Patent 11296665 - Amplifier arrangement > Description
This application claims the benefit of the foreign priority of German Patent Application No. 10 2019 130 691.4, filed on Nov. 14, 2019, the entirety of which is incorporated herein by reference.
The invention relates to an amplifier arrangement, in particular to a device for amplifying analog audio signals.
For converting analog audio signals captured by a microphone into digital audio data, the analog audio signals are first amplified in a pre-amplifier and then fed to an analog-to-digital converter (ADC). The pre-amplifier and the ADC as well as further components, e.g. a transmitter and batteries or accumulators for power supply, can be included in a wireless microphone. The dynamic range to be captured by the microphone is often so large (e.g. more than 120 decibels) that it is necessary to switch or regulate the gain factor of the pre-amplifier, depending on the input signal level. However, manual switching or regulating is inconvenient for the user, and furthermore harbors the risk of an incorrect operating. A dual pre-amplifier circuit is known that automatically switches between two amplifier branches with different gain factors. This allows the dynamic range of the ADC to be used better, so that the resolution of the digital signal is improved at low volume. A block diagram is shown in
An analog audio signal at an input IN coming e.g. from a microphone capsule is fed to both, a first amplifier branch comprising a first amplifier with high gain G1 and a second amplifier branch comprising a second amplifier with lower gain G2. The two amplified signals OUT1, OUT2 are digitized in an ADC and filtered with separate, but equal digital high-pass filters HP1,HP2 for removing the DC component. The ADC is e.g. a stereo ADC that samples both channels simultaneously. Then, the higher gain of the first amplifier branch is digitally compensated by multiplying the values with G2/G1. Now it is possible to switch by means of a switch SEL between the two signals for selecting an output signal to be provided at the output OUT, depending on the volume level of the input signal. At a low volume level of the input signal, the digital output signal of the first branch is selected, because it has a better resolution and higher signal-to-noise ratio. At a larger volume level of the input signal however, the first amplifier branch may enter a saturation range where the amplification becomes non-linear, which generates interferences. Therefore, the switch SEL is used to switch to the digital output signal of the second amplifier branch in this case.
There are coupling capacitors CG1i,CG1o,CG2i,CG2o between the single stages of the analog processing. Usually, the capacitors' values are subject to relatively large tolerances. Since these values have an influence on the phase of the signals, there are often differences in the phases of the signals of the two branches while switching, particularly at low frequencies. This leads to signal interference while switching, such as e.g. clicking noise.
This problem may be solved by DC coupling the amplifiers, as shown in
An object of the present invention is to solve this problem. It is noted that the above description is not an admission of prior art for the present invention, unless expressly stated.
The object is solved by an amplifier arrangement according to claim 1.
According to the invention, an amplifier arrangement comprises an input for connecting an analog input audio signal, which is then converted into an audio signal with a predetermined DC voltage component. The amplifier arrangement also comprises a first amplifier branch that has a first gain and provides a first output voltage, a second amplifier branch that has a second gain and provides a second output voltage, wherein the second gain is lower than the first gain, and a differentiating element or differentiator generating a voltage difference between the first and second output voltages. From this voltage difference, a control voltage for controlling the DC voltage component of the input signal is obtained in order to thereby minimize a DC component of the voltage difference between the output voltages. That is, the control voltage determines the predetermined DC voltage component of the input signal. At the same time, the DC components of the output voltages may be set to a desired value, as is required, for example, for certain ADCs. Both the first and second amplifier branches receive the same audio signal with the predetermined DC voltage component as input signal, and both are DC coupled.
Since, due to the regulation according to the invention, both output voltages always have the same DC voltage component, it is possible to switch without interference between the channels after the subsequent analog-to-digital conversion.
Further advantageous embodiments are disclosed in the dependent claims.
Further details and advantageous embodiments are depicted in the drawings, showing in
The invention can be used in a variety of different devices, e.g. wireless microphones, wireless headsets or pocket transmitters for headsets, so-called body packs.
VDC1o and VDC2o occur in the amplifier branch after the actual amplification, e.g. due to component tolerances or non-idealities, and they are also uncorrelated and specific for each of the branches. However, often they are only marginal. The DC component of the input voltage VDCC influences the bias voltage of both amplifier branches. It may already cause an amplifier that has a high gain to be clipping, since the actual audio signal is superimposed as an AC voltage to the DC voltage components considered here. However, such clipping needs to be avoided. A further problem is that the DC voltage components VOUT1,VOUT2 of the two amplifier branches may vary independently from each other, which is a problem for the subsequent analog-to-digital conversion.
