Patent Description:
A radiation detector may be applied as a component for detecting radiation (ionizing radiation or non-ionizing radiation), such as gamma rays, X-rays, ultraviolet (UV) radiation, visible radiation or charged particle radiation, e.g. in an analyzer device, in a spectrometer or in an electron microscope. A radiation detector typically serves to output an electrical output signal that is descriptive of the detected level of radiation. In the following, we refer to the electrical output signal from a radiation detector as a detector signal.

A non-limiting example of a radiation detector is a semiconductor drift detector (SDD), where a set of field electrodes is arranged to create a transversal electric field inside a block of semiconductor material, which electric field drives radiation-induced signal charges on one surface of the block of semiconductor material to a collection electrode that is typically located on the opposite surface of the block of semiconductor material. Consequently, the detector signal that is descriptive of the level of radiation detected by the SDD can be read out from the collection electrode.

Development of radiation detectors aims at achieving increased sensitivity, higher energy resolution, lower electronic noise and larger active detector area. While characteristics of a radiation detector as such play an important role in the resulting detection performance, a further crucial element in this regard is a preamplifier that is applied to amplify the detector signal before passing it for further processing by a signal processing system. In general, important characteristics of an applicable preamplifier include small physical size, low noise level, small rise and settling times and a linear response across a desired range of input signal levels.

Solid state charge sensitive amplifiers (CSA) have been widely used as preamplifiers for amplification of the electrical output signals from radiation detectors. A CSA enables amplification of an input current with a gain that is independent of the source capacitance. Therefore, CSAs are well suited to serve as preamplifiers in X-ray and particle detector applications for measuring charge pulses generated in the detector signals output from a radiation detector. A CSA may be provided, for example, by connecting an amplifier element in parallel with a feedback capacitor, where the amplifier element may be provided e.g. as a transimpedance amplifier.

<FIG> schematically illustrates an example of using a CSA as a component of a preamplifier circuit for amplification of a detector signal from a radiation detector. In particular, <FIG> depicts a detector assembly <NUM> that includes a radiation detector element <NUM> and a preamplifier circuit <NUM>, where a detector signal from the radiation detector element <NUM> is coupled to an input of the preamplifier circuit <NUM>, which generates an amplified detector signal at its output that may be coupled to a signal processing system that is applied for processing the amplified detector signal. The input of the preamplifier circuit <NUM> is coupled to an input of an amplifier <NUM>, whereas an output of the amplifier <NUM> is coupled to the output of the preamplifier circuit <NUM>. In the preamplifier circuit <NUM>, a feedback capacitor Cf and a reset switch S<NUM> are coupled in parallel with the amplifier <NUM>. In other words, both the feedback capacitor Cf and the reset switch S<NUM> are coupled between the input and output of the amplifier <NUM>. In the example of <FIG>, the amplifier <NUM> and the feedback capacitor Cf constitute the CSA. When the reset switch S<NUM> is open, the charge generated in the detector element <NUM> and provided to the input of the preamplifier circuit <NUM> is accumulated into the feedback capacitor Cf. Hence, the preamplifier circuit <NUM> basically operates as an integrator. For proper operation of the preamplifier circuit <NUM>, the charge accumulated into the feedback capacitor Cf needs to be periodically discharged to avoid saturation of the voltage at the output of the preamplifier circuit <NUM>. In the example of <FIG>, discharging of the feedback capacitor Cf may be carried out by periodically closing the reset switch S<NUM> for a relatively short period of time.

The output of the preamplifier circuit <NUM> may be, optionally, coupled to the signal processing system that is applied for processing the amplified detector signal via a pull-down circuit or a pull-up circuit (not shown in <FIG>). An advantage arising from usage of the pull-down/pull-up circuit is that it may contribute towards reducing electrical noise in the amplified detector signal and/or it may serve to provide electrostatic discharge (ESD) protection.

Still referring to the example of <FIG>, a time period during which the reset switch S<NUM> is closed (and hence the charge accumulated to the feedback capacitor Cf is discharging) may be referred to as a reset period (of the preamplifier circuit <NUM>). During a reset period, the output of the preamplifier circuit <NUM> may be undefined and it may not be applicable as the amplified detector signal. For this reason, the reset period constitutes a 'dead time' of the output of the preamplifier circuit <NUM>. Consequently, it is advantageous to keep the reset period as short as possible. Moreover, since a reset period provides an interruption to the amplification operation of the preamplifier circuit <NUM>, prompt resumption of the normal operation after a reset period is highly desirable in order to keep the disturbance to detection performance arising from the reset periods as small as possible.

In this regard, the approach according to the example of <FIG> provides a simple solution for implementing the reset periods, while on the other hand straightforward application of such an approach results in oscillations as a result of a high loop gain during the reset period and any attempt to reduce the oscillations by electronic filtering typically results in a relatively slow reset and/or a period of oscillating signal at the output of the preamplifier circuit <NUM>. As discussed above, both these aspects are disadvantageous and improved solutions for implementing and/or controlling the reset periods in preamplifier solutions like the one schematically described in <FIG> are highly desirable.

