High quiescent current control circuit and amplifier

Abstract

Circuits (300) are provided. In some examples, the circuit includes a first transistor (336) having a gate and a drain coupled together and a current source (334) coupled to the drain of the first transistor (336). A second transistor (308) has a drain coupled to a source of the first transistor (336). A gate of a third transistor (338) is coupled to a gate of the first transistor (336). A drain of a fourth transistor (310) is coupled to a source of the third transistor (338) and a gate of the fourth transistor (310) is coupled to a gate of the second transistor (308). In some examples, the third transistor (338) is configured to limit a first current (326) between the third transistor (338) and the fourth transistor (310) based on an output voltage (318).

Description

Technical Field

[0001] This application generally relates to high quiescent current control in amplifiers, and more specifically to circuitry for generating a current feedback loop to drive an amplifier stage to achieve class AB bias, and to an amplifier incorporating such circuitry. Background Technology

[0002] There are several different categories of amplifiers, including Class A, Class B, and Class AB (or "A / B"). Class A amplifiers maintain a single active element (such as a transistor) throughout the entire 360° of the input waveform. 0 The entire input signal is reproduced by applying power during the cycle. As a result, Class A amplifiers may have high power consumption.

[0003] Class B amplifiers use two active components, such as two transistors. Each transistor operates at 180°. 0 The transistor is turned on in the input waveform. For example, the first transistor is turned on at 0° of the input waveform. 0 Up to 180 0 When the input waveform is turned on, it is at 181. 0 To 360 0 It turns off at time. Then, the second transistor will turn off at 0 of the input waveform. 0 Up to 180 0 Close at time, and at 181 of the input waveform 0 To 360 0 In a push-pull configuration, one output transistor acts as a current source, while the other acts as a current sink. This configuration can be called a "push-pull" configuration because the first branch of the output stage "pushes" current to or supplies current to the load, while the second branch "pulls" current from or sinks current from the load. Class B amplifiers consume less power than Class A amplifiers, but they can be susceptible to crossover distortion due to the mismatch between the turn-on of one output transistor and the turn-off of the other.

[0004] Class AB amplifiers combine the characteristics of Class A and Class B amplifiers. Class AB amplifiers avoid the high power consumption of Class A amplifiers by using two transistors, similar to Class B amplifiers. To avoid crossover distortion, Class AB amplifiers bias both transistors to a slight conduction, even when there is no input signal. This small bias arrangement ensures that both transistors conduct simultaneously for more than 50% but less than 100% of the input cycle during a very small portion of the input waveform.

[0005] Low-voltage Class AB operational amplifiers (Op-Amps) may include current feedback loops to facilitate Class AB control. The current feedback is based on the current from the output transistors, and to determine the current magnitude, transistors that sense the current from the respective output transistors (sometimes called "measuring transistors") can be used. These current-sensing transistors can be coupled in a manner that forms a current-sensing branch within the operational amplifier.

[0006] When the output transistor operates outside its linear range, this can cause the current in the current sensing branch to increase far beyond its typical value, even under no-load conditions. This is because the operational amplifier's current feedback loop becomes malfunctioning outside its linear range. In operational amplifiers with both N-MOS and P-MOS transistors, the current feedback loop can be associated with either the N-MOS or P-MOS transistor of the operational amplifier.

[0007] When a current feedback loop is associated with an operational amplifier's N-MOS transistor, high quiescent current appears in the operational amplifier's sensing branch as the output common-mode approaches ground. This occurs because the operational amplifier's gain is low when the amplifier output is close to ground, and the voltage on the gate of one or more output transistors is close to the positive supply rail. This causes the current in the operational amplifier's sensing branch to increase.

[0008] When the current feedback loop is associated with the P-MOS transistors of an operational amplifier, high quiescent current appears in the sense branch of the operational amplifier as the output common-mode approaches the positive supply rail. This occurs because the operational amplifier gain is low when the amplifier output is close to the positive supply rail, and the voltage on the gate of one or more output transistors is close to ground. This causes the current in the sense branch of the operational amplifier to increase.

[0009] For example, the high current in the current sensing branch of a low-voltage Class AB operational amplifier is inefficient and generates excessive heat. These drawbacks are amplified in applications that use a large number of operational amplifiers. Summary of the Invention

[0010] In some examples, circuitry is provided to limit the quiescent current in the current-sensing branch of the current feedback loop used in the Class AB operational amplifier when the operational amplifier output is near or at the power / ground rail. When the operational amplifier output is near or at the power / ground rail, the current-sensing branch associated with one or more output transistors may go high. Transistors can be used to limit the current in the sensing branch based on the operational amplifier's output voltage. For this purpose, a voltage clamp can bias the current-limiting transistor, and the voltage clamp can adjust the bias of the current-limiting transistor based on the output voltage coupled to the voltage clamp.

