Patent Description:
A class B amplifier uses two active elements, such as two transistors. Each transistor is turned on for <NUM><NUM> of the input waveform. For example, the first transistor is on for <NUM>° to <NUM>° of the input waveform and off for <NUM>° to <NUM>° of the input waveform. The second transistor would then be turned off for <NUM>° to <NUM>° of the input waveform and on for <NUM>° to <NUM>° of the input waveform. That is, one output transistor operates as a current source, and the other output transistor operates as a current sink. This configuration may be referred to as a "push-pull" configuration because a first branch of the output stage "pushes" or sources currents to a load while a second branch of the output stage "pulls" or sinks current from the load. A class B amplifier has lower power consumption than a class A amplifier, but a class B amplifier may be susceptible to crossover distortion due to the turn-on of one output transistor not matching the turn-off of the other output transistor.

A class AB amplifier combines characteristics from both class A and class B amplifiers. Class AB amplifiers avoid the high power consumption of a class A amplifier by employing two transistors like class B amplifiers. To avoid the crossover distortion, class AB amplifiers bias both transistors into slight conduction, even when no input signal is present. This small biasing arrangement ensures that both transistors conduct simultaneously during a very small part of the input waveform by more than <NUM> per cent of the input cycle, but less than <NUM> per cent.

Low voltage class AB operational amplifiers (Op-Amp) may include a current feedback loop to facilitate class AB control. Current feedback is based on the current from the output transistors, and to determine the amount of current, transistors (sometimes referred to as "measuring transistors") may be used that sense the current from the respective output transistors. These sense transistors that sense the current may be coupled in such a way that a current sense branch forms in the Op-Amp.

When the output transistors operate outside the linear range, this may cause the current in the current sense branch to increase much beyond the typical value, even in the no load scenario. This is because the Op-Amp current feedback loop becomes dysfunctional outside the linear range. In an Op-Amp having N-MOS and P-MOS transistors, the current feedback loop can be associated with either the N-MOS transistors or the P-MOS transistors of the Op-Amp.

When the current feedback loop is associated with the N-MOS transistors of the Op-Amp, a high quiescent current occurs in the sense branch of the Op-Amp as the output common mode approaches ground. This occurs because when the amplifier output is close to ground, the gain of the Op-Amp is low, and the voltage on the gates of the one or more output transistors approaches the positive supply rail. This causes the current in the sense branch in the Op-Amp to go high.

When the current feedback loop is associated with the P-MOS transistors of the Op-Amp, a high quiescent current occurs in the sense branch of the Op-Amp as the output common mode approaches a positive supply rail. This occurs because when the amplifier output is close to the positive supply rail, the gain of the Op-Amp is low, and the voltage on the gates of the one or more output transistors approaches ground. This causes the current in the sense branch of the Op-Amp to go high.

A high current in the current sense branch in a low voltage class AB OP-Amp is, for example, inefficient, and it creates excess heat. These shortcomings, among others, are magnified in applications that use numerous Op-Amps. <CIT> discloses a current sensing circuit and a boost converter including the current sensing circuit. Further, <CIT> discloses a rail-to-rail class AB output stage having output bias current and linear performance. Moreover, <CIT> discloses a low voltage amplifier having a class-AB control circuit.

Features of the present disclosure may be understood from the following detailed description and the accompanying drawings.

Specific examples are described below in detail with reference to the accompanying figures. It is understood that these examples are not intended to be limiting, and unless otherwise noted, no feature is required for any particular example.

The embodiments of the present disclosure may be implemented as a circuit, integrated circuit, or other suitable configuration. The present disclosure may be implemented separately or integral with an Op-Amp. For example, an Op-Amp may be an integrated circuit, and the present disclosure may be incorporated in the integrated circuit of the Op-Amp to improve the Op-Amp.

Of course, these advantages are merely examples, and no advantage is required for any particular embodiment. Examples of the present disclosure are described with reference to the figures below.