In related art,<NPL> discloses a <NUM> CMOS front-end readout circuit for CdZnTe particle detector with tens of pF capacitance. It uses switched-reset system and <NUM>th order complex pole semi-gaussian shaper. The input MOSFET is optimized with the consideration of the properties of deep submicron technologies and the limitation of power dissipation. And a swing reduction technique is used on the switch signal to improve parasitic effects. The post-layout simulation result shows that the readout ASIC has about 300e ENC at 20pF detector capacitance and 10mW power dissipation.

Further in related art, <CIT> discloses a charge amplifier for converting a charge signal from a piezoelectric sensor into a voltage signal includes: a series connection of a first analog switch and a second analog switch connected in parallel with an integrating capacitor connected to an output terminal of an operational amplifier and an operational amplifier input end; a third analog switch connected between the connection point of the first analog switch and the second analog switch and a ground point; and the reset circuit for discharging the charge signal stored in the integrating capacitor through the first and second switches.

Further in related art, <CIT> discloses a charge amplifier circuit with a control loop for resetting, which for minimizing the leakage currents that falsify the measuring results includes two diodes, connected antiparallel in the loop and constituting input protection diodes of an integrated circuit. In the operating (measuring) phase the diode resistance is extremely high and the voltage drop over the diode pair is small, so that no significant leakage current gets through onto the charge amplifier input. In the reset phase the diode resistance is practically nil.

It is therefore an object of the present invention to provide an improved reset arrangement for a preamplifier circuit that allows for short reset periods and prompt resumption of normal operation of the preamplifier circuit after a reset period.

The invention is defined by the appended independent claims and preferred embodiments are defined by the dependent claims.

In the following a simplified summary of some embodiments of the present invention is provided in order to facilitate a basic understanding of an improved CSA design.

In accordance with an example embodiment, a preamplifier circuit is provided, the preamplifier circuit comprising an amplifier arranged in a first current path between an input node and an output node of the preamplifier circuit; a feedback capacitor arranged in a second current path between said input node and said output node; a feedback circuit having an adjustable transfer function arranged in a third current path between said input node and said output node; a reset switch arranged in said third current path to enable selectively coupling the output of the feedback circuit to the input of the amplifier and decoupling the output of the feedback circuit from the input of the amplifier; and a loop controller arranged to one of open or close the reset switch to, respectively, set the preamplifier circuit in one of a normal operating mode or in a reset mode in dependence of an external reset signal, in accordance with a voltage at said output node or in accordance with a voltage at the output of the feedback circuit, wherein the loop controller is arranged to select a predefined transfer function for the feedback circuit in response to the preamplifier circuit operating in the normal operating mode and select a second predefined transfer function in response to the preamplifier circuit operating in the reset mode, wherein the first transfer function and the second transfer function differ from each other in amplitude response and/or in frequency response, and wherein the loop controller is arranged to set the gain of the amplifier during the reset mode to a value that is smaller than that applied in the normal operating mode of the preamplifier circuit.

In accordance with another example embodiment, a radiation detector assembly is provided, the radiation detector assembly comprising a preamplifier circuit according to the example embodiment described in the foregoing; and a radiation detector element having its output coupled to the input node of the preamplifier circuit.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

Along the lines described in the foregoing with references to <FIG>, the preamplifier circuit <NUM> relying on a CSA provides good and reliable amplification performance only when the voltage of the signal at the output of the preamplifier circuit <NUM> is kept within a predefined range that depends on characteristics of the preamplifier circuit <NUM>. When used as a preamplifier for amplifying the detector signal from the radiation detector element <NUM>, the output of the preamplifier circuit <NUM> is basically descriptive of a combined effect of the radiation-induced signal charge accumulated in the feedback capacitor Cf and the charge accumulated therein due to a leakage current in the radiation detector element <NUM> and, therefore, when the detector assembly <NUM> as applied to detect radiation in its environment, the preamplifier circuit <NUM> needs to be reset before the charge accumulated in the feedback capacitor Cf reaches a certain charge threshold (and hence before the amplified detector signal at the output of the preamplifier circuit <NUM> reaches a corresponding voltage threshold). Further along the lines described in the foregoing, straightforward approach of keeping the reset switch S<NUM> open for normal operation of the preamplifier circuit <NUM> and periodically keeping the reset switch S<NUM> closed for resetting the preamplifier circuit <NUM> for a short period of time leads either to slow reset of the preamplifier circuit <NUM> and/or to oscillations (also called ringing) in the output signal of the preamplifier circuit <NUM>, which both are detrimental for the overall performance of the preamplifier circuit <NUM>.