[0011] In some examples, a circuit is provided that includes a first transistor having a gate and a drain coupled together. The circuit also includes a current source coupled to the drain of the first transistor. The circuit further includes a second transistor having a drain coupled to the source of the first transistor. The circuit also includes a third transistor, the gate of which is coupled to the gate of the first transistor. The circuit further includes a fourth transistor having a drain coupled to the source of the third transistor, and the gate of the fourth transistor being coupled to the gate of the second transistor.

[0012] In some examples, the electronic device includes a current source. The electronic device also includes a first transistor coupled to the current source and an output voltage. The electronic device further includes a second transistor coupled to the first transistor. The electronic device also includes a third transistor coupled to the first transistor, wherein the first transistor is configured to bias the third transistor based on the output voltage and the current source. The electronic device also includes a fourth transistor coupled to the second transistor, and the fourth transistor is coupled in series with the third transistor, wherein the third transistor is configured to limit a first current between the third and fourth transistors based on the output voltage.

[0013] In some examples, the amplifier includes an input stage configured to receive an input voltage signal and provide a drive signal. The amplifier also includes an amplifier stage configured to receive the drive signal and provide an amplified signal in response to a bias signal and the drive signal. The amplifier further includes an output stage configured to receive the amplified signal and provide an output voltage. The amplifier also includes control circuitry coupled to the amplifier stage and the output stage, the control circuitry being configured to provide a bias signal to the amplifier stage. The control circuitry includes a current source. The control circuitry also includes a voltage sensor coupled to the current source, the voltage sensor being configured to sense the output voltage. The control circuitry also includes a current limiter coupled to the voltage sensor, the current limiter being configured to limit a first current based on the output voltage. Attached Figure Description

[0014] The features of the invention will be understood from the following detailed description and accompanying drawings.

[0015] Figure 1 This is a block diagram of a Class AB operational amplifier according to some aspects of the present invention.

[0016] Figure 2 This is a circuit diagram of the control circuit and output stage of a low-voltage Class AB operational amplifier, where the feedback loop is on the N-MOS side of the amplifier stage.

[0017] Figure 3 This is a circuit diagram of the control circuit and output stage of a low-voltage Class AB operational amplifier according to some aspects of the present invention, wherein the feedback loop is on the N-MOS side of the amplifier stage.

[0018] Figure 4 This is a circuit diagram of the control circuit and output stage of a low-voltage Class AB operational amplifier with feedback loop on the P-MOS side of the amplifier stage.

[0019] Figure 5 This is a circuit diagram of the control circuit and output stage of a low-voltage Class AB operational amplifier according to some aspects of the present invention, wherein the feedback loop is on the P-MOS side of the amplifier stage. Detailed Implementation

[0020] Specific embodiments will now be described in detail with reference to the accompanying drawings. It should be understood that these examples are not intended to be limiting, and unless otherwise stated, no particular example requires any specific feature.

[0021] Embodiments of the present invention can be implemented as circuits, integrated circuits, or other suitable configurations. The present invention can be implemented alone or integrated with an operational amplifier. For example, the operational amplifier can be an integrated circuit, and the present invention can be incorporated into the integrated circuit of the operational amplifier to improve the operational amplifier.

[0022] Of course, these advantages are merely examples and do not imply that any particular embodiment possesses these advantages. Examples of the invention are described below with reference to the accompanying drawings.

[0023] Figure 1 This is a block diagram of an example Class AB operational amplifier 100 according to some aspects of the present invention. The Class AB operational amplifier 100 may include an input stage 104 configured to receive an input 102. Input 102 (Vin) may be, for example, a differential input voltage signal or a single-ended input voltage signal. When the input stage 104 is configured to receive a differential input signal, the input stage 104 may allow the common-mode voltage to drop to and below the negative supply rail. The input stage 104 provides a drive signal 106.

[0024] Input stage 104 is coupled to amplifier stage 112, which is configured to receive drive signal 106 and provide amplified signal 114 in response to drive signal 106 and bias signal 110. Amplifier stage 112 may include a current mirror with cascode (not shown) to provide higher output impedance and reduce the effect of Miller capacitance. When input 102 includes a differential input signal, the current mirror may sum the inverted signal of input 102 to drive the gate of the output transistor (not shown) in output stage 116. In this example, the cascode circuit may also be configured to provide a level shift between input stage 104 and output stage 116 via level shift drive signal 106, thereby providing a low supply voltage for the low-voltage Class AB operational amplifier.

