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1. WO2020141022 - MULTIPLE RESONANCE NETWORK FOR AN AMPLIFIER

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

MULTIPLE RESONANCE NETWORK FOR AN AMPLIFIER

TECHNICAL FIELD

The present invention relates generally to the technical field of broad-band amplifiers for high data-rate wired/optical communications. The invention presents in this technical field a multiple resonance network. Further, the invention presents an amplifier, particularly a broad-band amplifier, trans-impedance amplifier (TIA), and/or driver amplifier, which comprises the multiple resonance network. The multiple resonance network can serve as a Radio Frequency (RF) building block, which is able to provide a programmable transfer function.

BACKGROUND

RF building blocks, which are able to provide a programmable transfer function, are widely requested to optimize the data link performance. In fact, the frequency-dependent transfer function of an analog electronic component (e.g. TIA and driver) is desired to have a programmable bandwidth and/or a programmable peaking, in order to compensate the non ideality coming from the link (e.g. photo-diode, modulators, etc.). The availability of analog devices with such tuning features would be a competitive advantage, since they may be used to fine-tune the electro-optical transfer function in a communication module, where the electrical component is paired with its optical counterpart (e.g. MZM modulator, photodiode, etc.).

In the following, two conventional solutions are described and examined in terms of their advantages and disadvantages.

1. The most common design technique for achieving a peaking control of the frequency- dependent transfer function is the resistive/capacitive (RC) degeneration of a differential pair. A resistive degeneration Rdeg is generally added, in order to achieve a good linearity performance in terms of total harmonic distortion (THD). If the resistive degeneration is paired to a capacitive degeneration, the resulting RC pole of the degeneration network leads to a zero in the transconductance (Gm) transfer function. The zero is generally located close to a cut-off frequency associated to the output resistive stage, which leads to an extension of the bandwidth and to the presence of an evident peaking in the overall transfer function. The capacitor can be easily made programmable, which adds flexibility to the frequency position of the zero, and the effect of the zero switching is reported in the transfer function.

The evident advantage of this design technique is the straightforward implementation in modem scaled Complementary Metal-Oxide-Semiconductor (CMOS) and Bi-CMOS technologies, due to the availability of excellent integrated active switches (e.g. MOS transistors). However a detailed analysis reveals also several disadvantages associated with this design solution: A first disadvantage is that the zero in the transfer function generally occurs in the low/mid frequency range, leading to limited effectiveness of the technique for very high frequency applications. A second disadvantage derives from the quantitative small effects on the controllability of the peaking amplitude of the overall transfer function. As previously mentioned, the zero in the transconductance transfer function usually nulls the effect of a pole associated to the output stage, so that the peaking in the overall transfer function results from the frequency misalignment of the doublet pole-zero. A third disadvantage comes as a consequence of the previous: this technique links the programmability of the zero frequency to the peaking amplitude resulting in limited flexibility of the programmability of the transfer function (i.e. changing the peaking we get a change in the pole-zero doublet and vice-versa).

2. Another quite common technique is the so-called active feedback: starting from a cascade of gain stages (in the minimal configuration there are 2 stages Gmi and Gm2 in cascade), bandwidth limitations are partially overcome incorporating an active feedback element (Gmfb). Straightforward analytical calculations show that f3dB and peaking of the resulting close-loop transfer function depend on the transconductance gain Gmib suggesting that the feedback element is eligible to become the tuning element.

Peaking amplitude and cut-off frequency are unfortunately both dependent on the tuning parameter Gmib, and therefore the programmability space is rigid, there is no flexibility between the peaking, and the frequency tuning is present. Moreover this design solution presents two further major disadvantages: Firstly, the design flexibility is added including the active element Gmib therefore increasing the power consumption of the RF building block. Secondly, the solution is based on a close-loop topology, so that it is intrinsically not well suited to applications covering extremely large frequency ranges.

In summary, the conventional techniques for implementing bandwidth and/or peaking control of the frequency-dependent transfer function suffer from severe limitations in terms of both maximum working bandwidth and limited flexibility of the tuning elements.

SUMMARY

In view of the above-mentioned challenges, embodiments of the present invention aim to improve the prior art in light of the above mentioned drawbacks.

The objective is achieved by embodiments of the invention as provided in the enclosed independent claims. Advantageous implementations of the embodiments of the invention are further defined in the dependent claims.

A first aspect of the invention provides a multiple resonance network, comprising: an input terminal and an output terminal, at least one resonant circuit including an inductance connected between the input terminal and the output terminal, and a tuning circuit connected in parallel to the inductance, wherein the tuning element includes at least one reactive circuit and/or at least one resistive circuit.