<FIG> schematically illustrates a detector assembly <NUM>, which includes the radiation detector element <NUM> having its output coupled to an input node of a preamplifier circuit <NUM>. Hence, the preamplifier circuit <NUM> is arranged to amplify the detector signal provided at its input node into an amplified detector signal for provision via its output node, which may be coupled to a signal processing system that is applied for processing the amplified detector signal. As a non-limiting example, the radiation detector element <NUM> may comprise a semiconductor radiation detector such as an SDD. In the following, for brevity and clarity of description, the input node of the preamplifier circuit <NUM> is referred to simply as an input (of the preamplifier circuit <NUM>) and the output node of the preamplifier circuit <NUM> is referred to simply as an output (of the preamplifier circuit <NUM>). Although examples provided in this disclosure describe the usage of the preamplifier circuit <NUM> for amplification of detector signals from the radiation detector element <NUM>, this is a non-limiting example chosen for clarity and brevity of description, while the preamplifier circuit <NUM> is applicable for amplification of signals from sources different from the radiation detector element <NUM>. Moreover, although designated in the disclosed examples as a preamplifier, this is a non-limiting example and the preamplifier circuit <NUM> is applicable for use as an amplifier circuit that does not necessarily serve as a preamplifier.

In the example of <FIG>, the preamplifier circuit <NUM> comprises an amplifier <NUM> having its input coupled to the input of the preamplifier circuit <NUM>, whereas the output of the amplifier <NUM> is coupled to the output of the preamplifier circuit <NUM>. As in the preamplifier circuit <NUM>, also in the example of <FIG> the preamplifier circuit <NUM> the feedback capacitor Cf is coupled in parallel with the amplifier <NUM>, in other words between the input and output of the amplifier <NUM>, the amplifier <NUM> and the feedback capacitor Cf thereby constituting a CSA. The amplifier <NUM> may comprise, for example, a folded cascode amplifier. The input stage of the amplifier <NUM> may comprise, for example, a field-effect transistor (FET), which may have a linear, round or convex polygon (e.g. octagon) form. The output of the amplifier <NUM> is further coupled to an input of a feedback circuit <NUM>, and a (signal) output of the feedback circuit <NUM> is coupled to the input of the amplifier <NUM> via a reset switch S<NUM>. The amplifier <NUM> comprises an amplifier circuit that has an adjustable gain. The feedback circuit <NUM> comprises a circuit arrangement that has an adjustable transfer function. In some examples, the feedback control circuit <NUM> may be arranged to adjust or modify bias of the reset switch S<NUM> in accordance with one or more control signals received from a loop controller <NUM> (which will be described in more detail via examples provided in the following). In this regard, the feedback circuit <NUM> may be arranged to control the bias of the reset switch S<NUM> via a control output of the feedback circuit <NUM>.

The reset switch S<NUM> may be applied to set the preamplifier circuit <NUM> to operate in a normal operating mode or in a reset mode. The reset switch S<NUM> may be implemented by using a suitable transistor arrangement, which in some examples may have an adjustable bias. Non-limiting examples of implementing the reset switch S<NUM> are described in the following. The feedback circuit <NUM> may adjust the bias of the reset switch S<NUM> under control of the loop controller <NUM> and the bias adjustment may be carried out at least partially in dependence of characteristics (e.g. voltage and/or current) of the signal received at the input of the feedback circuit <NUM>. As a particular example, the feedback circuit <NUM> may set or adjust the bias of the reset switch S<NUM> differently in dependence of the current operating mode of the preamplifier circuit <NUM> and/or in dependence of the current transfer function of the feedback circuit <NUM>.

The output signal from the amplifier <NUM> is processed through the feedback circuit <NUM>. As described in the foregoing, the feedback circuit <NUM> has a selectable transfer function and the (signal) output of the feedback circuit <NUM> may be derived on basis of the output signal from the amplifier <NUM> in accordance with the currently applied transfer function. These and other aspects pertaining to an internal structure and/or operation of the feedback circuit <NUM> are described via examples in the following.

In the example of <FIG>, the preamplifier circuit <NUM> further includes a loop controller <NUM> having the output of the feedback circuit <NUM> and the output of the preamplifier circuit <NUM> (and hence the output of the amplifier <NUM>) coupled thereto. The loop controller <NUM> is arranged to control the reset switch S<NUM> based at least in part on voltage at the output of the feedback circuit <NUM> and/or the voltage at the output of the amplifier <NUM>, and the loop controller <NUM> is further arranged to adjust operation of the amplifier <NUM> and/or operation of the feedback circuit <NUM> at least in part in dependence of the current operating mode of the preamplifier circuit <NUM>. The adjustment in this regard may comprise, for example, one or more of the following:.

In other words, each of the reset switch S<NUM>, the amplifier <NUM> and the feedback circuit <NUM> may operate at least partially under control of the loop controller <NUM>. The loop controller <NUM> may provide control of the reset switch S<NUM> via one or more control signals <NUM> (shown as a dashed line in <FIG>). Along similar lines, the loop controller may provide adjustment of the amplifier <NUM> and/or the feedback circuit <NUM> via one or more respective control signals <NUM>, <NUM> (likewise shown as respective dashed line in <FIG>). The control signal(s) <NUM> issued to the reset switch S<NUM> may comprise one or more control signals that cause setting the reset switch S<NUM> into the open state or setting the reset switch S<NUM> to the closed state. The control signal(s) <NUM> issued to the amplifier <NUM> may comprise one or more control signals that cause setting or adjusting the gain of the amplifier <NUM> accordingly. Along similar lines, the control signal(s) <NUM> issued to the feedback circuit <NUM> may comprise one or more control signals that cause adjusting or selecting the transfer function of the feedback circuit <NUM> accordingly. The control signal(s) <NUM> may comprise, for example, an indication of the current operating mode of the preamplifier circuit <NUM>.

