Amplifier circuit

Abstract

The present application relates to a circuit for monitoring amplifier instability. The amplifier (100) has a circuit between the amplifier input terminal (IN N ) and the amplifier output (V OUT ) and a feedback path starting from the output terminal to form a feedback loop with at least a portion of the first signal path. The comparator (212) has: a first input terminal configured to receive a first signal (IN) derived from a first amplifier node as part of the feedback loop N ); and a second input terminal configured to receive a second signal (IN p ), the second amplifier node varies with the signal at the amplifier input but does not form part of the feedback loop. The comparator is configured to compare the first signal with the second signal and to generate a comparison signal (COMP), wherein in the event of amplifier instability, the comparison signal includes a characteristic indicative of amplifier instability.

Description

Technical Field

[0001] The field of representative embodiments of the present disclosure relates to methods, apparatus, and / or implementations related to or relating to amplifier circuits, and in particular to apparatus and methods for monitoring and detecting amplifier instability. Background Art

[0002] Many electronic devices have the ability to provide audio drive signals to audio output transducers (e.g., speakers). In some cases, an electronic device may be able to provide audio drive signals to an accessory or peripheral device (e.g., a set of headphones or earbuds, etc.) that, during use, may be removably connected to the electronic device via some kind of wired connection. For example, many electronic devices may have a receptacle or socket, such as a 3.5 mm jack socket, for receiving a corresponding mating plug of an accessory device. The audio drive circuitry (e.g., an audio codec) of the electronic device may be operable to provide an analog audio drive signal to drive the speaker of the accessory device when connected.

[0003] There are a wide variety of different audio accessory devices that can be connected to such electronic devices during use, and at least some of these audio accessory devices can present relatively high impedance loads. For example, some headphone accessories can have a DC load impedance on the order of hundreds of ohms. Given the diversity of accessories and the various ways in which accessories can be connected, in some cases, the connected load can have significant, undesirable capacitance and / or inductance.

[0004] For performance reasons, such as providing good power supply rejection ratio (PSRR) and total harmonic distortion (THD) quality, such audio driver circuits (e.g., headphone amplifiers) are typically arranged to have relatively high open-loop gain and high bandwidth. Such amplifiers are designed to remain stable under the expected load conditions. However, under certain operating conditions, the amplifiers may become unstable.

[0005] Amplifier instability can cause various negative effects, such as audible noise during silent periods or even during playback. Amplifier instability can also cause the amplifier to draw relatively large amounts of current. This can be particularly undesirable in the case of portable devices, which draw power from a limited power source, such as a battery. Summary of the Invention

[0006] Embodiments of the present disclosure relate to methods, devices, and systems for monitoring and detecting amplifier instability.

[0007] According to one aspect of the present disclosure, there is provided a circuit for monitoring amplifier instability, the circuit comprising:

[0008] An amplifier comprising:

[0009] a first signal path between the amplifier input and the amplifier output; and

[0010] a feedback path beginning at the amplifier output to form a feedback loop with at least a portion of the first signal path; and

[0011] A comparator comprising:

[0012] a first input configured to receive a first signal derived from a first amplifier node as part of the feedback loop; and

[0013] a second input configured to receive a second signal derived from a second amplifier node that varies with the signal at the amplifier input but does not form part of the feedback loop;

[0014] Wherein the comparator is configured to compare the first signal with the second signal and to generate a comparison signal, and wherein in the event that the amplifier is unstable, the comparison signal includes a characteristic indicative of amplifier instability.

[0015] The characteristic indicative of amplifier instability may include a characteristic frequency.The circuit may further include a processing module configured to receive the comparison signal and process the comparison signal to detect the characteristic frequency.

[0016] In some examples, the characteristic frequency may include a frequency within a defined frequency range. The defined frequency range includes a frequency corresponding to a unity gain bandwidth of the amplifier.

[0017] In some implementations, the characteristic frequency may include a frequency exceeding a defined threshold. In some cases, the defined threshold may be based on a maximum input signal frequency value and a scaling factor. The processing module may be configured such that at least one of the maximum input signal frequency value and the scaling factor is configurable.

[0018] In some examples, the processing module may include a counter configured to receive a clock signal, and the processing module may be configured to determine a cycle period of the comparison signal as a value indicating a frequency of the comparison signal. In some examples, the processing module may include a counter configured to receive a clock signal, and the processing module may be configured to determine a count value of the number of cycles of the comparison signal in a count period defined by the clock signal.

[0019] The processing module may be configured to determine that the comparison signal includes a characteristic indicative of amplifier instability if the characteristic frequency persists for a predetermined time period. The predetermined time period may be configurable.