[0025] In the Class AB operational amplifier 100, the output voltage 118 at the output stage 116 can be drawn from a positive voltage source (or power supply rail) (V). dd(not shown) Swings to the ground rail (V) SS (Not shown). Output stage 116 may include two transistors (not shown) in a push-pull configuration to provide output voltage 118 at the output stage. In the example, the transistors of output stage 116 may be N-MOS and P-MOS transistors.

[0026] like Figure 1 As shown, the Class AB operational amplifier 100 may include control circuitry 108 to implement a feedback loop to provide Class AB bias. For example, control circuitry 108 may receive a current feedback signal 120 from output stage 116. Current feedback signal 120 may include a current signal for each transistor in output stage 116. Control circuitry 108 may include one or more transistors to sense (or determine) the level (or amount) of each current signal in current feedback signal 120 from output stage 116. The current-sensing transistors may be arranged to provide a current-sensing branch in control circuitry 108.

[0027] Based on the current feedback signal 120, the control circuit 108 provides a bias signal 110 to the amplifier stage 112 to bias the amplifier stage 112 for Class AB operation. This completes the current feedback loop between the amplifier stage 112, the output stage 116, and the control circuit 108. The amplifier stage 112 may include a "P-MOS side" including a P-MOS transistor for providing the amplified signal 114 to the P-MOS output transistor (not shown) in the output stage 116, and the amplifier stage 112 may also include an "N-MOS side" including an N-MOS transistor for providing the amplified signal 114 to the N-MOS output transistor (not shown) in the output stage 116. The bias signal 110 as feedback can be applied to either the N-MOS side or the P-MOS side of the amplifier stage 112.

[0028] When the output voltage 118 at output stage 116 is outside the linear range, the current feedback loop can become malfunctioning. For example, if the feedback is applied to the N-MOS side of amplifier stage 112, the feedback loop may become malfunctioning when the output voltage 118 is close to ground. In another example, if the feedback is applied to the P-MOS side of amplifier stage 112, the feedback loop may become malfunctioning when the output voltage 118 is close to the positive voltage supply rail.

[0029] Because of the low gain of the operational amplifier, this dysfunction in the current feedback loop causes a high quiescent current in the current sensing branch of the control circuit 108. For example, when the output voltage 118 is close to ground, the gate of the N-MOS transistor in the output stage 116 is close to the positive supply voltage. Alternatively, when the output voltage 118 is close to the positive supply voltage, the gate of the P-MOS transistor in the output stage 116 is close to ground. In either case, this causes the current in the current sensing branch of the control circuit 108 to increase beyond the value of the Class AB operational amplifier 100 within its linear range, even under no-load conditions. This high current in the current sensing branch is inefficient and generates excessive heat.

[0030] To address the aforementioned issues, in some examples (and explained in more detail below), control circuitry 108 includes a circuit system for limiting high quiescent current in the current sensing branch of the feedback loop. Control circuitry 108 may include a voltage clamp configured to sense the output voltage 118 from output stage 116. The voltage clamp may include a transistor or other suitable component connected as a diode to sense the output voltage 118. The voltage clamp generates a gate voltage proportional to the output voltage 118. In some examples, the gate voltage is coupled to the gate of a current-limiting transistor configured to limit current in the sensing branch of control circuitry 108. When the operational amplifier operates in a non-linear region, the current-limiting transistor may limit the current; however, when the operational amplifier operates in a linear range, the voltage clamp may reduce or not limit the current in the sensing branch. Further description of the aforementioned circuit elements is given below.

[0031] Figure 2 This is a circuit diagram of the control circuit and output stage of a low-voltage Class AB operational amplifier with feedback loop on the N-MOS side of the amplifier stage. Circuit 200 discloses an output stage including a P-MOS output transistor 206 and an N-MOS output transistor 208, capacitors 228 and 230, and a voltage output (Vout) 218. The drain of transistor 206 is coupled to the drain of output transistor 208. The voltage output Vout 218 is located at the drains of output transistors 206 and 208. Capacitors 228 and 230 are coupled in series between the gates of output transistors 206 and 208, and capacitors 228 and 230 can be referred to as Miller capacitors. Miller capacitors 228 and 230 provide compensation for the Miller effect, which is the increase in the equivalent input capacitance to output transistors 206 and 208 in the output stage.

[0032] The control circuit portion of circuit 200 includes N-MOS transistors 210 and 220, and P-MOS transistors 216, 212, and 214. Transistors 216, 212, and 214 form a current selector 232, which is part of a current feedback loop that drives an amplifier stage to achieve class AB bias. Current selector 232 is configured to output the smaller of two currents, and current selector 232 may be referred to as the minimum current selector.