Therefore, an amplifier arrangement according to the invention comprises a comparator or differentiating member that detects differences between the DC voltage components of the output signals, as well as a DC voltage feedback that regulates the DC voltage component of the input voltage by using the detected difference. This ensures that the output signals of both amplifier branches have the same DC component.
The DC voltage component of the voltage difference is now integrated over time by an integrating member INT and then fed back via a feedback resistor Rbias as a control voltage Vbias to the common input of the two amplifier branches. Correspondingly, there is now a DC voltage Vk at the common input that influences the operating points of both amplifier branches. This DC voltage Vk is the above-mentioned predetermined DC component, as it is determined by the control voltage Vbias. The feedback of the control voltage Vbias creates a closed control loop that can be considered as an active bias control. The integrating member INT may be designed e.g. with a gain factor a (or a/s in the frequency domain) such that the control loop is stable and has a cutoff frequency of 5 Hz or below, e.g. around 2 Hz.
An advantage of the invention is that it may be used even if at least one of the two characteristic curves is non-linear. For this, the position of the two curves relative to each other may be modified by a control voltage that is added to the input signal of one of the two amplifier branches, such as the reference voltage VREF shown in
The above-described switching technology offers several advantages, e.g. as compared to “automatic gain control” (AGC): first, the usable dynamic range is increased, while an ADC with low bit depth may be used however, and wherein no control of the gain factor of the analog circuit is required that might lead to problems in transient states. E.g. an energy-saving and inexpensive 16-bit ADC can be used, which is particularly advantageous for usage in battery operated devices. Compared with a simple amplifier, the effective dynamic range is increased by the ratio G1/G2 of the two gain factors. Gain factors that differ significantly are particularly advantageous, for example 0 decibels (dB) for G2 and 20 dB to 30 dB for G1. The gain factors G1,G2 should differ at least by 10 dB (corresponding to a factor of ten). Second, both channels are sampled simultaneously and the switching is done in the digital domain. This means that most of the component-related deviations can be corrected or compensated for, so that most of the artifacts that would otherwise be hearable when switching between the channels are eliminated. Correspondingly, it is also possible to use inexpensive hardware components although they have high tolerances; these are compensated by the regulation.
In this example, the first amplifier branch with the larger gain factor G1 may be implemented by a particularly low-noise transistor as amplifying element, while for the second amplifier branch with the lower gain factor G2 a lower SNR is admissible, so that it may be implemented with a simple operational amplifier OPA2. Here, the control voltage or reference voltage Vref2 respectively may be fed directly to the second input of the operational amplifier. In principle, however, each of the two branches may be implemented by one or more discrete transistors and/or operational amplifiers. Usually it is better though to achieve a certain amplification factor with a certain high SNR by discrete transistors than by operational amplifiers. Due to the active regulation, not only JFET transistors may be used, but also bipolar transistors, whose operating point can be adjusted more easily and more stable. This holds also for particularly low-noise amplifiers that require a very high input impedance. Further, each of the two branches may in principle also be controlled by adjusting the reference voltage.
After the audio signals have been digitized, they are digitally normalized. For this, they are filtered with digital high-pass filters HP1,HP2 in order to remove possible DC components, and the gain of the first amplifier branch is digitally compensated by multiplication with G2/G1. One of the digital signals is selected by means of a switch SEL, depending on the power of the input signal. Since both have the same DC component now, it is assured that the values of the two audio signals arriving at the switch SEL do not differ so much that the switching would lead to interfering noise. The digital high-pass filtering HP1,HP2, the multiplication by G2/G1 and the switching SEL may be implemented by one or more microprocessors DSP, possibly with suitable configuration software.
An advantage of the invention is that the dynamic range of ADCs can be utilized better, even when using relatively simple and inexpensive ADCs. An improved resolution of their digital output values is achieved, particularly for input values with low power. In principle, the quantization steps of the analog-to-digital conversion are reduced for input values with low power. Compared with an ADC having a higher resolution, this has the advantage that a simple stereo-ADC may be used, which may have a relatively low resolution but requires significantly less power. At the same time, a particularly low-noise pre-amplification as described above can be used. The invention ensures optimal interaction between the analog amplifier and the digital processing.
Combinations of the above-described embodiments as well as numerous further modifications of the invention are possible. For example, an additional AC coupled buffer may be used that is before the input capacitor Cin and thus outside the regulation loop, in order to make the regulation completely independent from the impedance RS of the audio source. Due to the high SNR, the invention is particularly advantageous for usage in wireless microphones, pocket transmitters for wireless headsets and other battery or accumulator powered mobile audio devices with a microphone or microphone input.