[0020] The circuit may further include a controller, wherein in response to the processing module detecting a characteristic indicative of amplifier instability, the processing module may be configured to output a detection signal to the controller. In response to receiving the detection signal, the controller may be configured to output a control signal. The control signal may control the amplifier to shut down or adjust at least one operating parameter of the amplifier.

[0021] In some implementations, the first amplifier node comprises a first differential input of the amplifier and the second amplifier node comprises a second differential input of the amplifier.

[0022] In some examples, the amplifier includes an input gain stage and further includes one or more subsequent gain stages, and the first amplifier node includes an input to one subsequent gain stage and the second signal path includes a reference for the subsequent gain stage.

[0023] The circuit may be implemented as an integrated circuit.

[0024] The circuit may form at least part of an audio codec.

[0025] One aspect also relates to an electronic device comprising a circuit as described in any of the variations herein. The electronic device may further comprise a connector for removably mating with an accessory device during use, wherein the circuit is configured to output at least one audio drive signal to electrical contacts of the connector. The electronic device may be at least one of the following: a portable device, a battery-powered device, a communication device; a mobile or cellular telephone device or a smartphone; a computing device; a tablet, notebook, laptop, or desktop computer; a wearable device; a smartwatch; or a voice-activated or voice-controlled device.

[0026] In another aspect, a circuit for detecting amplifier instability is provided, the circuit comprising: an amplifier; and a processing module configured to receive a first signal from the amplifier and process the first signal to detect a characteristic frequency indicative of amplifier instability.

[0027] In another aspect, a circuit for detecting instability of an amplifier is provided, the circuit comprising:

[0028] a first input terminal configured to receive a first input signal derived from the amplifier;

[0029] a second input terminal configured to receive a second input signal derived from the amplifier; and

[0030] A comparison module is configured to compare the first input signal to the second input signal and detect a characteristic indicative of instability of the amplifier based on the comparison.

[0031] It should be noted that any feature described herein may be implemented in combination with any one or more of the other described features, unless expressly stated to the contrary herein or otherwise clearly indicated as incompatible. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] For a better understanding of the examples of the present disclosure, and to more clearly illustrate how these examples may be implemented, reference will now be made, by way of example only, to the following drawings, in which:

[0033] Figure 1 An example of an amplifier is shown;

[0034] Figure 2 An example of a circuit for monitoring amplifier instability according to one embodiment is shown;

[0035] Figure 3 shows that in the case of amplifier instability Figure 2 Examples of waveforms exhibited by the circuit;

[0036] Figures 4a to 4c Further example waveforms are shown;

[0037] Figure 5 Another example of a circuit for monitoring amplifier instability according to one embodiment is shown; and

[0038] Figure 6 shows that in the case of amplifier instability Figure 5 Examples of waveforms exhibited by the circuit. DETAILED DESCRIPTION

[0039] The following description sets forth example embodiments according to the present disclosure. Further example embodiments and implementations will be readily apparent to those skilled in the art. Furthermore, those skilled in the art will recognize that various equivalent techniques may be employed in place of or in conjunction with the embodiments discussed below, and all such equivalent techniques are intended to be included in this disclosure.

[0040] As discussed above, the host device may include audio driver circuitry, such as an audio codec, for outputting audio drive signals to the audio transducer. The audio driver circuitry may be capable of outputting the audio drive signals to an accessory device that is removably connected to the host device in use, and the audio driver circuitry may therefore include headphone amplifier circuitry.

[0041] Figure 1 An example of a differential input, single-ended output amplifier 100 is shown. Figure 1 In the example, the differential voltage signal V INN and V INPare supplied to the input resistor R1 to provide the first signal IN N and the second signal IN P As an input to the amplifier, it should be understood that other arrangements are possible. For example, as will be appreciated by those skilled in the art, any of a variety of different amplifier topologies may be implemented, such as nested-miller, Gm-c, nested Gm-c (NGCC), multi-path nested-miller, or transconductance with capacitive feedback compensation (TCFC) arrangements. The principles of instability monitoring described herein may be implemented for any such amplifier topology.

[0042] The differential amplifier 100 provides an output signal V OUT , in use driven by a load resistor R LOAD Indicates the audio payload. Figure 1 It is also shown that the load will have some load capacitance CL.

[0043] The amplifier 100 will be designed to be stable for a set of expected operating conditions and a set of expected load parameters (e.g., load resistance and / or load capacitance). The amplifier 100 can generally be configured to have high open-loop gain and high bandwidth to provide low distortion and high PSRR.