As illustrated in <FIG>, the loop controller <NUM> may comprise a voltage tracker <NUM> and a reset controller <NUM>. The operation provided by these two elements of the loop controller <NUM> is described in the following via non-limiting examples. With respect to control of the reset switch S<NUM>, the loop controller <NUM> is arranged to control setting the preamplifier circuit <NUM> in one of the normal operating mode and the reset mode via setting the reset switch S<NUM> into one of an open state and a closed state: setting the reset switch S<NUM> in the open state results in operating the preamplifier circuit <NUM> in the normal operating mode, during which the amplifier <NUM> of the preamplifier circuit <NUM> serves to amplify the detector signal into the amplified detector signal, whereas setting the reset switch S<NUM> in the closed state results in operating the preamplifier circuit <NUM> in the reset mode, during which the output of the amplifier <NUM> may be undefined and hence may not be applicable as the amplified detector signal. A period of operating the preamplifier circuit <NUM> in the reset mode may be referred to, as described in the foregoing, as a reset period and it may be considered as 'dead time' of the output of the preamplifier circuit <NUM>.

<FIG> schematically illustrates a variation of the detector assembly <NUM> depicted in <FIG>, where only part of the loop controller <NUM> is provided as part of the preamplifier circuit <NUM>. In particular, in the example of <FIG> the reset controller <NUM> is provided as part of the preamplifier circuit <NUM> while the voltage tracker <NUM> is provided outside the preamplifier circuit <NUM>. Consequently, in the example of <FIG> the reset controller <NUM> provided within the preamplifier circuit <NUM> receives control signals from the voltage tracker <NUM> that is outside the preamplifier circuit <NUM> and derives respective control signals <NUM>, <NUM>, <NUM> for adjusting the operation of the amplifier <NUM> and/or the operation of the feedback circuit <NUM> as well as a control signal for setting the reset switch S<NUM> into one of the open state and closed state.

The preamplifier circuit <NUM> may comprise further elements in addition to those depicted in the illustrations of <FIG> and <FIG>. Hence, in general terms the preamplifier circuit <NUM> comprises the amplifier <NUM> arranged in a first current path between the input and output nodes of the preamplifier circuit <NUM> and the feedback capacitor Cf is arranged in a second current path between the input and output nodes of the preamplifier circuit <NUM>. Moreover, the preamplifier circuit comprises the feedback circuit <NUM> and the reset switch S<NUM> arranged between a third current path between the input and output nodes of the preamplifier circuit <NUM>, the reset switch S<NUM> thereby enabling selectively coupling the output of the feedback circuit <NUM> to the input of the amplifier <NUM> (to set the preamplifier circuit <NUM> to operate in the reset mode) or decoupling the output of the feedback circuit <NUM> from the input of the amplifier <NUM> (to set the preamplifier circuit <NUM> to operate in the normal operating mode).

In the normal operating mode of the preamplifier circuit <NUM>, i.e. when the reset switch S<NUM> is open, the gain of the amplifier <NUM> may be controlled (e.g. in response to the control signal(s) <NUM> from the loop controller <NUM>) such that the amplifier applies a predefined static gain that serves to provide a desired amplification performance in view of the (expected) characteristics of the signal provided at the input of the preamplifier circuit <NUM> (e.g. the detector signal from the radiation detector element <NUM>) and/or in view of characteristics of a circuit or system coupled to the output of the preamplifier circuit <NUM> (e.g. the signal processing system intended for processing the amplified detector signal). As a non-limiting example, the gain of the amplifier <NUM> during the normal operating mode may be a suitable value that, for example, results in desired amplification in a range from a few decibels (dB) up to <NUM> dB.

In an example, in the normal operating mode of the preamplifier circuit <NUM>, the transfer function of feedback circuit <NUM> may be selected or adjusted in a predefined manner. The transfer function applied in the normal operating mode of the preamplifier circuit <NUM> may be arranged to reduce or minimise the leakage current through the reset switch S<NUM>, from a terminal (e.g. an 'input node' and/or an 'output node') of the reset switch S<NUM> to the (semiconductor) substrate and/or from a bulk node of the reset switch S<NUM> to the (semiconductor) substrate. As an example in this regard, the transfer function of the feedback circuit <NUM> during the normal operating mode may be a fixed predefined transfer function. In another example, the transfer function during the normal operating mode may be (further) selected or adjusted in dependence of the voltage at the output of the feedback circuit <NUM> and/or in dependence of the voltage at the output of the preamplifier circuit <NUM>.