[0033] The gate of transistor 216 in current selector 232 is coupled to the gate of transistor 206 in the output stage, enabling transistor 216 to measure (or act as a current sensor) the current associated with transistor 206. Transistor 216 senses the current associated with transistor 206 because its gate voltage changes when the output current in transistor 206 changes. Because the gates of transistors 216 and 206 are coupled together, the gate voltage at transistor 216 will change proportionally with the change in the gate voltage of transistor 206, thus enabling transistor 216 to measure (or act as a current sensor) the current output of transistor 206.

[0034] In current selector 232, the drain of transistor 216 is coupled to the source of transistor 220. The source of transistor 216 is coupled to the drain of transistor 214. The source of transistor 214 is coupled to the positive power rail 202. The gate of transistor 214 is coupled to the gate of transistor 212, and the source of transistor 212 is also coupled to the positive power rail 202. Transistor 212 can be configured as a diode (or “transistor connected as a diode”), wherein the gate of transistor 212 is coupled to the drain of transistor 212. The drain of transistor 212 is also coupled to the drain of transistor 210. The source of transistor 210 is coupled to ground rail 204. The gate of transistor 210 is coupled to the gate of output transistor 208, such that transistor 210 can measure (or act as a current sensor) the current associated with output transistor 208 in a manner similar to that described above with respect to transistors 216 and 206.

[0035] Current selector 232 determines the smaller of two currents: the current associated with output transistor 206 and the current associated with output transistor 208. The output of current selector 232 is the feedback current I. FB 224, this feedback current is the smaller of the two currents. Feedback current I FB 224 flows through transistor 220, which is configured as a diode by coupling its source and gate together. The feedback current I through the diode-connected transistor 220... FB 224 provides bias feedback voltage V FB222, the bias feedback voltage bias (in Class AB mode) above regarding Figure 1 Amplifier stage 112 is discussed.

[0036] Now refer to Figure 1 and Figure 2 Explain the operation of the current feedback loop. Amplifier stage 112 receives bias signal 110 (this bias signal includes the bias feedback voltage V). FB 222), and amplifier stage 112 is based on bias feedback voltage V FB 222 Adjust the output.

[0037] Bias feedback voltage V FB 222 Based on the feedback current I at transistor 220 FB 224, the transistor 220 is configured as a diode-like transistor. Feedback current I FB 224 can be determined by current selector 232, which selects the smaller of two currents. Current selector 232 includes transistors 212, 214, and 216. In the current feedback loop, current selector 232 operates to maintain a minimum current in output transistors 206 and 208 to prevent turn-on delay and thus prevent crossover distortion.

[0038] The current selector 232 outputs the smaller of two currents: the current associated with output transistor 206 and the current associated with output transistor 208. When output transistor 206 delivers a large current output, its gate-source voltage will be large and positively supplying the rail (V). dd The voltage between the source of transistor 202 and transistor 216 causes transistor 214 to operate in saturation. When transistor 214 operates in saturation, transistors 214, 216, and 212 act as cascode current mirrors to mirror the current of transistor 210 to the feedback current I through transistor 220. FB 224. Therefore, the current selector 232 outputs the smaller of the currents associated with output transistors 206 and 208, and biases the feedback voltage V. FB 222 is based on the current associated with output transistor 208, which is lower than the current associated with output transistor 206.

[0039] Conversely, when output transistor 208 delivers a large output current, transistor 214 operates within its linear range and pulls the source of transistor 216 high to saturation. Transistor 216 now mirrors the current of output transistor 206, which is smaller compared to the output current of output transistor 208. Therefore, current selector 232 receives the feedback current I via the output of transistor 216. FB 224) is the smaller of the currents associated with output transistors 206 and 208, and the bias feedback voltage VFB 222 is based on the current associated with output transistor 206, which is lower than the current associated with output transistor 208.

[0040] The sensing branch of the control circuit is located between transistors 212 and 210, I SENS Current 226 represents the current in the sensing branch.

[0041] When V out When the output at 218 is outside the linear range (approximately ground (or zero volts)), the I on the sensing branch between transistors 212 and 210 is... SENS Current 226 becomes high. This occurs because when the amplifier output is close to ground rail 204, the operational amplifier gain is low, and the gate voltage of output transistor 208 becomes high. The high gate voltage on output transistor 208 causes the gate voltage of transistor 210 to become high, which leads to I... SENS The current 226 increases. For example, when V out When the voltage at 218 is approximately 110 μV, the gate voltage of the output transistor 208 can be 3.1 V, resulting in I SENS The current 226 is approximately 660 μA. Conversely, when V out When the output at 218 is within the linear range, then for example, the output voltage V is 300 mV. out 218 can cause the gate voltage of the output transistor 208 to be 0.7 V and I SENS The current is 3.5 μA. When operating outside the linear range, high I... SENS A current of 226 is inefficient, and this inefficiency is amplified in low-voltage, low-power applications. Furthermore, the extra current generates additional heat.