[0044] However, as mentioned above, under certain operating conditions, the amplifier may become unstable. This is particularly true for headphone amplifier circuits, where a variety of different devices may be connected to the amplifier output in various ways during use, and the load parameters may vary. For example, when a user controls the load connected to the output of amplifier 100, a high capacitive load may be connected to amplifier 100.

[0045] The amplifier 100 has an output terminal V OUT to one of these inputs (in this example, input IN N ) feedback. Thus, there is a first signal path between the input and the amplifier output and a feedback path starting from the amplifier output to form a feedback loop with at least a portion of the first signal path. As will be understood by those skilled in the art, in order to have A OL The open-loop gain and feedback factor β of the amplifier remain stable when |A OL *When β|>1, the phase shift of the loop gain should be less than 180 degrees.

[0046] As will be described in more detail below, amplifier 100 may include multiple gain stages. Such an amplifier system will have a certain unity gain bandwidth (UGB), which may be defined, for example, by the parameters of the input stage and the feedback from the output to the input. The system may also exhibit one or more other important poles, for example, due to other gain stages, and such poles may generally be designed to be relatively far away from the UGB. If a high capacitive load is connected to the output of amplifier 100, this may cause one such pole to move closer to the unity gain bandwidth (UGB) of the amplifier and may be located at A OL This results in a phase reversal of 180 degrees or more before crossing the 0 dB point. This causes the amplifier to become unstable, which, as discussed above, can lead to various negative effects such as noise or audio artifacts and / or potentially large inrush currents.

[0047] Embodiments of the present disclosure relate to amplifier circuits, and in particular to amplifier circuits capable of monitoring and detecting amplifier instability.

[0048] Figure 2 An example of an amplifier circuit 200 capable of monitoring and detecting amplifier instability is shown. The circuit 200 includes an amplifier 100 that can be used with a Figure 1 The operation is essentially the same as discussed above. Therefore, there is again a N ) and the amplifier output (V OUT ) and a feedback path that originates at the amplifier output and forms a feedback loop with at least a portion of the first signal path. Circuit 200 further includes a comparison module 210, which includes a comparator 212. Comparator 212 is configured to compare a first signal derived from a first node of amplifier 100 and a second signal derived from a second node of amplifier 100. The first node is a node of the amplifier that is within the feedback loop and therefore exhibits instability effects. The second node is a node of the amplifier that is not within the feedback loop but can be effectively used as a reference. Preferably, the second node is a node that varies with the amplifier input signal.

[0049] In the embodiment shown, the comparator 212 can be conveniently configured to compare the first differential input signal IN N and the second differential input signal IN P However, as will be described in more detail below, the comparator 212 may compare the first signal and the second signal from any suitable node in the signal path of the amplifier 100. The comparator 212 compares the first input signal IN N and the second input signal IN P To generate a comparison signal COMP.

[0050] As discussed above, under some operating conditions, amplifier 100 may exhibit instability, for example, if a load with large capacitance is connected to the output stage of amplifier 100. If the load capacitance causes a phase reversal between the amplifier output and input, this may cause the amplifier to oscillate.

[0051] The oscillations caused by such amplifier instability will be at a frequency close to the UGB of amplifier 100. Since UGB is a known factor of amplifier 100 and is typically significantly higher than the maximum frequency of any signal content of the audio amplifier, UGB can be used to detect when amplifier 100 becomes unstable.

[0052] Figure 3 shows the first input signal IN during the instability of the amplifier 100 N , the second input signal IN P and the waveform of the comparison signal COMP. If the amplifier 100 becomes unstable, the amplifier will begin to oscillate at a frequency of approximately UGB. Therefore, the output of the amplifier will include an oscillating signal component at this frequency. Since UGB is a known characteristic of the amplifier 100, oscillation around this frequency will become a characteristic of the amplifier 100 becoming unstable. If the amplifier 100 becomes unstable and oscillates, the oscillating signal component at the output will be fed back to the inverting input of the amplifier 100 and will modulate the second input signal IN. N ,like Figure 3 shown.

[0053] The second input signal IN P is supplied as an input to the comparator 212 and is coupled to the first input signal IN N For comparison. Figure 2 In the amplifier shown, the second input signal IN P can be considered as a reference to the comparator 212, which sets the input common mode voltage. In normal, stable operation, the first input signal and the second input signal track each other. However, if the first input signal IN N Due to amplifier instability, the amplifier starts to oscillate, which will cause the first input signal IN N With respect to the second input signal IN P The first input signal IN N The second input signal IN P The comparison signal COMP from the comparator 212 will oscillate around the value of IN N Relative to IN P The comparison signal COMP switches between its different output states (e.g., high state and low state) at the intersection of the two states. Therefore, the comparison signal COMP will cycle between the two states with a frequency f osc (equal to the cycle period Tosc The inverse of the first input signal I NN oscillation frequency.

