Patent Application
20090102551
I. Field of the Invention
The invention relates generally to the field of amplifiers and more particularly to low voltage class AB operational amplifiers.
II. Description of Related Art
Nowadays, electronic accessories are a ubiquitous part of modern day life. Users of portable phones, organizers, and dedicated music and videos players demand electronics offering high performance, convenient size, and sufficient battery life.
A key component of many electronic accessories are the low-power signal processors and audio coder-decoders (CODECS), which are usually battery driven. Low-power, high performance operation is desirable for extending battery life. These goals may be achieved, in part, by using a low voltage supply (less than 2V) for memory, signal processors, and the CODECs used in electronic accessories.
Sometimes CODECs must provide 1 or 2 V RMS (root mean square) outputs to external loads. In this case, the internal amplifiers must operate over a larger supply voltage range, typically 3.6 V. While line outs have a standard impedance of 10 kΩ, the audio amplifier must provide considerably larger currents to headphones which typically have an input impedance of only 32Ω.
Because portable audio and video players are increasingly being used with high fidelity speaker systems, such as in docking stations, automobiles, and home theaters, users have increased expectations for signal to noise ratio (SNR) and total harmonic distortion (THD) performance. For example, headphones may have signal-to-noise ratios of about 70 dB, but on a high-fidelity audio system, amplifier noise (SNR) and distortion (THD) in excess of 100 dB may be noticeable. The player's amplifier, a fundamental aspect of the design, must be able to drive various kinds of loads while dissipating very low power.
Operational-amplifiers (op-amps) have long played a fundamental role in the design of electronic devices of all types. Performance tradeoffs are approached differently by different designs, while occasionally, new technologies and approaches allow performance enhancement in all design considerations.
Class AB op-amps, represent a tradeoff between the inefficient class A design, and the highly distorting class B design. The class A designs have a maximum theoretical operational efficiency of 25%, while the maximum theoretical efficiency of a class B device is 78.5%. Class AB designs generally fall somewhere between the two. Low-voltage op-amp designs require a minimum supply voltage of one gate-sources voltage and two saturation voltages.
A feedback class AB op-amp attempts to monitor and feedback the currents on the output stage of the amplifier. Previous low-voltage designs have failed to accurately monitor the amplifier's output currents with the accuracy needed to achieve SNR and THD performance of 100 dB or better.
An apparatus and method are provided for a low-power, low-voltage amplifier with improved signal to noise ratio (SNR) and total harmonic distortion (THD).
In some aspects, embodiments relate to an amplifier having an input stage, output stage and monitoring stage. The output stage has a first transistor with an input terminal and a second transistor with an input terminal. The monitoring stage has a first current sensing transistor with an input terminal and a first common terminal. The first current sensing transistor's input terminal is connected to the first transistor's input terminal and the first common terminal of the first current sensing transistor is connected to a first voltage rail. The monitoring stage also has a second current sensing transistor with an input terminal and a first common terminal. The second current sensing transistor's input terminal is connected to the second transistor's input terminal and the first common terminal of the second current sensing transistor is connected to a second voltage rail.
In another aspect, embodiments relate to a method having a determining step and a connecting step. In the determining step it is determined whether a tap voltage corresponds to an enable state or a disable state. The tap voltage is related to a source voltage for an amplifier. In the connecting step, an additional output stage is connected to a primary output stage of the amplifier when the determining asserts the tap voltage corresponds to the enable state.
In yet another aspect, embodiments relate to an operational amplifier comprising a primary output stage, a plurality of comparators, and a plurality of additional output stages. Each of the plurality of comparators is adapted to output a control signal in response to comparing a reference voltage to a tap voltage. Each additional output stage is adapted to connect to or disconnect from the primary output stage in response to the control signal of a corresponding comparator among the plurality of comparators.
The invention and embodiments thereof will be better understood when the following detailed description is read in conjunction with the accompanying figures. In the figures, elements are not necessarily drawn to scale. In general, like elements appearing in multiple figures are identified by a like reference designation. In the drawings:
The inventors have appreciated the need for operational amplifiers with excellent power, SNR, and THD performance, such that they are suitable for portable audio players connected to high fidelity amplifiers and speaker systems. Some embodiments include a low-voltage feedback class AB output stage operational amplifier. With proper component selection and biasing, embodiments may provide SNR and THD performance better than 100 dB. Embodiments offer superior monitoring of the output stage currents, improving the accuracy of the feedback loop.