[0042] Figure 3 This is a circuit diagram of the control circuit and output stage of a low-voltage Class AB operational amplifier according to some aspects of the present invention, wherein the feedback loop is on the N-MOS side of the amplifier stage. Figure 3 In circuit 300, similar to Figure 2 The circuit 200, the output stage includes transistors 306 and 308, Miller capacitors 328 and 330, and an output voltage (V). out Terminal 318. The control circuit portion of circuit 300 includes N-MOS transistors 310 and 320, and P-MOS transistors 316, 312, and 314. Transistors 316, 312, and 314 form a current selector 332, which is part of a current feedback loop driving an amplifier stage to obtain class AB bias. Finally, current source 334, N-MOS transistors 336, and 338 may also be included in the control circuit.

[0043] One terminal of the current source 334 is coupled to V dd 302, and the second terminal of current source 334 is coupled to the drain of transistor 336. Any suitable equivalent element can be conceived as a current source. In integrated circuits, current sources may be superior to other elements because they occupy less area than other elements.

[0044] Transistor 336 is an N-MOS transistor configured as a diode, with its gate coupled to its drain. Using transistor 336 in a diode configuration is an exemplary embodiment. Alternative components are contemplated to provide the functionality of transistor 336, including but not limited to the use of one or more diodes. The source of transistor 336 is coupled to the output voltage (Vout) terminal 318. The gate of transistor 336 is coupled to the gate of transistor 338. Transistor 338 is an N-MOS transistor. The drain of transistor 338 is coupled to the drain of transistor 312. The source of transistor 338 is coupled to the drain of transistor 310.

[0045] exist Figure 3 In the exemplary configuration, transistor 336 acts as a voltage clamp to provide a bias voltage at the gate of transistor 338. Transistor 336 is coupled to output voltage (Vout) terminal 318 to sense the output voltage generated at output voltage (Vout) terminal 318 by output transistor 308. Sensing the output voltage may include directly or indirectly measuring the voltage, detecting the voltage, or otherwise receiving an indication of the output voltage. By sensing the output voltage at output voltage (Vout) terminal 318, the voltage at the gate of transistor 336 can track the output voltage at output voltage (Vout) terminal 318. Therefore, as the voltage at output voltage (Vout) terminal 318 increases, the voltage at the gate of transistor 336 increases, and because the gate of transistor 336 is coupled to the gate of transistor 338, the voltage at the gate of transistor 338 also increases. Conversely, as the voltage at output voltage (Vout) terminal 318 decreases, the voltages at the gates of transistors 336 and 338 also decrease.

[0046] Transistor 338 is biased by transistor 336 to limit current I. SENS 326 Flow in the current sensing branch. When the gate voltage at transistor 338 increases, transistor 338 reduces the current I. SENS The limitation of 326. Conversely, the lower the output voltage at the output voltage (Vout) terminal 318, the lower the gate voltage at transistor 338. When the gate voltage at transistor 338 is low, transistor 338 operates to increasingly limit the current I. SENS The flow of 326.

[0047] Current source 334 provides bias current to transistor 336, which acts as a voltage clamp. Current source 334 biases transistor 336 so that transistor 336 can provide a gate voltage to transistor 338 to bias transistor 338 and limit current I. SENS 326. When current source 334 is supplied to transistor 336, when the voltage at output voltage (Vout) terminal 318 is approximately zero volts (or ground), transistor 338 will be biased by current source 334 to resist current I. SENS 326 flows in the current sensing branch. Therefore, the voltage clamp is configured to bias transistor 338 based on current source 334 and sensed output voltage at output voltage (Vout) terminal 318.

[0048] As an example, if the voltage at the output voltage (Vout) terminal 318 is as low as approximately 25 μV, then the gate voltage of 308 could be 3.1 V. Figure 3 In the example, the current I SENS 326 is limited by transistor 338, generating a current I SENS 326 is likely 10.8 µA. A current value of 10.8 µA in the sensing branch is significantly lower than... Figure 2 The current value in the sensing branch 226 without current limitation. As mentioned above regarding... Figure 2 As described in the example, when the output voltage V out At a low voltage (110μV), the current in sensing branch 226 will be significantly higher, at 660μA. Return to Figure 3 Because transistor 336 tracks the output voltage at output voltage (Vout) terminal 318, the current limit decreases as the output voltage at output voltage (Vout) terminal 318 increases to prevent current I... SENS 326 is overly limited. For example, if the output at the output voltage (Vout) terminal 318 is 300 mV, then the gate voltage of 308 can be 0.7 V and the current I... SENS 326 can be 3.3 μA. As this example shows, the disclosed embodiments help reduce wasted current outside the linear range.