<FIG> is a block diagram of an example class AB Op-Amp <NUM> according to some aspects of the present disclosure. Class AB Op-Amp <NUM> may include an input stage <NUM> configured to receive an input <NUM>. Input <NUM> (Vin) may be, for example, a differential input voltage signal or a single-ended input voltage signal. When input stage <NUM> is configured to receive a differential input signal, input stage <NUM> may allow common-mode voltages down to and below the negative supply rail. The input stage <NUM> provides a drive signal <NUM>.

Input stage <NUM> is coupled to the amplifier stage <NUM>, which is configured to receive the drive signal <NUM> and to provide an amplified signal <NUM> in response to the drive signal <NUM> and bias signal <NUM>. The amplifier stage <NUM> may comprise a current mirror with cascodes (not shown) to provide higher output impedance and reduce the effect of Miller capacitance. A current mirror may sum the opposite-phase signals of input <NUM>, when input <NUM> comprises a differential input signal, in order to drive the gates of the output transistors (not shown) in the output stage <NUM>. In an example, the cascode circuit may be further configured to provide a level shift between the input stage <NUM> and the output stage <NUM> by level shifting the drive signal <NUM> thereby providing a low supply voltage for a low voltage class AB Op-Amp.

In a class AB Op-Amp <NUM>, the output voltage <NUM> at the output stage <NUM> may swing from the positive voltage source (or supply rail) (Vdd) (not shown) to the ground rail (Vss) (not shown). The output stage <NUM> may include two transistors in a push-pull configuration (not shown) to provide the output voltage <NUM> at the output stage. In an example, the transistors of the output stage <NUM> may be an N-MOS and a P-MOS transistor.

As shown in <FIG>, class AB Op-Amp <NUM> may include control circuit <NUM> to implement a feedback loop to provide for class AB biasing. For example, control circuit <NUM> may receive current feedback signal <NUM> from output stage <NUM>. The current feedback signal <NUM> may include a current signal for each transistor in the output stage <NUM>. The control circuit <NUM> may include one or more transistors to sense (or determine) the level (or amount) of each current signal in the current feedback signal <NUM> from output stage <NUM>. The transistors that sense the current may be arranged to provide a current sense branch in the control circuit <NUM>.

Based on the current feedback signal <NUM>, the control circuit <NUM> provides the bias signal <NUM> to the amplifier stage <NUM> to bias the amplifier stage <NUM> for class AB operation. This completes the current feedback loop between the amplifier stage <NUM>, the output stage <NUM>, and the control circuit <NUM>. The amplifier stage <NUM> may comprise a "P-MOS side" that includes P-MOS transistors for providing the amplified signal <NUM> to the P-MOS output transistor (not shown) in the output stage <NUM>, and the amplifier stage <NUM> may comprise an "N-MOS side" that includes N-MOS transistors for providing the amplified signal <NUM> to the N-MOS output transistor (not shown) in the output stage <NUM>. The bias signal <NUM>, which is the feedback, may be applied to either the N-MOS side or the P-MOS side of the amplifier stage <NUM>.

When the output voltage <NUM> at the output stage <NUM> is outside the linear range, the current feedback loop can become dysfunctional. For example, if feedback is applied to the N-MOS side of the amplifier stage <NUM>, the feedback loop may become dysfunctional as the output voltage <NUM> approaches ground. In another example, if feedback is applied to the P-MOS side of the amplifier stage <NUM>, the feedback loop may become dysfunctional when the output voltage <NUM> approaches the positive voltage supply rail.

This dysfunction in the current feedback loop causes the quiescent current on the current sense branch in the control circuit <NUM> to go high due to the gain of the Op-Amp being low. For example, when the output voltage <NUM> approaches ground, the gate of the N-MOS transistor in the output stage <NUM> approaches the positive supply voltage. Alternatively, when the output voltage <NUM> approaches the positive supply voltage, the gate of the P-MOS transistor in the output stage <NUM> approaches ground. In either case, this causes the current on the current sense branch in control circuit <NUM> to increase beyond the value when the class AB Op-Amp <NUM> is within its linear range, even in the no load scenario. This high current on the current sense branch is inefficient and can generate excess heat.