The tuning element provides the possibility to influence the resonance frequencies in the multiple resonance network and thus the frequency-dependent transfer function. In particular, a reactive circuit is able to influence the cut-off frequency of the transfer function and thus the bandwidth. A resistive circuit is able to influence the quality factor of the resonances of the transfer function and thus the peaking.

In an implementation form of the first aspect, the tuning circuit is configured to change an absolute value and/or a quality factor of one or more resonances of the multiple resonance network.

Accordingly, the tuning circuit can be used to provide programmability of the peaking. The quality factor is a parameter that describes the resonance behavior of a resonance of the multiple resonance network. A higher quality factor means that the multiple resonance network will resonate at that frequency with greater amplitude if driven sinusoidally at the resonant frequency, and that the multiple resonance network resonates in a smaller frequency range

around the resonance frequency, i.e. the multiple resonance network has a smaller bandwidth at that resonance.

In an implementation form of the first aspect, the tuning circuit includes at least one switchable or tunable reactive circuit and/or at least one switchable or variable resistor.

Switching the reactive circuit or variable resistor on and off, changes the transfer function with respect to cut-off frequency or quality factor, and thus provides the desired programmability.

In an implementation form of the first aspect, the tuning circuit includes a plurality of capacitors and a plurality of switches for selectively connecting or disconnecting each of the capacitors in parallel to the inductance.

Connecting and disconnecting the capacitors, respectively, changes the cut-off frequency of the frequency-dependent transfer function, and thus allows manipulating the bandwidth.

In an implementation form of the first aspect, the tuning circuit includes a plurality of resistors and a plurality of switches for selectively connecting or disconnecting each of the resistors in parallel to the inductance.

Connecting and disconnecting the resistors, respectively, changes the quality factor of at least one resonances of the transfer function, and thus allows peaking control.

In an implementation form of the first aspect, the plurality of switches includes a plurality of integrated MOS devices.

In an implementation form of the first aspect, the multiple resonance network is configured to receive, as an input, a current, particularly a RF current, and provide, as an output, an RF voltage.

In an implementation form of the first aspect, the input terminal is an input terminal of a first transistor, and the output terminal is a control terminal of a second transistor.

In an implementation form of the first aspect, the multiple resonance network is configured to receive, as an input, a voltage, particularly a RF voltage, at a control terminal of the first

transistor, and provide, as an output, a current, particularly RF current, flowing between an input terminal and an output terminal of the second transistor.

In an implementation form of the first aspect, the resonant circuit includes the inductance, a first capacitor and a second capacitor, the first capacitor is connected with one of its terminals between the inductance and the input terminal of the first transistor and with its other terminal to ground, and/or the second capacitor is connected with one of its terminals between the inductance and the control terminal of the second transistor and with its other terminal to ground.

In an implementation form of the first aspect, the first transistor is directly connected with its input terminal to the inductance and is connected with its output terminal to ground, and the second transistor is directly connected with its control terminal to the inductance and is connected with its output terminal to ground.

In an implementation form of the first aspect, the first transistor is connected with its input terminal to a further resonant circuit, which particularly includes a resistor and an inductance connected in series.

In an implementation form of the first aspect, the first transistor is connected through the further resonant circuit to a voltage supply, particularly a DC voltage supply.

A second aspect of the invention provides an amplifier, particularly a broad-band amplifier, trans-impedance amplifier and/or driver amplifier, comprising a multiple resonance network according to the first aspect or any of its implementation forms.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

FIG. 1 shows a multiple resonance network according to an embodiment of the invention.

FIG. 2 shows a multiple resonance network according to an embodiment of the invention.

FIG. 3 shows a multiple resonance network according to an embodiment of the invention.

FIG. 4 shows a multiple resonance network according to an embodiment of the invention.

FIG. 5 shows a frequency-dependent transfer function of the multiple resonance network of FIG. 4 and illustrates a bandwidth tuning.

FIG. 6 shows a multiple resonance network according to an embodiment of the invention.

FIG. 7 shows a frequency-dependent transfer function of the multiple resonance network of FIG. 6 and illustrates a peaking control.

FIG. 8 shows an example of a triple resonance network.

FIG. 9 shows a frequency-dependent transfer function of the triple resonance network of FIG. 9.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the invention aim at providing a multiple resonance network, for example an RF building block, with a programmable transfer function, i.e. the possibility to change the frequency response of the transfer function. In particular, bandwidth and/or peaking should be programmable. Thereby, the RF building block should not suffer from limitations in terms of maximum working bandwidth or limited tuning flexibility.