[0049] Furthermore, because the exemplary embodiment tracks the output voltage (Vout) terminal 318, the exemplary embodiment does not interfere with the feedback loop of the Class AB operational amplifier. For example, when the output transistor 308 delivers a large output current, the current I... SENS 326 will go high (and will be limited), but transistor 338 is biased by current source 334 and transistor 336 so as not to over-limit I. SENS 326 makes it less than the current associated with transistor 306. Therefore, current selector 332 outputs the current associated with output transistor 306 to transistor 320 (as current I).FB 324), because the current associated with transistor 306 will be the smaller of the currents associated with transistors 306 and 308. Therefore, based on current I FB 324 bias feedback voltage V FB 322 is not affected by current I SENS The impact of the 326 restriction.

[0050] Conversely, when the output transistor 306 delivers a large output current, the current I... SENS 326 will be low because it is associated with output transistor 308. Due to current I SENS 326 is a low current (such as between the currents associated with transistors 306 and 308), and current selector 332 will select current I. SENS 326 as current I FB 324 is provided to transistor 320 to provide bias feedback voltage V FB 322. The current I in circuit 300 SENS The value of 326 will not interfere with the bias feedback voltage V. FB The determination of 322 is due to the current I. SENS 326 will have almost no current limitation on transistor 338 because the output voltage at the output voltage (Vout) terminal 318 will be very high, causing the voltage clamp to provide a high gate voltage to transistor 338, which will reduce or eliminate the current I. SENS 326 has no current limitations. Therefore, in either case, the disclosed embodiments do not interfere with the feedback loop and the bias feedback voltage V. FB The determination of 322.

[0051] Figure 4 This is the circuit diagram of the control circuit and output stage of a low-voltage Class AB operational amplifier, where the feedback loop is on the P-MOS side of the amplifier stage. When the feedback is on the P-MOS side of the amplifier stage, the output at the output voltage Vout = 418 is approximately the positive supply voltage V. dd At 402, the current I SENS High current will occur in 426. Figure 4 The current feedback loop is described below. The output stage includes a P-MOS transistor 406 and an N-MOS transistor 408, Miller capacitors 428 and 430, and an output voltage Vout 418. The drain of transistor 406 is coupled to the drain of transistor 408. The output voltage Vout 418 is located across the drains of transistors 406 and 408. Capacitors 428 and 430 are Miller capacitors connected in series between the gates of transistors 406 and 408.

[0052] The control circuit portion of circuit 400 includes P-MOS transistors 410 and 420, and N-MOS transistors 416, 412, and 414. Transistors 416, 412, and 414 form a current selector 432, which is part of a current feedback loop driving an amplifier stage to achieve Class AB bias. Current selector 432 is configured to output the smaller of the current associated with transistor 406 and the current associated with transistor 408. The output of current selector 432 is the feedback current I through transistor 420 connected as a diode. FB 424, transistor 420 provides the bias above regarding Figure 1 The bias feedback voltage V of the AB class amplifier stage 112 under discussion FB 422. Amplifier stage receive bias feedback voltage V FB 422, and the amplifier stage is based on the bias feedback voltage V. FB 422 Adjust the output.

[0053] The function of current selector 432 will now be explained. When output transistor 408 delivers a large current output, its gate-source voltage will be large, and the voltage between ground 404 and the source of transistor 416 causes transistor 414 to operate in saturation. When transistor 414 operates in saturation, transistors 416, 414, and 412 act as a cascode current mirror to mirror the current of measuring transistor 410 to transistor 420, which outputs a bias feedback voltage V based on the lower output current associated with output transistor 406. FB 422.

[0054] Conversely, when output transistor 406 delivers a large output current, transistor 414 operates within its linear range and pulls the source of 416 high to saturation. Transistor 416 now mirrors the current of output transistor 408, which is smaller compared to the output current of transistor 406. Therefore, current selector 432 receives current via the output of transistor 416 (current I). FB 424) is the smaller of the currents associated with transistors 406 and 408, and the bias feedback voltage V FB 422 is based on the current associated with output transistor 408, which is lower than the current associated with output transistor 406.

[0055] Current I SENS 426 represents the current in the sensing branch of the control circuit between transistors 412 and 410.