To address the above situations, in some examples (and as explained in more detail below), the control circuit <NUM> includes circuitry to limit the high quiescent current on the current sense branch of the feedback loop. The control circuit <NUM> may include a voltage clamp configured to sense the output voltage <NUM> from output stage <NUM>. A voltage clamp may comprise a diode connected transistor or other suitable components to sense the output voltage <NUM>. The voltage clamp generates a gate voltage proportional to the output voltage <NUM>. In some examples, the gate voltage is coupled to the gate of a current limiting transistor configured to limit the current in the sense branch in the control circuit <NUM>. The current limiting transistor may limit the current when the Op-Amp is operating in the non-linear region, but the voltage clamp may reduce or not limit the current in the sense branch when the Op-Amp is operating in the linear range. A further description of the above circuit elements is set forth below.

<FIG> is a circuit diagram of the control circuit and output stage of a low voltage class AB Op-Amp in which the feedback loop is on the N-MOS side of the amplifier stage. Circuit <NUM> discloses an output stage comprising P-MOS output transistor <NUM> and N-MOS output transistor <NUM>, capacitors <NUM> and <NUM>, and voltage output (Vout) <NUM>. The drain of transistor <NUM> is coupled to the drain of output transistor <NUM>. Voltage output Vout <NUM> is located on the drains of output transistors <NUM> and <NUM>. Capacitors <NUM> and <NUM> are coupled in series between the gate of output transistor <NUM> and the gate of output transistor <NUM>, and capacitors <NUM> and <NUM> may be referred to as Miller capacitors. Miller capacitors <NUM> and <NUM> provide compensation for the Miller effect, which is an increase in the equivalent input capacitance to output transistors <NUM> and <NUM> in the output stage.

The portion of circuit <NUM> comprising the control circuit includes N-MOS transistors <NUM> and <NUM>, and P_MOS transistors <NUM>, <NUM>, and <NUM>. Transistors <NUM>, <NUM>, and <NUM> form current selector <NUM> that is part of a current feedback loop driving the amplifier stage to obtain class AB biasing. Current selector <NUM> is configured to output the lesser of two currents, and current selector <NUM> may be referred to as a minimum current selector.

The gate of transistor <NUM> in current selector <NUM> is coupled to the gate of transistor <NUM> in the output stage, which enables transistor <NUM> to measure (or function as a current sensor of) the current associated with transistor <NUM>. Transistor <NUM> senses the current associated with transistor <NUM> because as the output current in transistor <NUM> changes, a change in gate voltage will occur. Because the gates of transistors <NUM> and <NUM> are coupled together, the gate voltage at transistor <NUM> will change proportionally as the gate voltage of transistor <NUM> changes, thus enabling transistor <NUM> to measure (or function as a current sensor of) the current output of transistor <NUM>.

In current selector <NUM>, the drain of transistor <NUM> is coupled to the source of transistor <NUM>. The source of transistor <NUM> is coupled to the drain of transistor <NUM>. The source of transistor <NUM> is coupled to the positive power rail <NUM>. The gate of transistor <NUM> is coupled to the gate of transistor <NUM>, and the source of transistor <NUM> is also coupled to the positive power rail <NUM>. Transistor <NUM> may be configured in a diode configuration (or "diode-connected transistor") in which the gate of transistor <NUM> is coupled to the drain of transistor <NUM>. The drain of transistor <NUM> is also coupled to the drain of transistor <NUM>. The source of transistor <NUM> is coupled to the ground rail <NUM>. The gate of transistor <NUM> is coupled to the gate of output transistor <NUM> enabling transistor <NUM> to measure (or function as a current sensor of) the current associated with output transistor <NUM> in a similar fashion as described above with respect to transistors <NUM> and <NUM>.

The current selector <NUM> determines the lesser of two currents: current associated with output transistor <NUM> and current associated with output transistor <NUM>. The output of current selector <NUM> is feedback current IFB <NUM>, which is the lesser of the two currents. The feedback current IFB <NUM> runs through transistor <NUM>, which is configured as a diode connected transistor by having the source and gate of transistor <NUM> coupled together. The feedback current IFB <NUM> through the diode connected transistor <NUM> provides for bias feedback voltage VFB <NUM> that biases (in class AB mode) the amplifier stage <NUM> discussed with respect to <FIG> above.

