Amplifier device

An electronic circuit arrangement is provided which comprises an input terminal configured to input an input signal to be amplified and an output terminal configured to output the amplified input signal as an output signal. A signal path is defined between the input terminal and the output terminal. An amplifier unit having an amplifier gain is provided and being configured to amplify the input signal and for generating the output signal. A variation of an operational current of the amplifier unit is configured to provide a variation of the amplifier gain. The amplifier unit is arranged within the signal path. Furthermore a gain control unit is configured to control the gain of the amplifier unit by adjusting the operational current of the amplifier unit. The gain control unit is arranged outside the signal path.

BACKGROUND

The present description generally relates to amplifier devices, and in particular relates to a transimpedance amplifier device.

In some applications amplifier units are used for amplifying a current, originating e.g. from a photodiode, in order to provide an output voltage. Such kind of amplifier units are called transimpedance amplifiers because the relation between the output signal (voltage) and the input signal (current) is defined in units of an impedance. The larger the impedance, the larger in principle is the gain of the transimpedance amplifier.

A major disadvantage of such an amplifier configuration is that parasitic capacitances are present due to a high value of the transimpedance such that a fast control loop cannot be provided.

In conventional transimpedance amplifiers the transimpedance element (e.g. an Ohmic resistor) is located in a signal path of the amplifier chain. Thus, it is not possible to realize high operational frequencies of the entire amplifier unit. High operational frequencies, however, are required in many applications where currents have to be transformed into output voltages, e.g. in the detection of light variations.

SUMMARY

According to a first aspect of the present description an electronic circuit arrangement is provided which includes an input terminal configured to input an input signal to be amplified, an output terminal configured to output the amplified input signal as an output signal, wherein a signal path is defined between the input terminal and the output terminal, an amplifier unit having an amplifier gain and being configured to amplify the input signal and to provide the output signal, wherein the amplifier unit is arranged within the signal path, and a gain control unit configured to control the gain of the amplifier unit, wherein the gain control unit is arranged outside the signal path.

According to a further aspect of the present invention an amplifier array is provided which includes at least two amplifier units each having an input terminal configured to input an input signal to be amplified, an output terminal configured to output the amplified input signal as an output signal, wherein a signal path is defined between the input terminal and the output terminal, and an amplifier gain configured to amplify the input signal and to provide the output signal, wherein the amplifier unit is arranged within the signal path, and a gain control unit configured to control the amplifier gain of the at least two amplifier units, wherein the gain control unit is arranged outside all signal paths.

According to a yet a further aspect of the present invention a method for controlling an amplifier gain of at least one amplifier unit is provided, including the steps of inputting an input signal to be amplified via an input terminal of the amplifier unit, amplifying the input signal and providing an output signal by the at least one amplifier unit having an amplifier gain, outputting the amplified input signal via an output terminal, wherein the at least one amplifier unit is arranged within a signal path defined between the input terminal and the output terminal, and controlling the gain of the at least one amplifier unit using a gain control unit which is arranged outside the signal path.

In the Figures, same reference numerals denote the same or similar parts or steps.

DETAILED DESCRIPTION

Reference will now be made in detail to the various exemplary embodiments, one or more examples of which are illustrated in the Figures. Thereby, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the present description includes such modifications and variations.

A number of embodiments will be explained below. In this case, identical structural features are identified by identical reference symbols in the Figures. The structures shown in the Figures are not depicted true to scale but rather serve only for the better understanding of the embodiments.

FIG. 1is a block diagram showing the interaction of an amplifier unit100and a gain control unit200. The amplifier unit100includes an amplifier core108, an input terminal I and an output terminal O. An input signal103is fed to the amplifier core108via the input terminal I, wherein the amplified input signal103is provided as an output signal104at the output terminal O.

In case the input signal103is an input current, i.e. from a photodiode, and the output signal104is an output voltage, the amplifier unit100performs a transimpedance transfer which is dependent on a transimpedance of the amplifier unit. The gain control unit200, which includes a gain control element201, provides a gain control signal208which is set to a gain control input G of the amplifier unit100.

It is noted that the gain control element201determines the transimpedance of the amplifier unit such that the gain control element201carries out the function of a transimpedance element. The transfer function of the amplifier unit100and the amplifier core108, respectively, is determined by the gain control unit200. The transfer function can be expressed as:
k=Uout/Iin
wherein Uout is the output voltage and Iin is the input current. As this transfer function is the ratio of a voltage to a current, the unit of the transfer function is in Ohm.