[0056] exist Figure 4 In circuit 400, when the output voltage Vout is in the linear range at 418 (approximately the positive power supply V), dd When 402) is outside, the current I SENS426 goes high. This happens because when the amplifier output is close to the positive supply Vdd 402, the operational amplifier gain is low, and the gate voltage of transistor 406 goes low. The low gate voltage on transistor 406 causes the gate voltage of transistor 410 to go low, which leads to a current I... SENS 426 goes high. For example, if the output voltage Vout at 418 is approximately 3.3 V, then the gate voltage of transistor 406 can be 0.2 V and can result in a current I of 600 μA. SENS 426. For example, using and Figure 2 A related example is the high current I that may occur when the operational amplifier operates outside its linear range. SENS The 426 is inefficient, and this inefficiency is amplified in low-voltage, low-power applications. Furthermore, the additional current generates additional heat.

[0057] Figure 5 This is a circuit diagram of the control circuit 108 and output stage 116 of a low-voltage Class AB operational amplifier according to some aspects of the present invention, wherein the feedback loop is on the P-MOS side of the amplifier stage 112. Except... Figure 4 In addition to the components disclosed in the document, Figure 5 One terminal of the current source 534 contains coupling to V. SS The current source 534 is provided for 504, and the second terminal of the current source 534 is coupled to the drain of transistor 536. Any suitable equivalent component can be envisioned as a current source.

[0058] Transistor 536 is a P-MOS transistor configured to have a gate coupled to its drain that is connected as a diode. Alternative components are envisioned to provide the functionality of transistor 536, including but not limited to the use of one or more diodes. The source of transistor 536 is coupled to the output voltage V. out 518. The gate of transistor 536 is coupled to the gate of transistor 538. Transistor 538 is a P-MOS transistor. The drain of transistor 538 is coupled to the drain of transistor 512. The source of transistor 538 is coupled to the drain of transistor 510.

[0059] exist Figure 5 In the exemplary configuration shown, transistor 536 can be used as a voltage clamp to provide voltage to bias transistor 538. Transistor 536 is coupled to the output voltage V. out 518 to sense output voltage V out 518. By sensing the output voltage V out The voltage on the gate of transistor 518 and 536 can track the output voltage V. out 518. Therefore, as the output voltage V... outAs 518 increases, the voltage at the gate of transistor 536 increases, and because the gate of transistor 536 is coupled to the gate of transistor 538, the voltage at the gate of transistor 538 also increases. Conversely, as the output voltage V... out As the voltage of transistor 518 decreases, the voltage at the gate of transistors 536 and 538 also decreases.

[0060] Transistor 538 is biased by transistor 536 to limit current I. SENS 526 Flow in the current sensing branch. When the gate voltage at transistor 538 increases, transistor 538 increases current I. SENS The limitation is 526. Conversely, the output voltage V... out The lower the voltage at 518, the lower the gate voltage at transistor 538. When the gate voltage at transistor 538 is high, transistor 530 operates to increasingly limit the current I. SENS The flow of 526.

[0061] Current source 534 biases transistor 536, which is used as a voltage clamp. Current source 534 biases transistor 536 so that transistor 536 can provide a gate voltage to transistor 538 to bias transistor 538 and limit current I. SENS 526. Therefore, transistor 536 is configured based on current source 534 and sensed output voltage V. out 518 bias transistor 538.

[0062] Return to reference about Figure 4 The example discussed, if the output voltage V out The voltage at 518 is approximately 3.3 V, so the gate voltage at 506 can be as low as approximately 0.2 V. In Figure 5 In example circuit 500, the current I SENS 526 is limited by transistor 538, and the resulting current I SENS 526 is likely to be around 10 µA, which is lower than Figure 4 There is no Figure 5 rate limiting configuration of I SENS The 600µA on branch 426 is due to voltage clamping tracking of the output voltage V. out 518, therefore the current I in the current sensing branch SENS The limitation of 526 depends on the output voltage V. out 518 decreases and decreases to prevent current I SENS 526 is overly restricted.

[0063] The exemplary embodiment does not interfere with the feedback loop of the Class AB operational amplifier. For example, when the output transistor 506 delivers a large output current, the current I... SENS 526 is high. Although transistor 538 will limit the current I.SENS 526, but it does not affect the generation of the bias feedback voltage V. FB The current feedback circuit of 522 has a bias feedback voltage V. FB 522 will not be based on current I SENS 526. When transistor 506 delivers a large output current, transistor 508 will produce a smaller output. Therefore, current selector 532 will output the current associated with output transistor 508 (such as I). FB 524) serves as the minimum current for transistor 520 to generate the bias feedback voltage V. FB 522.