The operation of the current feedback loop will now be explained with respect to <FIG>. The amplifier stage <NUM> receives the bias signal <NUM> (which comprises bias feedback voltage VFB <NUM>), and the amplifier stage <NUM> adjusts the output based on the bias feedback voltage VFB <NUM>.

The bias feedback voltage VFB <NUM> is based on the feedback current IFB <NUM> at transistor <NUM>, which is configured as a diode connected transistor. Feedback current IFB <NUM> may be determined by current selector <NUM>, which selects the lesser of two currents. Current selector <NUM> comprises transistors <NUM>, <NUM>, and <NUM>. In the current feedback loop, the current selector <NUM> operates to maintain a minimum current in output transistors <NUM> and <NUM> to prevent turn-on delay and thus crossover distortion.

The current selector <NUM> outputs the lesser of two currents: current associated with output transistor <NUM> and current associated with output transistor <NUM>. When output transistor <NUM> delivers a large output of current, its gate-source voltage will be large, and the voltage between the positive supply rail (Vdd) <NUM> and the source of transistor <NUM> operates transistor <NUM> in saturation. When transistor <NUM> operates in saturation, transistors <NUM>, <NUM>, and <NUM> function as a cascoded current mirror to mirror the current of transistor <NUM> to feedback current IFB <NUM> through transistor <NUM>. Thus, the current selector <NUM> outputs the lesser of the currents associated with output transistors <NUM> and <NUM>, and the bias feedback voltage VFB <NUM> is based on the current associated with output transistor <NUM>, which is lower than the current associated with output transistor <NUM>.

Conversely, when output transistor <NUM> delivers a large output current, transistor <NUM> operates in the linear range and pulls the source of transistor <NUM> high into saturation. Transistor <NUM> now mirrors the current of output transistor <NUM>, which is small in comparison to the output current of output transistor <NUM>. Thus, the output (feedback current IFB <NUM>) of current selector <NUM> via transistor <NUM> is the lesser of the currents associated with output transistors <NUM> and <NUM>, and the bias feedback voltage VFB <NUM> is based on the current associated with output transistor <NUM>, which is lower than the current associated with output transistor <NUM>.

The sense branch of the control circuit is between transistor <NUM> and <NUM>, and ISENS current <NUM> represents the current on the sense branch.

When the output at Vout <NUM> is outside the linear range (approximately ground (or zero volts)), the ISENS current <NUM> on the sense branch between transistor <NUM> and transistor <NUM> goes high. This happens because when the amplifier output is close to ground rail <NUM>, the gain of the Op-Amp is low, and the gate voltage of output transistor <NUM> goes high. The high gate voltage on output transistor <NUM> causes the gate voltage of transistor <NUM> to be high, which causes the ISENS current <NUM> to be high. For example, when the voltage at Vout <NUM> is approximately <NUM>µV, then a gate voltage of output transistor <NUM> may be <NUM> V resulting in an ISENS current <NUM> of approximately <NUM>µA. Conversely, when the output at Vout <NUM> is within the linear range, then, for example, an output voltage Vout <NUM> of <NUM> mV may result in a gate voltage of output transistor <NUM> at <NUM> V and ISENS current <NUM> to be <NUM>µA. The high ISENS current <NUM> when operating outside the linear range is inefficient, and in low voltage low power applications, such inefficiency is even more magnified. In addition, extra current causes additional heat.

<FIG> is a circuit diagram of the control circuit and output stage of a low voltage class AB Op-Amp in which the feedback loop is on the N-MOS side of the amplifier stage, according to some aspects of the present disclosure. In the circuit <NUM> of <FIG>, like circuit <NUM> of <FIG>, the output stage comprises transistors <NUM> and <NUM>, Miller capacitors <NUM> and <NUM>, and output voltage (Vout) <NUM>. The portion of circuit <NUM> comprising the control circuit includes N-MOS transistors <NUM> and <NUM>, and P_MOS transistors <NUM>, <NUM>, and <NUM>. Transistors <NUM>, <NUM>, and <NUM> form current selector <NUM> that is part of a current feedback loop driving the amplifier stage to obtain class AB biasing. Finally, current source <NUM>, N-MOS transistors <NUM> and <NUM> may also be included in the control circuit.