FIG. 2further elucidates the operation of an amplifier unit which is based on the transimpedance effect. The principle arrangement of a transimpedance amplifier TA includes a transimpedance T provided as a resistor R which is variable. The output voltage Vout depends on the input current Idiode according to the following formula:
Vout=(Idiode·R)+Vref
wherein Vref is a reference voltage applied to a non-inverting input terminal of the transimpedance amplifier TA.

Using the configuration shown inFIG. 2, incident light L is transformed into a diode current Idiode by means of a photodiode element PD. Then, the photodiode current Idiode is transformed into the output voltage Vout as a function of the transimpedance element T having a preset value of R.

According to a typical embodiment, the transimpedance is provided by the gain control unit200such that the transimpedance itself is not contained within a signal path from the input terminal I to the output terminal O of the amplifier unit100.

FIG. 3is a detailed circuit diagram of a typical embodiment of the transimpedance amplifier. As shown inFIG. 3, the circuit arrangement essentially consists of three circuit blocks, i.e. the amplifier unit100, the gain control unit200and a sensor device300. In the typical embodiment shown inFIG. 3, the sensor device300consists of a photodiode301, which provides an input signal103in dependence of an incident light beam IL.

A supply voltage105(VDD) is provided with respect to ground potential106(VSS). Four transistors TP1, TP2, TP3and TP4control respective currents from the supply voltage terminal105to the respective circuit parts. The source (emitter) of each of the control transistors TP1-TP4is connected to the supply voltage105, wherein the drain (collector) of each of these transistors is connected to the respective circuit.

The gates (bases) of the transistors TP1-TP4are connected to each other and are connected to an output terminal OA of an operational amplifier203included in the gain control unit200.

In order to derive a transfer function (transimpedance) of the circuit arrangement shown inFIG. 3, at first the function of the bandgap reference contained in the gain control unit200will be described. Two transistors TBG1and TBG2control input currents into the operational amplifier203. An emitter area of the transistor TBG2to an emitter area of the transistor TBG1corresponds to a ratio of n. The emitter of the transistor TBG1is connected to the transistor TP3and an inverting input (−) of the operational amplifier203, wherein the emitter of the transistor TBG2is connected to a gain control element201(an Ohmic resistor in this case, RBG), the other end of which is connected to the transistor TP4and the non-inverting input (+) of the operational amplifier203.

A reference element202, which in the present embodiment corresponds to the gain control element201, is connected between the base of the transistor TBG1and ground potential106. The Ohmic resistance value of the reference element202(RBG) corresponds to the Ohmic resistance value of the gain control element201(RBG). Both transistors TP3and TP4of the gain control unit200are loaded by identical currents, i.e. by the currents IC+IB. A potential difference between the base-emitter voltages of the transistors TBG1and TBG2thus have to be equalized by a potential difference which is provided at the connection terminals of the gain control element201(RBG).

The input signal103which is provided as an input current Iinis dependent on the incident light intensity IL incident onto the photodiode301(PD) and flows as a collector current of transistors TV2and TV4provided in the amplifier core108of the amplifier unit100. It is advantageous if the collector currents of the transistors TV2and TV4are transferred into an appropriate output signal104, i.e. into an output voltage Ua. As shown inFIG. 3(right side), a current which is the sum of the collector current ICof the transistor TBG2(second control transistor205) and the base current IBof the transistor TBG2, flows through the gain control element201(RBG).

A potential difference between the non-inverting input (+) of the operational amplifier203(Amp1) and the ground potential106is represented by the sum of two partial voltages, i.e. a base-emitter voltage UBE2of the second control transistor205(TBG2) and the product of the sum of the base current IBand the collector current ICof the second control transistor205and the resistance value of the gain control element201(RBG).

The potential difference between the inverting input (−) of the operational amplifier203is the sum of the base-emitter voltage UBE1of the transistor TBG1(first control transistor204) and a potential drop occurring at the reference element202. The base current IBof the first control transistor204flows through the reference element202causing a respective voltage drop of IB·RBG. The operational amplifier203(Amp1) adjusts the difference between its inverting input (−) and its non-inverting input (+) to be zero, such that the following equation holds:
UBE1+IB·RBG=UBE2+(IB+IC)·RBG(1)

As can be seen from equation (1), the base current IBof the transistors TBG1and TBG2is eliminated. The voltage difference between the base-emitter voltages of the transistors TBG1and TBG2is thus:
UBE1−UBE2=IC·RBG(2)

A collector current of any transistor may be evaluated as a function of the saturation current ISand the exponential function of the ratio of the respective base-emitter voltage and the thermal voltage UT. The thermal voltage UTis given by UT=kT/e, wherein k corresponds to the Boltzmann constant, T corresponds to the absolute temperature and e corresponds to the elementary charge.