Still referring to operation of the preamplifier circuit <NUM> in the normal operating mode, the feedback circuit <NUM> may be arranged to set or adjust the bias of the reset switch S<NUM> in order to reduce or minimize the leakage current through the reset switch S<NUM>, from a terminal (e.g. an 'input node' and/or an 'output node') of the reset switch S<NUM> to (semiconductor) substrate and/or from a bulk node of the reset switch S<NUM> to the (semiconductor) substrate. In this regard, the bias control may comprise, for example, the feedback circuit <NUM> issuing, via the control output of the feedback circuit <NUM>, one or more bias control signals <NUM> that cause setting or adjusting the bias voltage and/or bias current of the reset switch S<NUM> accordingly. As a non-limiting example in this regard, the bias voltage of the reset switch S<NUM> may be set to track the input voltage of amplifier <NUM> to minimize the voltage over the reset switch S<NUM>. Another non-limiting example comprises setting the bias voltage to a value causing a small leakage current through the reset switch S<NUM> in order to compensate voltage change at the input of the amplifier <NUM> due to any other current, for example one caused by the detector element <NUM> coupled to the input of the preamplifier circuit <NUM>.

<FIG> schematically illustrates another variation of the detector assembly <NUM> depicted in <FIG>, where the bias control of the reset switch S<NUM> via the bias control signal(s) <NUM> is provided by the loop controller <NUM> (e.g. by the reset controller <NUM>) instead of the feedback circuit <NUM>. Even though illustrated in <FIG> as a variation of the detector assembly <NUM> according to the example of <FIG>, a similar modification with respect to provision of the bias control of the reset switch S<NUM> may be applied in the detector assembly <NUM> according to the example of <FIG> as well.

Herein, the bias control signal(s) <NUM> are to be construed broadly, encompassing e.g. an actual bias voltage and/or bias current or respective control signal(s) that enable connecting the reset switch S<NUM> to a voltage source providing a bias voltage in accordance with the respective control signal(s) and/or providing a bias current in accordance with the respective control signal(s). Regardless of the manner of implementing the bias control signal(s) <NUM>, the voltage source for providing the bias voltage may be provided as part of the preamplifier circuit <NUM> or an external voltage source may be applied. Along similar lines, alternatively or additionally, regardless of the manner of implementing the bias control signal(s) <NUM>, the current source for providing the bias current may be provided as part of the preamplifier circuit <NUM> or an external current source may be applied.

<FIG> illustrates a block diagram of some logical components of the feedback circuit <NUM> according to a non-limiting example. In the schematic illustration of <FIG>, the output of the amplifier <NUM> is coupled to a first transfer function F<NUM>(s) and to a second transfer function F<NUM>(s), whereas one of the first and second transfer functions F<NUM>(s), F<NUM>(s) is selectively coupled to the output of the feedback circuit <NUM> by setting a switch <NUM> in accordance with the control signal(s) <NUM> received from the loop controller <NUM> (e.g. from the reset controller <NUM>). Therein, for example, the first transfer function F<NUM>(s) may be selected in response to the control signal(s) <NUM> indicating the normal operating mode of the preamplifier circuit <NUM> and the second transfer function F<NUM>(s) may be selected in response to the control signal(s) <NUM> indicating the reset mode of the preamplifier circuit <NUM>. The first and second transfer functions F<NUM>(s), F<NUM>(s) may differ from each other in amplitude and/or frequency response they serve to provide. In this regard, the amplitude resulting from the second transfer function F<NUM>(s) may be smaller than that resulting from the first transfer function F<NUM>(s) and/or the frequency response of the second transfer function F<NUM>(s) may extend over a wider range of frequencies than that resulting from the first transfer function F<NUM>(s).

<FIG> illustrates a block diagram of some logical components of the feedback circuit <NUM> according to another non-limiting example, where also the biasing of the reset switch S<NUM> is provided via the feedback circuit <NUM>. Therein, in addition to the operation described above with references to <FIG>, selection of the transfer function of the feedback circuit <NUM> further implies selection of bias control signal(s) <NUM> accordingly in accordance with the control signal(s) <NUM> received from the loop controller <NUM> (e.g. from the reset controller <NUM>), thereby adjusting the bias of the reset switch in <NUM> in dependence of the current operating mode of the preamplifier circuit <NUM>.

It should be noted, however, that the description of the first and second transfer functions F<NUM>(s), F<NUM>(s) with references to the examples of <FIG> and <FIG> is a conceptual one: even though shown in <FIG> and <FIG> as separate and independent logical blocks for graphical clarity of the illustration, the first and second transfer functions F<NUM>(s), F<NUM>(s) may be provided by respective circuitries that share one or more components and/or one or more of the (shared) components of the circuitry that serves to provide the first and second transfer functions F<NUM>(s), F<NUM>(s) is adjusted to operate differently in the normal operating mode and in the reset mode.

An advantage arising from controlling the bias of the reset switch S<NUM> and/or selecting or adjusting the transfer function of the feedback circuit <NUM> during the normal operating mode as described in the foregoing of the preamplifier circuit <NUM> is reduced noise level in the (amplified) signal at the output of the preamplifier circuit <NUM> due to reduced or even completely eliminated leakage current through the reset switch S<NUM> (e.g. through the transistor arrangement that may be applied to implement the reset switch S<NUM>).