Also disclosed is a method of continuous time adaptive supply voltage compensation. Design tradeoffs must be made when using a single output stage for an op-amp used under varying voltage supply conditions. By selectively connecting additional output stages to the primary output stage, excellent performance can be achieved from the minimum supply voltage to the maximum supply voltage at the lowest power.
An embodiment is the low-voltage feedback class AB output stage operational amplifier 100 illustrated as a simplified block diagram in
The input stage 110 provides a low noise, high input impedance, differential input. The input stage receives input signals V+ and V− on terminals 102 and 104, respectively. The input stage 110 is connected via connection 112 to the class AB feedback amplifier stage 140.
The feedback amplifier stage 140 amplifies the input signal received from the input stage 110. The amplified signal is output to the output stage 130 via connections 116 and 118 and monitored by the class AB monitoring and feedback stage 120. The feedback amplifier stage 140 receives feedback from the monitoring and feedback stage 120 via connection 114.
The output stage 130 drives the output signal Vout, on terminal 106 of the operational amplifier 100. In some embodiments, the output stage comprises a pair of output drivers 132 and 134. In some embodiments, the output driver 132 is a p-type transistor, and the output driver 134 is an n-type transistor. In an example embodiment, the p-type transistor is an PMOS transistor, and the n-type transistor is an NMOS transistor, however, any type of complementary transistor pair may be used. The input terminal (gate for FETs) for transistors 132 and 134, indicated by “>” in
The compensating network 150 supports the output stage 130. In some embodiments, the compensating network 150 may be considered part of output stage 130.
The monitoring stage 120 accurately measures the output currents for the output drivers, imposes the class AB operating rule, and completes a feedback loop with feedback amplifier 140. The monitoring stage comprises current monitors 124 and 126, and class AB controller 122.
Because current monitors 124 and 126 are connected directly to VDD and VSS respectively, without any intervening transistors, the output currents of the current monitors accurately reproduce the currents at the output stage 130 of op-amp 100. Accurately monitoring the output currents provides feedback for the amplification of the input signals, V+ and V−, The class AB operation is enforced by the feedback configuration of op-amp 100.
Having provided a block diagram of an embodiment, a further embodiment of op-amp 100 is shown schematically in the circuit diagram of
The op-amp 100 has many features. For example, the minimum voltage of operation is one gate to source voltage 192 and two saturation voltages 194 and 196. The monitoring stage 120 provides accurate control for class AB operation. Accurate control for class AB operation results in high performance and low distortion. The circuit can operate over a wide supply voltage range and is capable of driving resistive and capacitive loads. It can also be manufactured within a very small footprint.
The input stage 110 comprises p-type transistors MP1, MP3, and MP4 and n-type transistors MN1-MN4. An n-type differential input stage is formed by MN1 and MN2 which take inputs V− and V+ at there input terminals, respectively.
Transistor MN3, has a voltage, Vbias<1>, applied to its input terminal. This transistor establishes a bias current for operation of the device. Voltage Vbias<1> may be generated by a biasing network (not shown), or any other suitable way and may adapt to varying circuit conditions.
The feedback amplifier stage 140 comprises p-type transistors MP2 and MP5, and n-type transistors MN5 and MN6. The feedback amplifier stage receives an input signal via connection 112, under proper operating conditions, voltage Vbias<2>, received on the gate of MP2 is replicated at the gate of MP5, connection 114.
The monitoring stage 120 comprises current monitors 124 and 126 and class AB controller 122 (
Current monitor 126 is embodied by transistor MP7. The input terminal of MP7 is connected directly to the input terminal of output driver 132 via connection 116, which is embodied as transistor MP8. The source (transistor terminal with an arrow) of MP7 is directly connected, with no intermediate transistor, to the voltage rail VDD. The drain terminal is connected to current mirror 122B. The current I2 through MP7 is proportional to the current through transistor MP8.