[0054] Thus, the oscillation frequency at the comparator output can be used as an indicator that amplifier 100 has become unstable. In the event of amplifier instability, the amplifier will oscillate at a frequency near its unity-gain bandwidth (UGB), which is a known characteristic of the amplifier. Therefore, a characteristic frequency indicative of amplifier instability can be predetermined for any amplifier based on the known UGB. Thus, comparison signal COMP can be monitored for characteristics indicative of instability. In some examples, the characteristic can be that the frequency of comparison signal COMP corresponds to a defined characteristic frequency, such as within a defined characteristic frequency of the UGB of a reference amplifier or within a defined frequency range based on the UGB. In some cases, the UGB of an amplifier may be significantly higher than the maximum expected signal content. Thus, a characteristic of instability may simply be the determination that the frequency of comparison signal COMP is above a defined threshold.

[0055] Reference again Figure 2 In one embodiment, circuit 200 may include a processing module 214 configured to receive a comparison signal COMP. Processing module 214 may be configured to process comparison signal COMP and detect when comparison signal COMP includes a characteristic indicating instability of amplifier 100. In one embodiment, the characteristic may include a characteristic frequency as discussed above.

[0056] As described above, during stable operation, the first input signal and the second input signal are expected to track each other, and thus the comparison signal is not expected to switch significantly. Therefore, the processing module 214 can be configured to detect a specific characteristic frequency that indicates amplifier instability. In one embodiment, the processing module 214 can include a digital signal processor (DSP) and / or a state machine and can be configured to determine the frequency of the comparison signal COMP and, based on the determined frequency, determine whether the comparison signal COMP exhibits characteristics that indicate instability.

[0057] In some implementations, the processing module 214 may determine the period (i.e., cycle period) between consecutive rising edges or falling edges in the comparison signal COMP, for example, by counting the number of clock cycles of the clock signal CLK. Therefore, the processing module 214 may include a counter 216 that receives the clock signal CLK. In one example, the circuit 200 may include a clock generator that generates the clock signal CLK. In another example, the clock signal may be provided from a clock generated "off-chip". In some implementations, the amplifier circuit may use a suitable clock signal for other reasons. The determined cycle period of the comparison signal is a value indicating the frequency of the comparison signal. The cycle period may be converted to a corresponding frequency, for example, via a suitable lookup table. Alternatively, the determined cycle period may be compared with a predetermined period or cycle period range corresponding to the frequency at the UGB of the amplifier.

[0058] To reduce the likelihood of false positive detections of the characteristic frequency, processing module 214 can be configured to detect a signal component corresponding to the characteristic frequency for a predetermined time period. This predetermined time period can be specified by clock signal CLK. In the event of instability, amplifier oscillation can be maintained relatively continuously. Noise or transients caused by amplifier operation can cause comparison signal COMP to switch at a frequency that appears to be the characteristic frequency, but not in a continuous manner. Therefore, detecting a signal component at the characteristic frequency for a predetermined time period can distinguish amplifier instability from transients or random noise.

[0059] In one example, processing module 214 can be configured to detect a characteristic frequency for a defined minimum number of cycles corresponding to a characteristic frequency, such as a UGB, to determine that amplifier 100 has become unstable. For example, processing module 214 can be configured to detect the characteristic frequency for a period corresponding to at least 10 cycles. In one example, the UGB of the amplifier can be approximately 1 MHz. Thus, processing module 214 can be configured to determine that amplifier 100 exhibits instability if the characteristic frequency persists for at least 10 ms (corresponding to a duration of 10 cycles at that frequency). However, those skilled in the art will appreciate that the predetermined time period can be any suitable length for a given application and can be configurable.

[0060] In some implementations, rather than counting the number of cycles of the relatively fast clock signal in a cycle period defined by the comparison signal COMP, the counter 216 may instead count the number of cycles of the comparison signal COMP in a count period defined by the clock signal CLK. The count period may correspond to a predetermined time period (e.g., a period corresponding to the defined number of cycles at a characteristic frequency). If the counter reaches a count value corresponding to the defined number of cycles during the count period, this may be considered an indication that the frequency of the comparison signal COMP is at the characteristic frequency.