The embodiments of the invention base on a multiple resonance network, which is an extension of the triple resonance network (TRN) presented in‘Galal, Razavi,“ 40-Gb/s amplifier and ESD protection circuit in 0.1 H-mhi CMOS technology”, IEEE Journal of Solid-State Circuits. Year: 2004, Volume: 39, Issue: 12’. This technique, together with its companion called inverse triple resonance network (I-TRN) presented in ‘Chih-Fan Liao, Shen-Iuan Liu, “ 40 Gb/s Transimpedance- AGC Amplifier and CDR Circuit for Broadband Data Receivers in 90 nm CMOS’ IEEE Journal of Solid-State Circuits, Year: 2008, Volume: 43, Issue: 3’ is particularly interesting to push the circuit design of driver and TIA to the absolute limits of the technology. In order to provide the correct background, a review of the TRN is given in the following and some considerations are derived, which laid the foundation of the present invention.

FIG. 8 shows an example of a TRN, and FIG. 9 shows schematically its frequency-dependent transfer function Considering FIG. 8 (wherein MOS devices are used only as reference, they could be replaced by bipolar transistors), without losing generality we can assume Ci = Ci = Co/2 and L2 = 2*Li.

Simple calculations lead to identify three resonance frequencies:


The resonant frequencies are indicated in the graph of the transfer function of the TRN shown in FIG. 9.

Considering the overall transfer function, the resulting bandwidth exceeds about 3 times the frequency limitation given by the resistive load RC. This TRN is the starting element of the multiple resonance network according to embodiments of the invention described in the following.

FIG. 1 shows a multiple resonance network 100 according to an embodiment of the invention. In particular, the multiple resonance network 100 has an adapted or adaptable a frequency-dependent transfer function. The multiple resonance network 100 may thus well serve as a RF building block providing programmability in a broad-band amplifier.

The multiple resonance network 100 comprises an input terminal 101 and an output terminal 102. The input terminal 101 and the output terminal 102 may be realized by transistors, but could also be realized by trans-conductors or generally current generators. Further, the multiple resonance network 100 comprises at least one resonant circuit, which includes an inductance

103 (and, for instance, two capacitors Cl and C2 as shown exemplarily in FIG. 1), wherein the resonant circuit is connected between the input terminal 101 and the output terminal 102.

Furthermore, the multiple resonance network 100 comprises a tuning circuit 104, which is connected in parallel to the inductance 103. The tuning circuit 104 includes at least one reactive circuit or element and/or at least one resistive circuit or element. In this way the tuning circuit

104 is configured to influence a frequency response of the transfer function of the multiple resonance network 100.

Notably, the multiple resonance network 100, when in operation, has different resonant frequencies. The resonance frequency that may primarily be influenced by the tuning circuit 104 may be w2 as described above with respect to FIG 8.

FIG. 2 shows a multiple resonant network 100 according to an embodiment of the invention, which builds on the multiple resonance network 100 shown in FIG. 1. Same elements in FIG.

1 and FIG. 2 share the same reference signs and function likewise.

In addition to the elements shown for the multiple resonance network 100 of FIG. 1 , the multiple resonance network 100 of FIG. 2 further includes a second resonant circuit, which is connected to the input terminal 101. For example, the further resonant circuit may include at least one resistor 202 and at least one inductance 203, which are connected in series. The further resonant circuit may be connected to ground. The further resonant circuit influences the frequency-dependent transfer function, and particularly the resonant frequencies of the network 100.

FIG. 3 shows a multiple resonance network 100 according to an embodiment of the invention, which builds on the multiple resonance network 100 shown in FIG. 2. Same elements in FIG. 2 and FIG. 3 share the same reference signs and function likewise.

In addition to the elements shown for the multiple resonance network 100 in FIG. 2, in the multiple resonance network 100 of FIG. 3 the input terminal 101 is an input terminal of a first transistor 204, and the output terminal 102 is a control terminal of a second transistor 205. The transistors 204, 205 may be field effect transistors (FETs) or bipolar junction transistors (BJTs).

In the multiple resonance network 100 shown in FIG. 3, the further resonant circuit may be connected between the first transistor 204 and a voltage supply. That is, the first transistor 204 may be connected through the further resonant circuit, e.g. the resistor 202 and inductance 203, to a voltage supply, particularly a DC voltage supply. Notably, this implementation is also possible for the multiple resonance network 100 shown in FIG. 2.