[0064] Conversely, when output transistor 508 delivers a large output current, output transistor 506 will be associated with a smaller current output. Therefore, current I SENS 526 will be the low current (such as current I) output of the current selector 532. FB 524), because it is associated with output transistor 506, and bias feedback voltage V FB 522 will be based on current I SENS 526 was used to determine this. Figure 5 In an exemplary embodiment, the current I SENS The selection of 526 will not affect the function of the current feedback loop because the current I... SENS 526 is not limited or has very few limitations. This is because the output voltage V out The voltage at 518 will be lower, causing transistor 536 to supply a low gate voltage to transistor 538, which will reduce or eliminate current I. SENS 526 has no current limitations. Therefore, in either case, the disclosed embodiments do not have any interference feedback loops.

[0065] The term "coupled" is used throughout this specification. This term may encompass a connection, communication, or signaling path that supports a functional relationship consistent with this description. For example, if device A generates a signal to control device B to perform an action, in the first example, device A is coupled to device B. Or, in the second example, if intermediate component C substantially does not alter the functional relationship between device A and device B, device A is coupled to device B via intermediate component C, such that control signals generated by device B through device A are controlled by device A.

[0066] Circuits 300 and 500, or other integrated circuit devices, can use any combination of dedicated hardware and instructions stored in a non-transitory medium. Therefore, elements of circuits 300 and 500 may include processing resources coupled to a non-transitory computer-readable medium. Processing resources may include one or more microcontrollers, ASICs, CPUs, GPUs, and / or other processing resources configured to execute instructions stored on the medium. Examples of suitable non-transitory computer-readable media include one or more flash memory devices, battery-powered RAM, SSDs, HDDs, optical media, and / or other storage devices suitable for storing instructions for the processing resources.

[0067] It should be understood that the present invention provides several exemplary embodiments and that modifications can be made to these embodiments. Such modifications are clearly within the scope of the present invention. Furthermore, applying these teachings to other environments, applications, and / or purposes is consistent with and contemplated by the present invention.

Claims

1. A circuit for generating a current feedback loop to drive an amplifier stage to obtain a class AB bias, comprising: The first transistor has a gate and a drain coupled together; A current source coupled to the drain of the first transistor; The second transistor has a gate for receiving a signal from the amplifier stage and a drain coupled to the source of the first transistor to form an output voltage node of the output stage of the Class AB operational amplifier, wherein the output voltage node is configured to provide an output voltage (Vout). A third transistor having a gate coupled to the gate of the first transistor; as well as A fourth transistor, having a drain coupled to the source of the third transistor and a gate coupled to the gate of the second transistor, is configured to sense the current associated with the second transistor. The current source is configured to provide bias current to the first transistor via the drain of the first transistor. The first transistor is configured to provide a gate voltage to the third transistor based on the provided bias current and the output voltage (Vout) to bias the third transistor, and The third transistor is configured to limit a first current between the third transistor and the fourth transistor based on the provided gate voltage.

2. The circuit according to claim 1, wherein: The first transistor, the second transistor, the third transistor, and the fourth transistor include N-MOS transistors.

3. The circuit according to claim 1, wherein: The first transistor, the second transistor, the third transistor, and the fourth transistor include P-MOS transistors.

4. The circuit according to claim 2, wherein: The current source is coupled to the voltage source.

5. The circuit according to claim 3, wherein: The current source is coupled to ground.

6. The circuit according to claim 1, further comprising: A fifth transistor forming part of the output stage of the Class AB operational amplifier, having a drain coupled to the drain of the second transistor.

7. The circuit according to claim 6, further comprising: A current selector coupled to the fifth transistor and the third transistor, wherein the current selector comprises three transistors.

8. The circuit according to claim 7, further comprising: The sixth transistor has a drain that is coupled to the output of the current selector.

9. The circuit of claim 7 or 8, wherein the current selector is configured to output the smaller of a second current associated with the second transistor and a third current associated with the fifth transistor.

10. The circuit according to claim 7 or 8, wherein: The current selector includes: A seventh transistor coupled to the third transistor, wherein the gate of the seventh transistor is coupled to the drain of the seventh transistor; An eighth transistor, wherein the gate of the eighth transistor is coupled to the gate of the seventh transistor; and The ninth transistor is coupled to the eighth transistor.

11. An amplifier comprising the circuitry of any one of claims 1 to 10 for generating a current feedback loop to drive an amplifier stage to obtain a class AB bias, the amplifier comprising: The input stage is configured to receive an input voltage signal (Vin) and provide a drive signal; An amplifier stage configured to receive the drive signal and provide an amplified signal in response to a bias signal and the drive signal; The output stage is configured to receive the amplified signal and provide an output voltage, wherein the second transistor is arranged in the output stage; as well as A control circuit coupled to the amplifier stage and the output stage, the control circuit being configured to provide the bias signal to the amplifier stage, wherein the current source, the first transistor, the third transistor, and the fourth transistor are arranged in the control circuit.

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