[0061] In some examples, the processing module 214 may determine that the amplifier 100 exhibits instability based on detecting oscillations at a frequency that is higher than the maximum signal frequency that the amplifier 100 may be expected to amplify during use. For example, for at least some audio applications, the maximum signal frequency that the amplifier 100 may be expected to drive may be approximately 96 kHz (based on the maximum common sampling rate of digital audio signals). Therefore, if a signal component with a frequency much higher than the maximum signal frequency is present at the output of the amplifier 100, it may be determined that this is not due to the signal being amplified, but rather is the result of amplifier instability. Therefore, in one example, the processing module 214 may not use f osc To match a specific characteristic frequency (eg, UGB) close to the amplifier 100, the processing module 214 may determine the oscillation frequency f osc Is it above a certain threshold value f TH , the threshold value can be defined relative to the maximum signal frequency. For example, the frequency threshold f TH Based on the maximum signal frequency f MAX Multiply by the scaling factor k to determine, that is, the processing module 214 can determine when f osc >f TH , where f TH =k*f MAX In some implementations, the scaling factor k may be configurable and, in one example, may be in the range of 5 to 10. However, those skilled in the art will appreciate that the scaling factor k may be set to any suitable value for a given application. Similarly, as described above, the processing module 214 may be configured to detect amplifier instability if the oscillation frequency remains above a threshold value for a predetermined time period specified by the clock signal CLK.

[0062] Reference again Figure 2In some examples, amplifier circuit 200 may further include a controller 218. If processing module 214 determines that amplifier 100 exhibits instability, processing module 214 may be configured to output a detection signal DET to controller 218. Controller 218 is operable to control one or more aspects of amplifier operation in response to detection signal DET indicating amplifier instability. In response to receiving detection signal DET, controller 218 may output a control signal CTL-A to amplifier 100. In one embodiment, control signal CTL-A may shut down amplifier 100 to prevent persistent instability. In some embodiments, control signal CTL-A may adjust one or more parameters of amplifier 100 to mitigate instability. For example, control signal CTL-A may adjust the value of an internal compensation capacitor of amplifier 100. In another example, control signal CTL-A may additionally or alternatively increase the bias current or supply current of amplifier 100 in an attempt to stabilize the amplifier. Additionally or alternatively, the controller 216 may generate at least a control signal CTL for communication to some other component that may or may not be integrated with the amplifier circuit. For example, the control signal CTL may be transmitted to an application processor of a host electronic device to indicate that the amplifier should be shut down or to communicate an error message to a user of the device, such as indicating that the audio load connected to the electronic device is not properly arranged. Note that in some implementations, the controller 216 may be integrated with the amplifier circuit 200, i.e., it may be an on-chip controller, but in some implementations, the controller may be an external or off-chip controller, i.e., not integrated with the amplifier circuit 200.

[0063] To further explain the principles of the present disclosure, Figures 4a to 4c The output signal V for different operating conditions of the amplifier 100 is shown. OUT , first current signal IN N , the second current signal IN P and the voltage waveforms of the comparison signal COMP. These example waveforms are obtained from an amplifier 100 configured in a transconductance with capacitive feedback compensation (TCFC) architecture. The amplifier 100 is designed to remain stable with a connected load capacitance of up to 1 nF. In this example, the UGB of the amplifier is approximately 1 MHz.

[0064] Figure 4a shows that during stable operation of the amplifier 100, when amplifying an audio signal having certain signal content, the output signal V OUT , the first input signal IN N , the second input signal IN P and example waveforms of the comparison signal COMP. Since the amplifier 100 is stable, the output signal V OUT behaves in a consistent manner and the first input signal INN Tracking the second input signal IN P Therefore, the first input signal IN N With the second input signal IN P There is no crossover point between , and therefore the comparison signal COMP does not switch between the high state and the low state. In this case, the comparison signal COMP remains in one state (low state in this example) and therefore does not exhibit characteristics indicating amplifier instability.

[0065] Figure 4b The output signal V during the unstable period of the amplifier 100 is shown. OUT , the first input signal IN N , the second input signal IN P and example waveforms of the comparison signal COMP. Figure 4b In the example shown in FIG, a load capacitor CL of 10 nF is connected to the output of the amplifier 100. The presence of such a high load capacitance may cause the amplifier to be unstable. Figure 4b An example with only static input (i.e. corresponding to no signal content or silent period) is shown. It can be seen that the output signal V OUT begins to exhibit significant oscillations, and these oscillations cause the first input signal IN N Therefore, if Figure 4b As shown, the first input signal IN N With the output signal V OUT As mentioned above, the second input signal IN P is used as a reference and is therefore not significantly affected by oscillations. Therefore, the first input signal IN N Around the second input signal IN P Oscillation, as shown in the figure.