As described in the foregoing, the loop controller <NUM> is coupled to the output of the feedback circuit <NUM> and to the output of the preamplifier circuit <NUM> (i.e. to the output of the amplifier <NUM>), and it hence receives respective output signals from the feedback circuit <NUM> and from the amplifier <NUM>. <FIG> illustrates a block diagram of some components of the loop controller <NUM> according to an example. As also illustrated in <FIG>, <FIG> and <FIG>, the loop controller <NUM> may comprise the voltage tracker <NUM> and the reset controller <NUM> and the loop controller <NUM> (e.g. the reset controller <NUM>) may issue the one or more control signals <NUM> for selectively opening or closing the reset switch S<NUM>, the one or more control signals <NUM> for setting or adjusting the gain of the amplifier <NUM> and the one or more control signals <NUM> for adjusting or selecting the transfer function of the feedback circuit <NUM>. Moreover, in examples where the bias control of the reset switch S<NUM> is provided by the loop controller <NUM>, the loop controller <NUM> (e.g. the reset controller <NUM>) may issue the one or more bias control signals <NUM>.

During the normal operating mode of the preamplifier circuit <NUM>, according to an example, the voltage tracker <NUM> compares the voltage at the output of the feedback circuit <NUM> to a predefined threshold voltage Vth,<NUM> and triggers a reset period in response to the voltage at the output of the feedback circuit <NUM> exceeding the voltage threshold Vth,<NUM>. Such triggering may involve the voltage tracker <NUM> issuing a trigger signal or trigger command to the reset controller <NUM>, which may apply the control signal(s) <NUM> to operate the reset switch S<NUM> accordingly. According to an example, the reset controller <NUM> closes the reset switch S<NUM> directly in response to triggering of the reset period, thereby initiating the reset period without an additional delay. According to another example, triggering of the reset period results in the reset controller <NUM> setting a timer to run for a first predefined time period and the reset controller <NUM> closing the reset switch S<NUM> in response to the timer elapsing, thereby initiating the reset period after the first predefined time period has elapsed.

In another example, the voltage tracker <NUM> may be arranged to monitor the voltage at the output of the amplifier <NUM> (i.e. at the output of the preamplifier circuit <NUM>) instead of the voltage at the output of the feedback circuit <NUM> and to trigger a reset period in response to the voltage at the output of the amplifier <NUM> exceeding a predefined voltage threshold Vth,<NUM>. In a further example, the voltage tracker may be arranged to monitor both the voltage at the output of the feedback circuit <NUM> and at the output of the amplifier <NUM> and to trigger a reset period in response to the voltage at the output of the feedback circuit <NUM> exceeding the voltage threshold Vth,<NUM> and/or the voltage at the output of the amplifier <NUM> exceeding the voltage threshold Vth,<NUM>.

In the above example of the voltage tracker <NUM> triggering a reset period in response to the voltage at the output of the feedback circuit <NUM> exceeding the voltage threshold Vth,<NUM> and/or the voltage at the output of the amplifier <NUM> exceeding the voltage threshold Vth,<NUM>, an underlying assumption that the respective output voltages saturate towards a certain respective maximum voltage (that depends on characteristics of the amplifier <NUM>). In an alternative example, the output voltages of the feedback circuit <NUM> and the amplifier <NUM> saturate towards a ground potential and in such an approach, during the normal operating mode of the preamplifier circuit <NUM>, the voltage tracker <NUM> compares the voltage at the output of the feedback circuit <NUM> to a predefined threshold voltage V'th,<NUM> and/or compares the voltage at the output of the amplifier <NUM> to a predefined threshold voltage V'th,<NUM> and triggers a reset period in response to the voltage at the output of the feedback circuit <NUM> failing to exceed the voltage threshold V'th,<NUM> and/or the voltage at the output of the amplifier <NUM> failing to exceed the voltage threshold V'th,<NUM>. Along the lines described above, triggering of the reset period may result in the reset controller <NUM> initiating the reset period without a delay or after a delay defined by the first time period via the control signal(s) <NUM>.

In a further example, the loop controller <NUM> may optionally further comprise an input for receiving an external reset signal (as illustrated in <FIG>, <FIG> and <FIG>). Reception of the external reset signal (e.g. a reset command or a reset pulse) via this input causes the loop controller <NUM> to trigger a reset period and, consequently, causes the reset controller <NUM> to initiate the reset period without a delay or after a delay defined by the first time period. In a yet further example, the loop controller <NUM> may omit (operation of) the voltage tracker <NUM> and trigger a reset period according to a predefined profile instead, e.g. at predefined time intervals. In this scenario, triggering of a reset period typically results in the reset controller <NUM> initiating the reset period without a delay.

