Patent Description:
For most systems, a single amplification stage does not provide sufficient gain or bandwidth, or it will not have the correct input or output impedance matching. The solution is to combine multiple amplification stages.

A multi-stage amplifier may be used to achieve the desired performance by cascading several amplification stages. Using a three-stage amplifier as an example, the output of the first amplification stage is used as the input of the second amplification stage, and the output of the second amplification stage is used as the input of the third amplification stage. The first amplification stage is the input stage, and the third amplification stage is the output stage. To achieve the desired SNR (signal-to-noise ratio), the input stage may be large in size. However, the larger the circuit size, the more power is consumed. The stability of the circuit may be also affected.

A high-SNR amplifier with limited power consumption, acceptable circuit size, and high stability is called for. <CIT> relates to an amplifier for photodiodes and includes (referring to <FIG> of <CIT>) a preamplifier <NUM>, an AC amplifier <NUM>, a signal delay circuit <NUM>, and semiconductor switches <NUM>, <NUM>.

An amplifier with multiple output modes is introduced.

An amplifier includes a plurality of amplification stages and a controller. The amplification stages are cascaded between an input terminal and an output terminal of the amplifier. An output stage among the plurality of amplification stages is switched between a normal mode and an attenuation mode. The controller switches the output stage from the normal mode to the attenuation mode in response to the amplifier input being lower than the threshold. In the attenuation mode, the output stage provides an attenuation resistor coupled in series with the load resistor of the amplifier. Noise is successfully attenuated by the attenuation-mode output stage.

The controller switches the output stage from the normal mode to the attenuation mode at zero-crossing points of the output signal of the amplifier.

The amplifier input is a digital signal from a digital analog front-end (AFE) circuit and is converted to an analog form to be amplified by the amplifier.

The controller controls the digital analog front-end circuit to digitally compensate for signal attenuation due to the attenuation resistor. The digital analog front-end circuit may digitally compensate for signal attenuation due to the attenuation resistor by a digital gain. When operating the output stage in the attenuation mode, the controller may increase the resistance of the attenuation resistor step-by-step and increase the digital gain accordingly.

The amplifier has a feedback circuit coupling the feedback signal of the output signal of the amplifier to an input stage among the plurality of amplification stages. The controller controls the feedback circuit to compensate for signal attenuation due to the attenuation resistor. The controller may control resistance of the feedback circuit to compensate for signal attenuation due to the attenuation resistor by an analog gain. When operating the output stage in the attenuation mode, the controller may increase the resistance of the attenuation resistor step-by-step and increase the resistance of the feedback circuit accordingly.

The controller switches the output stage from the attenuation mode to the normal mode in response to the amplifier input reaching the threshold. The controller may switch the output stage between the normal mode and the attenuation mode at zero-crossing points of the output signal of the amplifier.

The resistance of the attenuation resistor is greater than the resistance of the load resistor, and the resistance of the second feedback resistor is greater than the first feedback resistor.

The resistance of the attenuation resistor (RA) is k times the load resistor (RL); resistance of the second feedback resistor (RFB2) is k times the first feedback resistor (RFB1); and k is a number greater than <NUM>.

The following description shows exemplary embodiments carrying out the invention. The scope of the invention is determined by the appended claims.

<FIG> depicts an amplifier <NUM> in accordance with an exemplary embodiment of the present invention. The amplifier <NUM> includes cascaded amplification stages, Amp1, Amp2. and AmpN, a controller <NUM>, and a feedback circuit <NUM>.