[0066] The comparator 212 compares the first input signal IN N and the second input signal IN P , and the resulting comparison signal COMP therefore switches between a high state and a low state, as shown. As discussed above, the frequency with which the comparison signal COMP switches may indicate that the amplifier 100 exhibits instability. Figure 4b In the example shown in FIG, the oscillation cycle period (e.g., the duration between consecutive rising edges (or falling edges)) of the comparison signal COMP is determined to be approximately 780 nanoseconds, indicating an oscillation frequency of 1.2 MHz. This frequency is significantly higher than the maximum possible signal frequency (e.g., 96 kHz) and is near the UGB of the amplifier. Therefore, the processing module 214 can determine that the amplifier 100 has become unstable based on the comparison signal COMP.

[0067] Figure 4c shows the output signal V during the instability period of the amplifier 100 OUT , the first input signal IN N , the second input signal IN P and another example of the waveform of the comparison signal COMP. Figure 4c The behavior of the amplifier 100 is also shown for a high capacitive load CL of 10 nF. Figure 4c The behavior of the waveform is shown for the case where the input signal applied to the amplifier 100 is to be driven, ie in the presence of some non-stationary signal content.

[0068] Figure 4c shows the output signal V OUT Due to the fact that the input signal is being driven to change in amplitude (increase in this example). Figure 4c The time period of the waveform shown is approximately 10 μs, and thus corresponds to a relatively short time period of the audio signal.

[0069] because Figure 4c The amplifier 100 in the example also exhibits instability, so V OUT Oscillation will occur in the first current signal I NN It can be seen that the first input signal IN N and the second input signal IN P Both of them change due to the audio signal being amplified, but the first input signal IN N There is also a high-frequency oscillation signal component in the first input signal IN. N Around the second input signal IN P Oscillation, as shown in the figure.

[0070] The comparator 212 can again compare the first input signal IN N and the second input signal IN P , and the resulting comparison signal COMP will toggle between high and low states at a characteristic frequency indicative of amplifier instability. In this example, the period between successive rising (or falling) edges in the comparison signal is approximately 750 ns, indicating a frequency of approximately 1.3 MHz. Therefore, the processing module 214 can process the comparison signal COMP to determine whether the amplifier 100 has become unstable by looking for such a high frequency.

[0071] Thus, in various embodiments, amplifier instability is detected by detecting the presence of high frequency signal components in the signal derived from the amplifier's signal path. In principle, any signal node that would exhibit an oscillating signal component during amplifier instability can be monitored, for example, the output signal V OUTThe monitoring signal is extracted and frequency analyzed, for example, using FFT. However, in general, for performance reasons, it may be preferable to avoid using the output signal, and in at least some implementations, the output drive signal V OUT It may be a relatively high voltage signal. Furthermore, implementing a frequency analyzer such as an FFT may be relatively complex.

[0072] Monitor the differential input signal (ie, the first input signal IN N and the second input signal IN P ) is advantageous because these signals track each other across any audio signal content and allow the use of a simple comparator to generate a comparison signal that can be used to detect instabilities. The comparison signal output from the comparator will switch based on changes in one of the input signals relative to the other, so in the event of any oscillations due to instability, the comparison signal will exhibit a frequency corresponding to the frequency of the oscillations.

[0073] As mentioned, the frequency of the comparison signal can be readily determined, for example, by determining the time between successive rising or falling edges in the comparison signal, such as by counting the number of clock cycles of a suitable clock signal. Thus, the cycle period, and therefore the frequency, of the comparison signal can be readily determined. The comparator and counter can be implemented as low-power and low-area circuits, and thus do not significantly increase the power or area of the amplifier circuit.

[0074] Although the input signal IN N and IN P The use of may be convenient, but other signal nodes of the amplifier may also be monitored, and two signals from any suitable signal path of amplifier 100 may be used as inputs to comparator 212 to monitor amplifier instability.

[0075] To illustrate, Figure 5 An amplifier 500 is schematically shown configured in a transconductance with capacitive feedback compensation (TCFC) architecture, which may be used with the amplifier 100 discussed above. Figure 5 The amplifier 500 in the example of FIG includes a series of three gain stages gm1, gm2, and gm3. However, those skilled in the art will appreciate that the amplifier 500 may include any suitable number of gain stages.

[0076] Input signal V IN The amplifier 500 is sequentially driven by the first gain stage gm1, the second gain stage gm2 and the third gain stage gm3 to output an output signal V OUTA feedback path comprising a first Miller capacitor Cc1 is provided between the amplifier output and the output of the first stage gm1. The amplifier 500 further comprises a transconductance gain stage gmt connected in series with a second Miller capacitor Cc2 which in this case is in a feedback path around the final gain stage gm3.