Compared to the TRN shown in FIG. 8, one difference of the multiple resonance networks 100 shown in Fig. 1-3 is the addition of the tuning circuit 104 (also referred to as Zoning, as labelled in the figures), which is able to modify the frequency-dependent transfer function of the multiple resonance network 100, in particular may add tuning features. According to embodiments of the invention, the following implementations are possible with respect to the tuning element 104:

• If Z tuning is a pure reactive circuit or element, the resonance frequencies of the multiple resonance network 100 can be modified, in particular may add frequency programmability of the circuit cut-off frequency.

• is a resistive circuit or element, it may act on the quality factor of the resonances

of the multiple resonance network 100. The macro effect is a variation of the amplitude of the peaking of the frequency-dependent transfer function.

Notably, the circuit position of the tuning circuit 104 (i.e. at a position without flowing of DC current) is advantageous and allows the implementation of switchable elements based on MOS devices.

FIG. 4 shows an example of a multiple resonance network 100 according to an embodiment of the invention, which builds on the multiple resonance network 100 shown in FIG. 3. Same elements in FIG. 3 and FIG. 4 share the same reference signs and function likewise. According to the previous considerations, a control of the bandwidth of the transfer function can be implemented with the multiple resonance network 100 shown in FIG. 4.

In particular, starting from the more general schemes of the multiple resonance networks 100 shown in FIG. 1-3, the tuning circuit 104 in FIG. 4 is realized by a bank of capacitors 400. Each capacitor 400 may be connected in parallel to the inductance 103 (also referred to as L2, as labelled in the figures) with integrated switches 401 (in the figure represented as ideal switches) that can be easily implemented with e.g. MOS devices. Adding capacitors 400 in parallel to the inductance 103, the resonance frequencies of the network change, the presence of switches 401 make the function completely tunable.

Schematic results according to simulations are shown in in FIG. 5. In particular, FIG. 5 shows a simplified graph reporting the mentioned bandwidth control capability. Acting on the switches 401, the frequency shape of the network 100 is preserved but the absolute value of the resonant frequencies are affected resulting an effective tuning of the bandwidth. Moreover, the cut-off frequency is changed, while the peaking is only slightly affected.

FIG. 6 shown an exemplary multiple resonance network 100 according to an embodiment of the invention, which builds on the network 100 shown in FIG. 3. Same elements in FIG. 3 and FIG. 6 share the same reference signs and function likewise. In particular, in FIG. 6 the tuning circuit 104 is now realized with a bank of resistors 600. Each resistor 600 can be connected in parallel to the inductance 103 with switches 601 that can be easily implemented e.g. with MOS devices. Adding resistors 600 in parallel to the inductance 103, the quality factor of the

resonances are changed leading to an action on the peaking of the frequency-dependent transfer function.

Schematic results according to simulations are shown in in FIG. 7. In particular, FIG. 7 shows a simplified graph reporting the mentioned peaking control capability.

In summary, embodiments of the invention combine the bandwidth extension solution, called TRN with a novel design technique that allows a flexible control of both bandwidth and peaking amplitude of the resulting transfer function. Starting from the basic TRN, the additional tuning circuit 104 is added such that the following may be achieved:

• The frequency-dependent transfer function can be tuned in terms of cut-off frequency and peaking.

• The cut-off frequency tuning is achieved by including capacitors 400 in parallel to the inductance 103.

• The peaking control is achieved including resistors 600 in parallel to the inductance 103.

• The position of the tuning circuit 104 is prone to implementation with integrated switching devices, e.g. MOS transistors.

• The control of cut-off frequency and peaking are orthogonal, allowing a large flexibility in the programmability of the frequency-dependent transfer function.

The embodiments of the proposed invention add flexibility to RF building blocks, and can be fruitfully used in e.g. TIAs or drivers for high-speed data links. Compared with the best performing conventional topologies, the embodiments of the present invention are based on an advanced design techniques, and can be used to achieve RF building blocks with extremely large bandwidth. Extending the comparison with the best-performing conventional topologies to the power consumption, the conclusion can be drawn that the embodiments of the present invention have no penalties in terms of power consumption.

The proposed embodiments are suitable for fully- integrated building blocks for TIA and driver requiring programmability in terms of bandwidth and peaking control. The proposed embodiments are suitable for integration in many different IC technologies including (but not limited to) Bi-CMOS, CMOS, and III-V compound technologies (e.g. InP or GaAs).

The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article“a” or“an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.