The first amplification stage Amp1 is an input stage. The final amplification stage AmpN is an output stage. An amplifier input Sin converted to an analog form Ain to be amplified by the amplifier <NUM> may be provided by a digital analog front-end (digital AFE) circuit <NUM>. The amplifier <NUM> has an input terminal receiving the analog form Ain of the amplifier input Sin, and has an output terminal outputting an output signal VOL to drive a load resistor RL. The feedback circuit <NUM> couples the output stage AmpN to the input stage Amp1 (e.g., via an adder <NUM>). The output stage AmpN provides the feedback circuit <NUM> with a feedback signal of the output signal VOL. The multi-stage amplification (Amp1. AmpN) is based on the amplifier input Sin as well as the feedback signal of the output signal VOL. Specifically, the output stage AmpN is specially designed and an accompanying controller <NUM> is required.

The output stage AmpN is switched between a normal mode and an attenuation mode. The controller <NUM> is coupled to the digital AFE <NUM> to get information about the amplifier input Sin. In response to the amplifier input Sin lower than a threshold Sth (e.g., -40dBFS, where FS means the full-scale signal), the controller <NUM> switches the output stage AmpN from the normal mode to the attenuation mode. Noise is effectively suppressed. However, compensation for the signal attenuation due to the attenuation-mode output stage is required. The controller <NUM> may control the digital AFE <NUM> for digital compensation or control the feedback circuit <NUM> for analog compensation.

The amplifier <NUM> uses the multi-mode output stage AmpN to achieve a high signal-to-noise ratio (SNR). There is no need to excessively enlarge the size of the input stage Amp1. The power consumption of the amplifier <NUM> is reasonable. The amplifier <NUM> is a low-cost design with high stability.

The controller <NUM> may switch the output stage AmpN from the attenuation mode back to the normal mode in response to the amplifier input Sin reaching the threshold Sth (e.g., -40dBFS). For large-signal amplification, the performance is mainly related to THD (total harmonic distortion). The normal-mode output stage is preferred.

<FIG> illustrates the details of the amplifier <NUM> in accordance with an exemplary embodiment of the present invention. The controller <NUM> outputs a selection signal Sel (Sel) to switch the output stage AmpN between the normal mode and the attenuation mode. The controller <NUM> further outputs a control signal CS to gradually attenuate the noise and gradually compensate for the signal attenuation when operating the output stage AmpN in the attenuation mode.

The output stage AmpN includes operational amplifiers <NUM> and <NUM> and an attenuation resistor RA. When the output stage AmpN is in the normal mode, the operational amplifier <NUM> is enabled and the operational amplifier <NUM> is disabled. The second-to-last amplification stage Amp(N-<NUM>) is coupled to the output terminal (referring to VOL) of the amplifier through the operational amplifier <NUM>. When the output stage AmpN is in the attenuation mode, the operational amplifier <NUM> is disabled and the operational amplifier <NUM> is enabled. The second-to-last amplification stage Amp(N-<NUM>) is coupled to the output terminal (referring to VOL) of the amplifier through the operational amplifier <NUM> and the attenuation resistor RA. The output stage AmpN provides the attenuation resistor RA to be coupled in series with the load resistor RL. Noise is attenuated by the series circuit that includes the attenuation resistor RA and the load resistor RL.

The attenuation resistor RA may be larger than the load resistor RL. For example, the resistance of the attenuation resistor RA may be k times the resistance of the load resistor RL, and k is a number greater than <NUM>. In an exemplary embodiment, k is <NUM>, so that the attenuation introduced by the attenuation resistor RA is -12dB. In another example, k is <NUM>, so that the attenuation introduced by the attenuation resistor RA is -18dB. Noise is effectively attenuated. However, compensation for signal attenuation is required.

The signal attenuation due to the attenuation-mode output stage may be compensated for in the digital domain or the analog domain. The compensation may depend on the resistance ratio, k, between the attenuation resistor RA and the load resistor RL.

In an exemplary embodiment, the controller <NUM> may control the digital AFE <NUM> to compensate for signal attenuation due to the attenuation resistor RA by a digital gain.

The attenuation resistor RA may be a variable resistor. When operating the output stage AmpN in the attenuation mode, the controller <NUM> increases the resistance of the attenuation resistor RA gradually (e.g., step-by-step) and, accordingly, increases the digital gain step-by-step.