The emitter current of the transistor TBG1thus is given by the following equation:
IC1=ISexp(UBE1/UT)  (3a)
wherein the collector current of the transistor TBG2is given by the following equation:
IC2=n·ISexp(UBE2/UT)  (3b)

The factor n in equation (3b) results from the different emitter areas of the transistors TBG2and TBG1. As mentioned above the emitter area of TBG2is n times as large as the emitter area of TBG1. Due to the control method using the transistors TP3and TP4, the collector currents IC1according to equation (3a) above and IC2according to equation (3b) above have to be identical such that the following relations hold:
IC1=IC2=ICandUT=kT/e(4)

By combining equations (3a), (3b), (4) and (2) and by applying a logarithmic calculus, the following equation (5) is obtained:

Thus, the collector current ICof the transistor TBG1and the transistor TBG2, respectively, is given by
IC=kTIn(n)/(eRBG)  (6)

Currents which are generated by means of the transistors TP3and TP4are mirrored into the amplifier unit100by means of the transistors TP1and TP2. An emitter area of the transistor TP1is designed to be m times the emitter area of the transistors TP3, TP4, wherein an emitter area of the transistor TP2is designed to be 6j times the emitter area of the transistors TP3and TP4, respectively. It is noted that j is given as a constant factor which may be adjusted by providing different emitter areas of the respective transistors. Thus, the transistor TP1mirrors a current of m(IC+IB) wherein the transistor TP2mirrors a current of 6j(IC+IB).

In the following, it is assumed that the output voltage Ua(output signal104) which is provided at an output terminal102of the amplifier unit100with respect to the ground potential106, is identical to a reference voltage Urefwhich is provided by a reference voltage source107. In this case (Ua=Uref), currents of 3j×ICflow via diodes TN2and TN3, respectively. The diodes TN2and TN3both have a area (W/L) of 3j with respect to the area of MOS diodes TN4and TN5(first and second MOS-diodes206and207, respectively) provided in the gain control unit200.

It is noted here that the reference voltage Urefprovided by the reference voltage source107may be adjusted in a large range. The reference voltage Urefmay be zero. A transistor TN1which is connected between the output terminal102and the ground potential106is scaled with respect to the diode TN4by a factor of m, such that a current of m (IC+IB) may flow through the transistor TN1.

A gate-source voltage of TN1corresponds to that of the MOS diode TN4such that, in case of Ua=Uref, the drain-source voltage of TN3exactly corresponds to that of the MOS-diode TN4such that the Early-effect is completely eliminated.

For reasons of symmetry, a MOS-diode TN2is provided corresponding to a transistor TN3which is connected between the collector of the core transistors TV2, TV4and ground potential106(VSS). The configuration of the amplifier core108shown in the amplifier unit100ofFIG. 3together with both resistors R results in an input current Iinfor an emitter area ratio of TV3to TV1and TV4to TV2, respectively of a factor of 1 in the following:

Iin=3/(2⁢(1+1))·akT·IC·j·(Ua-Uref)(7)
By combining equations (6) and (7) above, the voltage difference between the output voltage Ua(output signal104) and the reference voltage Urefwhich is provided by the reference voltage source107may be expressed as:
Ua−Uref=(2(1+1))/3jRBG·Iin/ln(n)≠f(T)  (8)

From equation (8) it can be seen that the output voltage and the voltage difference between the output voltage and the reference voltage, Uaand Urefrespectively, does not depend on the temperature. Thus, the voltage difference Ua−Urefis represented as a constant factor TI (transimpedance) multiplied by the input signal (input current Iinprovided by the photodiode301) as depicted in equation (9) below:
Ua−Uref=TI·Iin(9)

By comparing equations (8) and (9) above, the transimpedance resulting from the circuit arrangement illustrated with respect toFIG. 3can be written as:
TI=(2(1+1))/3jRBG/ln(n)  (10)