One terminal of current source <NUM> is coupled to Vdd <NUM>, and a second terminal of current source <NUM> is coupled to the drain of transistor <NUM>. Any suitable equivalent elements that function as a current source are contemplated. In an integrated circuit, current sources may be preferable to other elements because current sources may occupy less area in the integrated circuit than other elements.

Transistor <NUM> is an N-MOS transistor configured as a diode connected transistor having the gate coupled to the drain. The use of transistor <NUM> in a diode configuration is an exemplary embodiment. Alternative components are contemplated to provide the function of transistor <NUM>, including but not limited to using one or more diodes. The source of transistor <NUM> is coupled to output voltage (Vout) <NUM>. The gate of transistor <NUM> is coupled to the gate of transistor <NUM>. Transistor <NUM> is an N-MOS transistor. The drain of transistor <NUM> is coupled to the drain of transistor <NUM>. The source of transistor <NUM> is coupled to the drain of transistor <NUM>.

In the exemplary configuration of <FIG>, transistor <NUM> functions as a voltage clamp to provide a bias voltage on the gate of transistor <NUM>. Transistor <NUM> is coupled to output voltage (Vout) <NUM> to sense the output voltage produced by output transistor <NUM> at output voltage (Vout) <NUM>. Sensing the output voltage may comprise either 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) <NUM>, the voltage on the gate of transistor <NUM> can track the output voltage at output voltage (Vout) <NUM>. Thus, as the voltage at output voltage (Vout) <NUM> increases, the voltage at the gate of transistor <NUM> increases, and because the gate of transistor <NUM> is coupled to the gate of transistor <NUM>, the voltage at the gate of transistor <NUM> also increases. Conversely, as the voltage at output voltage (Vout) <NUM> decreases, the voltage at the gates of transistors <NUM> and <NUM> also decreases.

Transistor <NUM> is biased by transistor <NUM> to limit the flow of current ISENS <NUM> on the current sense branch. When the gate voltage at transistor <NUM> increases, transistor <NUM> reduces the limitation of current ISENS <NUM>. Conversely, the lower the output voltage at output voltage (Vout) <NUM>, the lower the gate voltage at transistor <NUM>. When the gate voltage at transistor <NUM> is low, transistor <NUM> operates to increasingly limit the flow of current ISENS <NUM>.

Current source <NUM> provides a biasing current for transistor <NUM>, which is acting as the voltage clamp. Current source <NUM> biases transistor <NUM> to enable transistor <NUM> to provide a gate voltage to transistor <NUM> to bias transistor <NUM> to limit current ISENS <NUM>. With current source <NUM> provided to transistor <NUM>, when output voltage (Vout) <NUM> is approximately zero volts (or ground), transistor <NUM> will be biased by current source <NUM> to resist the flow of current ISENS <NUM> in the current sense branch. Thus, the voltage clamp is configured to bias transistor <NUM> based on current source <NUM> and the sensed output voltage at output voltage (Vout) <NUM>.

As an example, if the voltage at output voltage (Vout) <NUM> is low at approximately <NUM>µV, then a gate voltage of <NUM> may be <NUM> V. In the example of <FIG>, the current ISENS <NUM> is limited by transistor <NUM>, and the resulting current ISENS <NUM> may be <NUM>µA. A current value in the sense branch of <NUM>µA is significantly lower than the current value in the sense branch <NUM> of <FIG> in which there is no current limiting. As set forth above in an example with respect to <FIG>, when the output voltage Vout <NUM> is low (<NUM>µV), the current in the sense branch <NUM> will be significantly higher at <NUM>µA. Returning to <FIG>, because transistor <NUM> tracks the output voltage at output voltage (Vout) <NUM>, the limitation of current decreases as the output voltage at output voltage (Vout) <NUM> goes up to prevent current ISENS <NUM> from being overly limited. For example, if the output at output voltage (Vout) <NUM> is <NUM> mV, the gate voltage of <NUM> may be <NUM> V and current ISENS <NUM> may be <NUM>µA. As this example demonstrates, the disclosed embodiments help reduce wasted current outside the linear range.

