, device A is coupled to device B by direct connection; or (b) in a second example, device A is coupled to device B through intervening component C if intervening component C does not alter the functional relationship between device A and device B, such that device B is controlled by device A via the control signal generated by device A.

[0024] Also, in this description, the recitation based on means based at least in part on. Therefore, if X is based on Y, then X may be a function of Y and any number of other factors.

[0025] As used herein, the terms terminal, node, interconnection, pin and lead are used interchangeably. Unless specifically stated to the contrary, these terms are generally used to mean an interconnection between or a terminus of a device element, a circuit element, an integrated circuit, a device or other electronics or semiconductor component.

[0026] A circuit or device that is described herein as including certain components may instead be adapted to be coupled to those components to form the described circuitry 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 the sources to form the described structure either at a time of manufacture or after a time of manufacture, for example, by an end-user and/or a third-party.

[0027] References may be made in the claims to a transistor's control input and its current terminals. In the context of a BJT, the control input is the base, and the current terminals are the collector and emitter.

[0028] Circuits described herein are reconfigurable to include additional or different components to provide functionality at least partially similar to functionality available prior to the component replacement. Components shown as resistors, unless otherwise stated, are generally representative of any one or more elements coupled in series and/or parallel to provide an amount of impedance represented by the resistor shown. For example, a resistor or capacitor shown 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 shown 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.

[0029] While certain elements of the described examples are included in an integrated circuit and other elements are external to the integrated circuit, in other example embodiments, additional or fewer features may be incorporated into the integrated circuit. In addition, some or all of the features illustrated as being external to the integrated circuit may be included in the integrated circuit and/or some features illustrated as being internal to the integrated circuit may be incorporated outside of the integrated. As used herein, the term integrated circuit means one or more circuits that are: (i) incorporated in/over a semiconductor substrate; (ii) incorporated in a single semiconductor package; (iii) incorporated into the same module; and/or (iv) incorporated in/on the same printed circuit board.

[0030] Uses of the phrase ground in the foregoing description include a chassis ground, an Earth ground, a floating ground, a virtual ground, a digital ground, a common ground, and/or any other form of ground connection applicable to, or suitable for, the teachings of this description. In this description, unless otherwise stated, about, approximately or substantially preceding a parameter means being within +/?10 percent of that parameter or, if the parameter is zero, a reasonable range of values around zero.

[0031] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.