In the reset mode, the loop controller <NUM> adjusts the gain of the amplifier <NUM> to a value that is smaller than that applied during the normal operating mode. As an example in this regard, the operation of the amplifier <NUM> may be adjusted by the reset controller <NUM>, via the one or more control signals <NUM>, such that the gain of the amplifier <NUM> is set or adjusted to a smaller value than the predefined static value applied during the normal operating mode of the preamplifier circuit <NUM>. The gain control may involve directly setting or adjusting the gain of the amplifier <NUM> or otherwise adjusting operation of the amplifier <NUM> such that its gain gets adjusted in a desired manner. In the reset mode, the gain of the amplifier <NUM> (e.g. an open loop gain) is brought down to a small value that is less than unity (e.g. less than <NUM> or less than <NUM> dB, depending on the exact manner of defining or expressing the gain of the amplifier <NUM>). Non-limiting examples in this regard include the reset controller <NUM> issuing one or more control signals that cause setting or adjusting the gain of the amplifier <NUM> into a second predefined static value that is smaller than the predefined static value applied during the normal operating mode of the preamplifier circuit <NUM>, the reset controller <NUM> issuing one or more control signals that result in setting or adjusting the gain of the amplifier in accordance with a predefined function of time, or the reset controller <NUM> issuing one or more control signals that result in adjusting the gain of the amplifier <NUM> during the reset mode in dependence of the voltage at the output of the amplifier <NUM> and/or in dependence of the voltage at the output of the feedback circuit <NUM>. Reducing the gain of the amplifier <NUM> during the reset mode contributes, for example, towards prompt completion of the reset period and/or towards reducing oscillations (so-called ringing effect) and/or other undesired non-linear effects in the output signal of the preamplifier circuit <NUM> during and immediately following the reset period.

Still referring to operation in the reset mode, the reset controller <NUM> operates to adjust operation of at least one aspect of the preamplifier circuit <NUM> to facilitate controlled discharging of the charge accumulated to the feedback capacitor Cf. In this regard, the reset controller <NUM> may be arranged to adjust operation of the feedback circuit <NUM> such that it sets or adjusts the bias of the reset switch S<NUM> such that it enables prompt discharging of the feedback capacitor Cf. In this regard, as described in the foregoing, the bias control may comprise, for example, the feedback circuit <NUM> issuing one or more bias control signals <NUM> that cause setting or adjusting the bias voltage and/or bias current of the reset switch S<NUM> accordingly. As a non-limiting example in this regard, the bias voltage of the reset switch S<NUM> may be set to the lowest potential available in the circuit to increase reset current and/or to reduce reset time. In another example, as also described in the foregoing, the bias control of the reset switch S<NUM> may be, alternatively, provided via the loop controller <NUM> (e.g. by the reset controller <NUM>).

As another example of the reset controller <NUM> adjusting operation of the preamplifier circuit <NUM> in the reset mode, the one or more control signals <NUM> from the reset controller <NUM> may result in setting or adjusting operation of the feedback circuit <NUM> during the reset period e.g. in one of the following ways:.

Hence, during the reset period, the transfer function of the feedback circuit <NUM> may vary over time. The changes in operation of the feedback circuit <NUM> during the reset period may result in changes in the overall transfer function of the feedback loop through the feedback circuit <NUM> and the amplifier <NUM> during the reset period. This, in turn, results in a reduction of the output voltage of the preamplifier circuit <NUM> and hence in the input voltage of the feedback circuit <NUM>, which may result in abnormal operation of the feedback circuit <NUM> due to its input voltage being outside its predefined input voltage range.

Like reduction of the gain of the amplifier <NUM> during the reset mode, also selection or adjustment of the transfer function of the feedback circuit <NUM> and/or bias control of reset switch S<NUM> during the reset mode of the preamplifier circuit <NUM> contribute, for example, towards prompt completion of the reset period and/or towards reducing oscillations (so-called ringing effect) and/or other undesired non-linear effects in the output signal of the preamplifier circuit <NUM> during and immediately following the reset period.

The reset period may have a fixed predefined duration, or the duration of the reset period may be variable and depend on a characteristic in a certain point of the preamplifier circuit <NUM>, e.g. on a voltage at the output of the amplifier <NUM> (i.e. the voltage at the output of the preamplifier circuit <NUM>). As an example of the former approach, the reset controller <NUM> may be arranged to set, upon initiation of the reset period, a timer to run for a second predefined time period and to open the feedback switch S<NUM> in response to the timer elapsing, thereby providing a reset period having the fixed predefined duration. As non-limiting examples of the latter approach, the reset controller <NUM> may be arranged to open the feedback switch S<NUM> in response to the voltage at the output of the amplifier <NUM> or in response to the voltage at some other predefined point of the feedback amplifier circuit <NUM> failing to exceed a predefined voltage threshold Vth,reset, thereby terminating the reset period after detecting (sufficient extent of) discharging of the feedback capacitor Cf having taken place.

As described in the foregoing, the reset switch S<NUM> may be provided using a suitable transistor arrangement. An advantageous design of the reset switch S<NUM> aims at minimizing the noise arising from a leakage current through the reset switch S<NUM> during operation in the normal mode, i.e. when the reset switch S<NUM> is in the open state. Typically, significant sources of noise in transistor arrangements relying on one or more FETs include resistive channel thermal noise and channel flicker (<NUM>/f) noise. Moreover, when targeting at very low noise solutions, further noise sources include thermal noise originating from gate and substrate resistances, leakage current induced shot noise from drain-substrate and source-substrate diodes. In this regard, usage of one or more isolated MOS devices (PMOS or NMOS) in implementing the reset switch S<NUM> is advantageous since they serve to reduce the effect of such noise sources.