Furthermore, because the exemplary embodiment tracks the output voltage (Vout) <NUM>, the exemplary embodiment does not interfere with the feedback loop of the class AB Op-Amp. For example, when the output transistor <NUM> delivers a large output current, the current ISENS <NUM> will go high (and will be limited), but transistor <NUM> is biased by current source <NUM> and transistor <NUM> so as to not overly limit ISENS <NUM> such that it is smaller than the current associated with transistor <NUM>. Therefore, current selector <NUM> will output to transistor <NUM> (as current IFB <NUM>) the current associated with output transistor <NUM> because the current associated with transistor <NUM> will be the lesser of the currents associated with transistors <NUM> and <NUM>. Thus, the bias feedback voltage VFB <NUM> which is based on current IFB <NUM> is not affected by the limitation to current ISENS <NUM>.

Conversely, when the output transistor <NUM> delivers a large output current, the current ISENS <NUM> will be low because it is associated with output transistor <NUM>. As the current ISENS <NUM> is the low current (as between the currents associated with transistors <NUM> and <NUM>), the current selector <NUM> will select the current ISENS <NUM> to be provided as current IFB <NUM> to transistor <NUM> to provide the bias feedback voltage VFB <NUM>. The current ISENS <NUM> value in circuit <NUM> will not interfere with the determination of the bias feedback voltage VFB <NUM> because current ISENS <NUM> will have little to no current limiting by transistor <NUM> as output voltage output voltage (Vout) <NUM> will be high causing the voltage clamp to provide a high gate voltage to transistor <NUM>, which will reduce or eliminate any current limiting of current ISENS <NUM>. Thus, in either case, the disclosed embodiments do not interfere with the feedback loop and the determination of the bias feedback voltage VFB <NUM>.

<FIG> is a circuit diagram of the control circuit and output stage of a low voltage class AB Op-Amp in which 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, high current in current ISENS <NUM> will occur when the output at output voltage Vout <NUM> is approximately the positive supply Vdd <NUM>. The current feedback loop is illustrated in <FIG>. The output stage comprises P-MOS transistor <NUM> and N-MOS transistor <NUM>, Miller capacitors <NUM> and <NUM>, and output voltage Vout <NUM>. The drain of transistor <NUM> is coupled to the drain of transistor <NUM>. Output voltage Vout <NUM> is located on the drains of transistors <NUM> and <NUM>. Capacitors <NUM> and <NUM> are Miller capacitors coupled in series between the gate of transistor <NUM> and the gate of transistor <NUM>.

The portion of circuit <NUM> comprising the control circuit includes P-MOS transistors <NUM> and <NUM>, and N_MOS transistors <NUM>, <NUM>, and <NUM>. Transistors <NUM>, <NUM>, and <NUM> form current selector <NUM> that is part of a current feedback loop driving the amplifier stage to obtain class AB biasing. Current selector <NUM> is configured to output the lesser of a current associated with transistor <NUM> and a current associated with transistor <NUM>. The output of current selector <NUM> is feedback current IFB <NUM> through diode connected transistor <NUM>, which provides the bias feedback voltage VFB <NUM> that biases the class AB amplifier stage <NUM> discussed with respect to <FIG> above. The amplifier stage receives bias feedback voltage VFB <NUM>, and the amplifier stage adjusts the output based on the bias feedback voltage VFB <NUM>.

The function of current selector <NUM> will now be explained. When output transistor <NUM> delivers a large output of current, its gate-source voltage will be large, and the voltage between ground <NUM> and the source of transistor <NUM> operates transistor <NUM> in saturation. When transistor <NUM> operates in saturation, transistors <NUM>, <NUM>, and <NUM> function as a cascoded current mirror to mirror the current of measuring transistor <NUM> to transistor <NUM> that outputs bias feedback voltage VFB <NUM> based on the lower output current associated with output transistor <NUM>.