Here and throughout, the terms source, drain, and gate terminals may be used with specific reference to a MOSFET embodiment, however, in embodiments where other technologies are used, connections may be made using the corresponding input and common terminals.
Current monitor 124 is embodied by transistor MN7. The input terminal of MN7 is connected directly to the input terminal of output driver 134 via connection 118, which is embodied as transistor MN10. The source terminal of MN7 is directly connected, with no intermediate transistor, to the voltage rail VSS. In the illustrated embodiment, the drain terminal is connected to the class AB controller current source 122A and current mirror 122B, via connection 114.
Current source 122A of the class AB controller 122 (
Current mirror 122B of the class AB controller 122 (
I
1
=I
2
+I
4.
Stated another way, the output currents through the output drivers 132 and 134 sum to a constant. This is insured by the connection of the transistors and by Kirchoff's current law. The class AB feedback loop formed by the feedback amplifier 140 and the monitoring and feedback stage 120 controls the output currents.
Output stage 130 comprises transistors MP8 and MN10 capacitors C1 and C2, and resistor R form the compensating network 150. The output voltage, Vout, of op-amp 100 is provided on terminal 106. The push-pull output configuration allows the output to source and sink currents to the load.
One of the challenges of operating and designing audio CODECs is meeting requirements to operate over a wide range of supply voltages. Traditional selection of the output transistors is a tradeoff between performance and distortion. When the supply increases, the output stage must have more drive capability since the output signal may swing over a larger range. If the op-amp were designed for the highest voltage of operation, however, it would be very inefficient at lower operating voltages where the extra current is not used to drive the output. If, on the other hand, the output stage is designed for the lowest voltage, distortion increases when the op-amp is used at its highest supply voltage, because it may lack sufficient drive capability.
Continuous time adaptive power supply compensation may be achieved by such a design, allowing for good performance and efficiency at low and high supply voltages, as well as intermediate values.
Op-amp 200 is shown with additional output stages 130-1 to 130-N for implementing continuous time adaptive supply voltage compensation. Like output stage 130, each additional output stage comprises a pair of output drivers 132-1 to 132-N and 134-1 to 134-N. In some embodiments, these are p-type transistors (output drivers 132-1 to 132-N) and n-type transistors (output drivers 134-1 to 134-N).
Output stages may be selectively enabled/disabled by controls 136-1 to 136-N, and controls 138-1 to 138-N. The controls for a given stage include or are connected to a monitor that determines if the stage should be enabled or disabled. In some embodiments, the monitor is a voltage monitor and makes the determination based on a voltage associated with the supply voltage. For example, if the output stage 130-1 is presently disabled, the voltage monitor associated with output stage 130-1 determines if the corresponding voltage has passed an enable threshold. If so, the controls 136-1 and 138-1 enter an enabled state. If, on the other hand, the output stage 130-1 is presently enabled, the voltage monitor associated with output stage 130-1 determines if the corresponding voltage has passed a disable threshold. If so, the controls 136-1 and 138-1 enter a disabled state. Output stages 130-2 to 130-N may operate similarly, however, different parameters may be monitored to determine the state of each stage. When an output stage 130-1, for example, is enabled, the input terminal of output driver 132-1 is connected to connection 116. This allows the monitoring stage 120 to also monitor the current through output driver 132-1. Like output driver 132, output driver 132-1 is connected to VDD and to the output terminal 106.
In a similar way, output driver 134 is connected to connection 118 when a stage 130-1 is enabled.
When additional output driver 130-1 is disabled, controls 136-1 and 138-1 connect the input terminals of 132-1 and 134-1 to the voltage rails VDD and VSS, respectively. This connection turns off the extra output drivers and minimizes the leakage current flowing through them.
Each of controls 136-1 to 136-N, and 138-1 to 138-N take two control signals. In additional output stage 130-1 for example, control 136-1 receives control signals S1<1> and S2<1> at internal switches W2<1> and W1<1>, respectively. Control 138-1 also receives the control signals S1<1> and S2<1> to switches W3<1> and W4<1>, respectively. In some embodiments, S1<1> and S2<1> are binary signals having the property.