[0077] In this example, the second stage gm2 may be additionally or alternatively monitored to detect amplifier instability. Specifically, the output V1 of the first gain stage, which is the input to the second gain stage, may be monitored. If the output signal V OUT If there are oscillations in the first gain stage gm2, these oscillations will therefore be fed back to the input node of the second gain stage gm2 and appear in the voltage V1. Therefore, the first gain stage output voltage V1 can be supplied as input to the comparator 212 as the first current signal IN N 's alternative.

[0078] The second input of the comparator 212 is a suitable reference signal, around which the first gain stage output voltage V1 oscillates in the event of amplifier instability. In this example, the second gain stage gm2 receives a bias voltage V for biasing the second gain stage gm2. BIAS The bias voltage V BIAS may be supplied as a second input to the comparator 212 for comparison with the first gain stage output voltage V1 .

[0079] As will be understood by those skilled in the art, in a TCFC amplifier, the voltage V1 output from the first stage and the bias voltage V BIAS In normal operation, they track each other, i.e., V1 and V BIAS track each other in the presence of any audio signal content. N and IN P In a similar way, if the amplifier oscillates, the voltage V1 will be at V BIAS Oscillates near V BIAS The common mode voltage that will be used as V1 oscillation. It will be clear to those skilled in the art how to generate a suitable bias voltage V using, for example, the bandgap voltage as a reference, or, for example, a resistor divider or the current into a diode. BIAS .

[0080] Figure 6 shows the output signal V during the instability period of the amplifier 500 OUT , the first gain stage output voltage V1, the bias voltage V BIAS and example waveforms of the comparison signal COMP. Due to instability, the output signal V OUT Oscillation, such as Figure 6 shown. Figure 6shows a period in which the audio signal content is initially at a quiescent level, so the output signal V OUT Oscillations around the output quiescent level then cause the audio signal content to increase the overall amplitude of the output signal.

[0081] Output signal V OUT The oscillations will be fed back to the input of the second gain stage gm2 and will modulate the output voltage V1 as discussed above. Therefore, the first gain stage output voltage V1 also exhibits oscillations during amplifier instability, which causes this voltage V1 to also oscillate around the bias voltage V BIAS oscillation.

[0082] Thus, the resulting comparison signal COMP will also switch between a high state and a low state in a similar manner as previously discussed, with a frequency corresponding to a characteristic frequency of amplifier instability. The comparison signal COMP may be supplied to a processing module 214 that processes the comparison signal COMP to detect and signal amplifier instability.

[0083] about Figure 5 and Figure 6 The description of FIG has shown that the signal path of the second gain stage gm2 can be monitored to detect amplifier instability. However, those skilled in the art will understand that according to the present disclosure, any two suitable signal paths of the amplifier can be monitored to detect amplifier instability.

[0084] As described above, the principles described herein can therefore be used to monitor and detect amplifier instability in a range of different amplifiers. Such monitoring may be particularly useful for audio driver circuits that can be removably connected to a range of different loads (e.g., headphone amplifier circuits, etc.) during use. However, amplifier instability monitoring circuits such as those described in any of the variations herein may be beneficially used in other applications, which may or may not be audio applications, and embodiments relate to amplifiers that can be used for any purpose.

[0085] Embodiments may be implemented as an integrated circuit, which in some examples may be a codec or the like. Embodiments may be incorporated into an electronic device, which may be, for example, a portable device and / or a device that can operate on battery power. The device may be a communication device, such as a mobile phone or smartphone, or the like. The device may be a computing device, such as a notebook computer, laptop computer, or tablet computing device. The device may be a wearable device, such as a smartwatch. The device may be a device with voice control or activation capabilities.

[0086] Those skilled in the art will recognize that some aspects of the above-described apparatus and methods (e.g., development and configuration methods) may be embodied as processor control code, for example, on a non-volatile carrier medium such as a disk, CD, or DVD-ROM, a program memory such as a read-only memory (firmware), or a data carrier such as an optical or electrical signal carrier. For many applications, the embodiments will be implemented on a DSP (digital signal processor), an ASIC (application specific integrated circuit), or an FPGA (field programmable gate array). Thus, the code may comprise conventional program code or microcode, or code for setting up or controlling an ASIC or FPGA, for example. The code may also comprise code for dynamically configuring a reconfigurable device such as a reprogrammable logic gate array. Similarly, the code may comprise code for a hardware description language such as Verilog TM ) or VHDL (Very High Speed Integrated Circuit Hardware Description Language). As will be appreciated by those skilled in the art, the code may be distributed among multiple coupled components that communicate with each other. Where appropriate, these embodiments may also be implemented using code running on a field (re)programmable analog array or similar device to configure the analog hardware.