Conversely, when output transistor <NUM> delivers a large output current, transistor <NUM> operates in the linear range and pulls the source of <NUM> high into saturation. Transistor <NUM> now mirrors the current of output transistor <NUM>, which is small in comparison to the output current of transistor <NUM>. Thus, the output (current IFB <NUM>) of current selector <NUM> via transistor <NUM> is the lesser of the currents associated with transistors <NUM> and <NUM>, and the bias feedback voltage VFB <NUM> is based on the current associated with output transistor <NUM>, which is lower than the current associated with output transistor <NUM>.

Current ISENS <NUM> represents the current on the sense branch of the control circuit which is between transistor <NUM> and <NUM>.

In the circuit <NUM> of <FIG>, when the output at output voltage Vout <NUM> is outside the linear range (approximately positive supply Vdd <NUM>), current ISENS <NUM> goes high. This happens because when the amplifier output is close to positive supply Vdd <NUM>, the gain of the Op-Amp is low, and the gate voltage of transistor <NUM> goes low. The low gate voltage on transistor <NUM> causes the gate of transistor <NUM> to be low, which causes current ISENS <NUM> to go high. For example, if the voltage at output voltage Vout <NUM> is approximately <NUM> V, then a gate voltage of transistor <NUM> may be <NUM> V and may cause a current ISENS <NUM> of <NUM>µA. As with the examples associated with <FIG>, the high current ISENS <NUM> that may occur when the Op-Amp is operating outside the linear range is inefficient, and in low voltage low power applications, such inefficiency is even more magnified. In addition, extra current causes additional heat.

<FIG> is a circuit diagram of the control circuit <NUM> and output stage <NUM> of a low voltage class AB Op-Amp in which the feedback loop is on the P-MOS side of the amplifier stage <NUM>, according to some aspects of the present disclosure. In addition to the elements disclosed in <FIG>, <FIG> incorporates a current source <NUM> coupled to Vss <NUM> on one terminal of current source <NUM>, and a second terminal of current source <NUM> is coupled to the drain of transistor <NUM>. Any suitable equivalent components that function as a current source are contemplated.

Transistor <NUM> is a P-MOS transistor configured as a diode connected transistor having its gate coupled to its drain. Alternative components are contemplated to provide the function of transistor <NUM>, including but not limited to using one or more diodes. The source of transistor <NUM> is coupled to output voltage Vout <NUM>. The gate of transistor <NUM> is coupled to the gate of transistor <NUM>. Transistor <NUM> is a P-MOS transistor. The drain of transistor <NUM> is coupled to the drain of transistor <NUM>. The source of transistor <NUM> is coupled to the drain of transistor <NUM>.

In the exemplary configuration of <FIG>, transistor <NUM> may function as a voltage clamp to provide a voltage to bias transistor <NUM>. Transistor <NUM> is coupled to output voltage Vout <NUM> to sense the output voltage Vout <NUM>. By sensing the output voltage Vout <NUM>, the voltage on the gate of transistor <NUM> can track the output voltage Vout <NUM>. Thus, as the output voltage Vout <NUM> increases, the voltage at the gate of transistor <NUM> increases, and because the gate of transistor <NUM> is coupled to the gate of transistor <NUM>, the voltage at the gate of transistor <NUM> increases. Conversely, as the output voltage Vout <NUM> decreases, the voltage at the gates of transistors <NUM> and <NUM> also decreases.

Transistor <NUM> is biased by transistor <NUM> to limit the flow of current ISENS <NUM> on the current sense branch. When the gate voltage at transistor <NUM> increases, transistor <NUM> increases the limitation of current ISENS <NUM>. Conversely, the lower the output voltage Vout <NUM>, the lower the gate voltage at transistor <NUM>. When the gate voltage at transistor <NUM> is high, transistor <NUM> operates to increasingly limit the flow of current ISENS <NUM>.

Current source <NUM> biases transistor <NUM>, which is acting as the voltage clamp. The current source <NUM> biases transistor <NUM> to enable transistor <NUM> to provide a gate voltage to transistor <NUM> to bias transistor <NUM> to limit current ISENS <NUM>. Thus, transistor <NUM> is configured to bias transistor <NUM> based on current source <NUM> and the sensed output voltage Vout <NUM>.