S1<1>=˜S2<1>.
Here “˜” is used to indicate that S2<1> is the complement of S1<1>. Thus when internal switches W2<1> and W3<1> are closed, internal switches W1<1> and W4<1> are open, and vice versa. The remaining additional output stages may operate in a similar way.
For example, when additional output stage 130-1 is in an enabled state, internal switches W2<1> and W3<1> are closed and internal switches W1<1> and W4<1> are open connecting the input terminal of MP<1> and MN<1> to the input terminal of MP8 and MN10, respectively. In embodiments where the additional output stages are connected to op-amp 100, closing internal switches W2<2> and W3<1> also connects MP<1> and MN<1> to the current monitors 126 and 124 (
The switch control signals S1<1> to S1<N>, and S2<1> to S2<N> may be generated in any suitable way. A supply voltage monitor 300 comprising N voltage monitors is illustrated in
V
<1>
≦V
<2>
≦ . . . ≦V
<N−1>
≦V
<N>
Each comparator 50-1 to 50-N receives a reference voltage, VREF, to compare the corresponding tap voltage to. The reference voltage may be generated in any suitable way, but should not vary with the supply voltage. In some embodiments, a bandgap reference is used. Very little power is drawn from the reference voltage source by the supply voltage monitors 300 since it is only connected to high impedance inputs.
The output of comparators 50-1 to 50-N generate output signals signal S1<1> to S1<N>, respectively. To generate output signals signal S2<1> to S2<N>, each respective output S1<1> to S1<N> is connected to a respective inverter 51-1 to 51-N. According to an embodiment where the resistors R<1> to R<N+1> all have non-zero resistances and comparators 50-1 to 50-N all behave in the same manner. As the supply voltage exceeds VREF, the additional output stage 130-1 to 130-N are enabled reverse sequentially (130-N first, followed by 130-(N−1)).
In some embodiments, the comparators 50-1 to 50-N are have hysteresis as illustrated symbolically. This allows for an enable threshold that is different than the disable threshold. One benefit of a comparator with hysteresis is that small variations in the supply voltage will not cause the comparator to quickly change output states.
The op-amp 100 and op-amp 200 have thus far been illustrated as single ended amplifiers.
Having described embodiments of single ended op-amp 200 and differential op-amp 500 a method for operating an amplifier with continuous time adaptive power supply compensation is now detailed with reference to
In some embodiments, the control parameter and a reference parameter are compared either directly or indirectly by step 706. A reference parameter, if necessary, may be acquired in step 704. In some embodiments, the reference parameter is a reference voltage, such as VREF in the embodiment shown in
When flow proceeds from step 706 to step 710, it is determined if the current state is the enabled state. If so, no change in state is necessary and flow returns to the start, via “A.” If however, the current state is not the enabled state, a state change is needed and flow continues to step 720.
In step 720 the additional output stage is connected. In some embodiments, connecting the additional output stage comprises steps 722 and 724. In step 722 the input terminal of the p-type output transistor of the additional output stage is connected to the p-type output transistor of the primary output stage. For example, in the embodiment of op-amp 200 show in
Similarly, in step 724 the input terminal of the n-type output transistor of the additional output stage is connected to the n-type output transistor of the primary output stage. For example, in the embodiment of op-amp 200 show in
After having connected the additional output stage to the primary output stage in step 730, the process returns to the start, via “A.”
If in step 706 it is determined that the additional output stage is to be disable, flow continues to step 708 where it is determined if the current state is the disable state. If so, no change in state is necessary and flow returns to the start, via “A.” If however, the current state is not the disable state, a state change is needed and flow continues to step 730.
In step 730 the additional output stage is disconnected. In some embodiments, disconnecting the additional output stage comprises steps 732 and 734. Continuing the example of op-amp 200 in
After having disconnecting the additional output stage to the primary output stage in step 730, the process returns to the start, via “A.”
Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.
The present application claims the benefit, under 35 U.S.C. §119(e), of U.S. Provisional Application Ser. No. 60/999,450, filed Oct. 18, 2007, entitled “Low Voltage Low Power Class AB Operational Amplifier,” which application is hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60999450 | Oct 2007 | US |