It is noted that the transimpedance TI may be designated as a virtual transimpedance, because the transimpedance element is not arranged in the signal path of the amplifier unit. As can be seen from equation (10), the transimpedance does not depend on the absolute temperature T and is not provided in the dynamic path (the amplifier unit100) of the circuit arrangement shown inFIG. 3, but in the static path (the gain control unit200) of the circuit arrangement shown inFIG. 3such that high operational frequencies of the amplifier unit100can be obtained. For a factor 1=2 of the emitter areas of TV3and TV4with respect to TV1and TV2, respectively, equation (10) may be rewritten as:
TI=2RBG/(j·ln(n)  (11)

As it is shown by equation (11), the voltage difference of the output voltages between Uaand Urefis directly proportional to the input current Iin(photodiode current, input signal103) such that a constant transimpedance is provided. It is a specific advantage of the circuit arrangement illustrated inFIG. 3that the effective transimpedance TI according to equations (10), (11) is constant and is not provided directly in the signal path, i.e. in a path between the input terminal101and the output terminal102of the amplifier unit100, but in a static circuit unit, i.e. the gain control unit200.

Thus the input current does not flow through the transimpedance, as shown inFIG. 1, but an operational point of the amplifier unit100is adjusted by the gain control element201of the gain control unit200. A parasitic capacitance of the transimpedance TI thus cannot occur within the signal path of the amplifier unit100such that very high operational frequencies of the amplifier unit can be achieved. An effective transimpedance TI according to equations (10), (11) is thus provided in virtually static circuit components such as the gain control unit200. By adjusting the ratios defined by the constants j, n and l, the value of the transimpedance can be appropriately adjusted.

As an example, n=8 is chosen such that in a circuit layout a transistor TBG1is surrounded by eight partial transistors TBG2. Using a value of j=0.25, an effective transimpedance results in
TI=3.847·RBG

Thus, the effective transimpedance is nearly four times as large as the Ohmic value of the resistance RBGof the gain control element201. Advantageously, this means that less circuit space has to be provided compared to conventional transimpedance amplifier arrangements.

It is noted that the circuit arrangement shown inFIG. 3is based on the use of NMOS transistors. The present invention, however, is not restricted to the use of NMOS transistors. It is possible to replace all NMOS transistors by PMOS transistors and vice versa. Furthermore, all NMOS transistors may be replaced by bipolar npn transistors and all PMOS transistors may be replaced by bipolar pnp transistors, such that the whole circuit is based on bipolar technology. Furthermore, all bipolar pnp transistors may replaced bipolar npn transistors.

As the gain of the entire amplifier unit is determined by the transimpedance element TI, an easy gain adjustment may be provided just by switching the gain control element201and the reference element202simultaneously to different values. Furthermore, it is possible to provide a reference element202with a fixed ratio with respect to an Ohmic resistance value as compared to the gain control element201.

FIG. 4is a block diagram of an amplifier array including a plurality of amplifier units100-1,100-2, . . . ,100-N which are controlled by a single gain control unit200via a gain control signal208.

The amplifier units100-1,100-2, . . . ,100-N each have an input terminal I for inputting an input signal103to be amplified and an output terminal O for outputting the amplified input signal as an output signal104. A signal path is defined between the input terminal I and the output terminal O. An amplifier gain for amplifying the input signal and for providing an output signal is adjusted, wherein a variation of an operational current of the amplifier unit provides a variation of the respective amplifier gain. All amplifier units are arranged within the respective signal paths.

As shown inFIG. 4, the gain control unit200for controlling the amplifier gain of the amplifier units100-1,100-2, . . . ,100-N is arranged outside all signal paths.

The inventive concept is applicable to other areas where a current-to-voltage conversion is required. The circuit arrangement may be adapted for sensors which provide a current, e.g. photodiodes which provide a current in dependence of an incident light intensity.

It is noted that, although the present invention has been described with respect to photodiode sensors, it is not limited to light conversion or sensor devices at all. The present invention may be employed in applications where a voltage-to-voltage conversion is required if it is possible to convert the input voltage input current, e.g. by using an input resistor or an input impedance.

The invention has been described on the basis of embodiments which are shown in the appended figures and from which further advantages and modifications emerge. However, the invention is not restricted to the embodiments described in concrete terms, but can rather be modified and varied in a suitable manner. It lies within the scope of the invention to combine individual features and combinations of features of one embodiment with features and combinations of features of another embodiment in a suitable manner in order to arrive at further embodiments according to the invention.

It will be obvious to those skilled in the art that based upon the teachings herein changes and modifications may be made without departing from the invention disclosed and its broader aspects. That is, all examples set forth herein above are intended to be exemplary and non-limiting.