Referring back to the example discussed with respect to <FIG>, if the output voltage Vout <NUM> is approximately <NUM> V, then a gate voltage at <NUM> may be low at approximately <NUM> V. In the example circuit <NUM> of <FIG>, the current ISENS <NUM> is limited by transistor <NUM>, and the resulting current ISENS <NUM> may be approximately <NUM>µA, which is lower than the <NUM>µA on the ISENS <NUM> branch in <FIG> without the current limiting configuration of <FIG>. Because the voltage clamp tracks the output voltage Vout <NUM>, the limitation of current ISENS <NUM> in the current sense branch decreases as the output voltage Vout <NUM> goes down to prevent current ISENS <NUM> from being overly limited.

The exemplary embodiment does not interfere with the feedback loop of the class AB Op-Amp. For example, when the output transistor <NUM> delivers a large output current, the current ISENS <NUM> is high. While transistor <NUM> will limit the current ISENS <NUM>, it does not affect the current feedback circuit generating bias feedback voltage VFB <NUM> because the bias feedback voltage VFB <NUM> will not be based on the current ISENS <NUM>. When transistor <NUM> delivers a large output current, transistor <NUM> will be producing a smaller output. Thus, the current selector <NUM> will output (as IFB <NUM>) the current associated with output transistor <NUM> as the minimum current to transistor <NUM> for producing bias feedback voltage VFB <NUM>.

Conversely, when the output transistor <NUM> delivers a large output current, output transistor <NUM> will be associated with the smaller current output. Thus, the current ISENS <NUM> will be the low current output (as current IFB <NUM>) by current selector <NUM> because it is associated with output transistor <NUM>, and the bias feedback voltage VFB <NUM> will be determined based on the current ISENS <NUM>. In the exemplary embodiment of <FIG>, the current ISENS <NUM> will not affect the functioning of the current feedback loop when selected because the current ISENS <NUM> will not be limited or have a very small limitation. This is because the voltage at output voltage Vout <NUM> will be low causing transistor <NUM> to provide a low gate voltage to transistor <NUM>, which will reduce or eliminate any current limiting of current ISENS <NUM>. Thus, in either case, the disclosed embodiments do not interfere with the feedback loop.

The term "couple" is used throughout the specification. The term 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, in a first example device A is coupled to device B, or in a second example device A is coupled to device B through intervening component C if intervening component C does not substantially 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.

Circuits <NUM> and <NUM> or other integrated circuit devices may use any combination of dedicated hardware and instructions stored in a non-transitory medium. Accordingly, elements of circuits <NUM> and <NUM> may include a processing resource coupled to a non-transitory computer-readable medium. The processing resource 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-backed RAM, SSDs, HDDs, optical media, and/or other memory devices suitable for storing the instructions for the processing resource.

Claim 1:
A circuit (<NUM>) for generating a current feedback loop driving an amplifier stage to obtain class-AB biasing, comprising:
a first transistor (<NUM>) having a gate and a drain coupled together;
a current source (<NUM>) coupled to the drain of the first transistor (<NUM>);
a second transistor (<NUM>) forming part of an output stage of a class-AB operational amplifier having a gate for receiving an amplified signal from an amplifier stage, having a drain coupled to a source of the first transistor (<NUM>) and directly coupled to an output voltage node (<NUM>) of the output stage, wherein the output voltage node (<NUM>) is configured to provide an output voltage (Vout);
a third transistor (<NUM>) having a gate coupled to the gate of the first transistor (<NUM>); and
a fourth transistor (<NUM>) having a drain coupled to a source of the third transistor (<NUM>), and a gate of the fourth transistor (<NUM>) is coupled to the gate of the second transistor (<NUM>) in order to function as a current sensor of the current associated with the second transistor (<NUM>)
wherein the current source (<NUM>) is configured to provide a biasing current to the first transistor (<NUM>) via the drain of the first transistor (<NUM>),
wherein the first transistor (<NUM>) is configured, based on the provided biasing current and the output voltage (Vout), to provide a gate voltage to the third transistor (<NUM>) to bias the third transistor (<NUM>), and
wherein the third transistor (<NUM>) is configured to limit a first current between the third transistor (<NUM>) and the fourth transistor (<NUM>) based on the provided gate voltage.