Usage of the isolated NMOS device enables biasing of a bulk terminal separately from the substrate of the circuit. This makes it possible to select bulk and substrate bias voltages in a manner that results in minimising leakage currents of a switch constructed using an isolated MOS transistor. Also noise coupling from the substrate can be minimized by the aid of an isolated MOS transistor, because an additional reverse biased pn-junction is formed between transistor bulk and circuit substrate. Moreover, isolated MOS devices also provide protection against electrostatic discharge (ESD) due to the diode-stack structure applied therein also in such MOS devices of small size that are typically applied to enable reaching low noise level. Non-limiting examples of such designs for the reset switch S<NUM> are described in the following:.

As schematically illustrated in <FIG>, the output of the preamplifier circuit <NUM> may be coupled to a circuit or signal processing system that is applied for processing the amplified detector signal including an output stage <NUM> that is provided for stabilizing the preamplifier circuit <NUM> and for reducing the time it takes to discharge the feedback capacitor Cf during a reset period. The output stage <NUM> basically serves as a circuit having a varying impedance coupled in parallel with the preamplifier circuit <NUM>, and advantageous effects arising from usage of the output stage <NUM> may (further) include reduced electrical noise in the amplified detector signal and/or (improved) ESD protection.

The output stage <NUM> may comprise a pull-down circuit or a pull-up circuit. Although illustrated in the example of <FIG> as an entity that is separate from the preamplifier circuit <NUM>, the output stage <NUM> may be provided as an element that is included in the preamplifier circuit <NUM>. In such an example, the output of the output stage <NUM> constitutes the output of the preamplifier circuit <NUM>. Non-limiting examples of pull-down and pull-up circuits that are applicable for use as the output stage <NUM> are schematically illustrated in <FIG> and briefly described in the following:.

Hence, the output stage <NUM> may comprise a circuitry that involves a(n isolated) NMOS device and a resistor. Such an output stage <NUM> may serve two purposes, i.e. a pull-down (or pull-up) circuit and an ESD protection circuit. As an example in this regard, an amplifier within the output stage <NUM> comprising e.g. a biased MOSFET together with a source degenerated resistor may serve a pull-down circuit, whereas parasitic diode(s) of the MOSFET may serve an ESD protection structure. Usage of an isolated MOSFET provides an additional advantage of biasing or connecting the parasitic diode(s) in an optimized manner, e.g. not only towards the (semiconductor) substrate. Herein, the amplifier within the output stage <NUM> provides a higher output impedance in comparison to a case where only a resistor is applied. In this regard, a high value resistor arranged in the (semiconductor) substrate would require a larger area that typically incurs an increased cost and extra parasitic capacitance to the output. The amplifier within the output stage <NUM> further enables optimizing (e.g. minimizing) (thermal) noise in the output in comparison to a case where a separate ESD device, such as a diode, is applied.

In the foregoing, references are made to selectively operating the preamplifier circuit <NUM> in the normal operating mode or in the reset mode. While the exact relationship between time periods spent in each of these two modes depends e.g. on the characteristics of the signal supplied in the input of the preamplifier circuit <NUM> (e.g. with respect to its current, voltage and/or variations thereof over time), on the characteristics of the components applied to implement the preamplifier circuit <NUM> and on desired performance of the preamplifier circuit <NUM>, in general a single period in the normal operating mode between two reset periods has typically a duration in the order of milliseconds, whereas a single period in the reset mode between two periods in the normal operating mode typically has a duration in the order of nanoseconds.

Claim 1:
A preamplifier circuit (<NUM>) for amplification of a signal, the preamplifier circuit (<NUM>) comprising:
an amplifier (<NUM>) arranged in a first current path between an input node (In) and an output node (Out) of the preamplifier circuit (<NUM>);
a feedback capacitor (Cf) arranged in a second current path between said input node (In) and said output node (Out);
a feedback circuit (<NUM>) having an adjustable transfer function arranged in a third current path between said input node (In) and said output node (Out);
a reset switch (S<NUM>) arranged in said third current path to enable selectively coupling the output of the feedback circuit (<NUM>) to the input of the amplifier (<NUM>) and decoupling the output of the feedback circuit (<NUM>) from the input of the amplifier (<NUM>); and
a loop controller (<NUM>) arranged to selectively one of open or close the reset switch (S<NUM>) to, respectively, set the preamplifier circuit (<NUM>) in one of a normal operating mode or in a reset mode in dependence of an external reset signal, in accordance with a voltage at said output node (Out) or in accordance with a voltage at the output of the feedback circuit (<NUM>),
wherein the loop controller (<NUM>) is arranged to select a first predefined transfer function (F<NUM>) for the feedback circuit (<NUM>) in response to the preamplifier circuit (<NUM>) operating in the normal operating mode and select a second predefined transfer function (F<NUM>) in response to the preamplifier circuit (<NUM>) operating in the reset mode, wherein the first transfer function (F<NUM>) and the second transfer function (F<NUM>) differ from each other in amplitude response and/or in frequency response, and
wherein the loop controller (<NUM>) is arranged to set the gain of the amplifier (<NUM>) during the reset mode to a value that is smaller than that applied in the normal operating mode of the preamplifier circuit (<NUM>).