[0087] It should be noted that the embodiments mentioned above illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, and "a" or "an" does not exclude a plurality, and a single feature or other unit may perform the functions of multiple units listed in a claim. Any reference number or label in a claim shall not be construed as limiting its scope.

Claims

1. A circuit for monitoring amplifier instability, the circuit comprising: An amplifier comprising: a first signal path between the amplifier input and the amplifier output; and a feedback path starting from the amplifier output to form a feedback loop with at least a portion of the first signal path; and A comparator comprising: a first input configured to receive a first signal derived from a first amplifier node as part of the feedback loop; and a second input configured to receive a second signal derived from a second amplifier node that varies with the signal at the amplifier input but does not form part of the feedback loop; wherein the comparator is configured to compare the first signal with the second signal and to generate a comparison signal, and wherein in the event of amplifier instability, the comparison signal comprises a characteristic comprising an oscillation frequency indicative of amplifier instability; and A processing module is configured to receive the comparison signal and process the comparison signal to detect the oscillation frequency. 2 . The circuit of claim 1 , wherein the oscillation frequency indicative of amplifier instability comprises a frequency within a defined frequency range. 3 . The circuit of claim 2 , wherein the defined frequency range includes frequencies corresponding to a unity-gain bandwidth of the amplifier.

4. The circuit of claim 2, wherein the oscillation frequency indicative of amplifier instability comprises a frequency exceeding a defined threshold. The circuit of claim 4 , wherein the defined threshold is based on a maximum input signal frequency value and a scaling factor. 6 . The circuit of claim 5 , wherein the processing module is configured such that at least one of the maximum input signal frequency value and the scaling factor is configurable. 7 . The circuit according to claim 1 , wherein the processing module comprises a counter configured to receive a clock signal, and the processing module is configured to determine the cycle period of the comparison signal as a value indicative of the frequency of the comparison signal.

8. The circuit according to any one of claims 1 to 6, wherein the processing module comprises a counter configured to receive a clock signal, and the processing module is configured to determine a count value of the number of cycles of the comparison signal in a counting period defined by the clock signal.

9. The circuit of any one of claims 1 to 6, wherein the processing module is configured to determine that the comparison signal includes the characteristic indicative of amplifier instability if the oscillation frequency indicative of amplifier instability persists for a predetermined time period.

10. The circuit of claim 9, wherein the predetermined time period is configurable.

11. The circuit of any one of claims 1 to 6, further comprising a controller, wherein in response to the processing module detecting the characteristic indicative of amplifier instability, the processing module is configured to output a detection signal to the controller. 12 . The circuit of claim 11 , wherein in response to receiving the detection signal, the controller is configured to output a control signal to control the amplifier to be turned off. 13 . The circuit of claim 11 , wherein in response to receiving the detection signal, the controller is configured to output a control signal to adjust at least one operating parameter of the amplifier.

14. The circuit of any one of claims 1 to 6, wherein the first amplifier node comprises a first differential input of the amplifier and the second amplifier node comprises a second differential input of the amplifier.

15. The circuit of any one of claims 1 to 6, wherein the amplifier comprises an input gain stage and one or more subsequent gain stages, and the first amplifier node comprises an input to one of the subsequent gain stages and the second amplifier node comprises a reference for the subsequent gain stage.

16. The circuit according to any one of claims 1 to 6, implemented as an integrated circuit.

17. The circuit of any one of claims 1 to 6, wherein the circuit forms at least part of an audio codec.

18. An electronic device comprising a circuit according to any preceding claim and a connector for removably mating with an accessory device in use, wherein the circuit is configured to output at least one audio drive signal to electrical contacts of the connector.

19. An electronic device comprising a circuit according to any one of claims 1 to 17, wherein the device is at least one of the following: a portable device, a battery-powered device, a communication device; a mobile or cellular telephone device or a smartphone; a computing device; a tablet computer, a notebook computer, a laptop computer or a desktop computer; a wearable device; a smart watch; a voice-activated or voice-controlled device.

20. A circuit for detecting instability of an amplifier, the circuit comprising: a first input terminal configured to receive a first input signal derived from the amplifier; a second input terminal configured to receive a second input signal derived from the amplifier; as well as A comparison module is configured to compare the first input signal with the second input signal and detect a characteristic indicative of instability of the amplifier based on the comparison, wherein the characteristic indicative of amplifier instability comprises a characteristic frequency comprising an oscillation frequency indicative of amplifier instability.

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