In another exemplary embodiment, the controller <NUM> performs analog compensation along the feedback path.

In <FIG>, there are two feedback resistors RFB1 and RFB2 and two switches SW1 and SW2, which form the feedback circuit <NUM> of <FIG>. The feedback resistor RFB1 and the switch SW1 are connected in series. The feedback resistor RFB2 and the switch SW2 are connected in series. When the output stage AmpN is in the normal mode, the switch SW1 is closed and the switch SW2 is open, and the output terminal (referring to VOL) of the amplifier is coupled to the input stage Amp1 via the feedback resistor RFB1. When the output stage AmpN is in the attenuation mode, the switch SW2 is closed and the switch SW1 is open, and an output terminal <NUM> of the operational amplifier <NUM> is coupled to the input stage Amp1 via the feedback resistor RFB2. The resistance of the feedback resistor RFB2 may be specially designed to compensate for the signal attenuation due to the attenuation resistor RA. In the example wherein the resistance of the attenuation resistor RA is k times the resistance of the load resistor (RA = k*RL), the resistance of the feedback resistor RFB2 is k times the resistance of the resistor RFB <NUM> (RFB2 = k*RFB1).

The signal attenuation due to the attenuation resistor RA is compensated for by an analog gain introduced by the feedback path.

The attenuation resistor RA and the feedback resistor RFB2 both can be variable resistors. When operating the output stage AmpN in the attenuation mode, the controller <NUM> increases the resistance of the attenuation resistor RA gradually (e.g., step-by-step) and, accordingly, increases the feedback resistor RFB2 step-by-step.

The amplifier shown in <FIG> is an I-V amplifier. However, the multi-mode output stage AmpN may be also applied on a V-V amplifier.

Specifically, the controller <NUM> may switch the output stage AmpN between the normal mode and the attenuation mode at zero-crossing points of the output signal VOL of the amplifier <NUM>. For example, when using the amplifier <NUM> as a headphone amplifier, switching the mode of the output stage AmpN at the zero-crossing points of the output signal Vo makes for a good user experience (i.e. there are no glitches).

<FIG> shows a waveform of the output signal VOL. The controller <NUM> may switch the output stage AmpN between the normal mode and the attenuation mode at zero-crossing points <NUM>, <NUM>, <NUM>, <NUM> and <NUM>.

The controller <NUM> may predict the zero-crossing points of the output signal VOL in accordance with the information, obtained from the digital AFE <NUM>, about the amplified signal Sin.

Claim 1:
An amplifier, comprising:
a plurality of amplification stages (Amp1, Amp2, ...AmpN), cascaded between an input terminal and an output terminal of the amplifier (<NUM>), wherein an output stage (AmpN) among the plurality of amplification stages is switched between a normal mode and an attenuation mode;
a controller (<NUM>), operable to switch the output stage from the normal mode to the attenuation mode in response to an amplifier input signal at the input terminal being lower than a threshold;
a feedback circuit (<NUM>) formed by:
a first feedback resistor (RFB1) and a first switch (SW1), connected in series; and
a second feedback resistor (RFB2) and a second switch (SW2), connected in series, wherein the amplifier is so configured that:
when the output stage (AmpN) is in the normal mode, the first switch (SW1) is closed and the second switch (SW2) is open, and the output terminal of the amplifier is coupled to an input stage (Amp1) among the plurality of amplification stages via the first feedback resistor (RFB1);
in the attenuation mode, the output stage (AmpN) provides an attenuation resistor (RA) coupled in series with a load resistor (RL) of the amplifier; and
when the output stage (AmpN) is in the attenuation mode, the second switch (SW2) is closed and the first switch (SW1) is open, and a first terminal of the attenuation resistor (RA) is coupled to the input stage (Amp1) via the second feedback resistor (RFB2) while a second terminal of the attenuation resistor (RA) is coupled to the load resistor (RL).