Patent Description:
Optical communication systems employ a variety of voltage-driven devices to generate light modulation. One of the most commonly used devices is the Mach-Zehnder interferometric modulator (MZM). The MZM has a wavelength-independent transfer function; besides, it can be operated in a dual electrode structure, that - in comparison to a single electrode modulator - requires lower driving voltages and provides chirp-free optical output.

For a SiPh MZM, a reverse-biased junction is typically used to generate the electro-optical effect. One drawback of this approach is that a reverse-biased junction has very small capacitance and therefore a long modulator with relatively high driving voltage and losses is needed to achieve the required extinction ratio. Travelling wave electrode or distributed amplifiers are the most common driving schemes.

To reduce the size and the losses of the modulator, a forward-biased junction is to be preferred. A forward-biased junction works as an injection device and - as such - it has very large capacitance (><NUM> pF) to create enough extinction ratio, given its short length. In order to drive this heavy load, a low impedance driver is required. The best driver would consist of a B-class output stage; also known as push-pull. In a CMOS technology this implementation is straightforward, but in bipolar technologies, where usually only lateral PNP transistors are available, the most suitable stage is the emitter follower (EF).

In an EF, when the output goes high not all the current from the top transistor is available to charge the load, because some of the current is lost by flowing through the bias transistor, thus slowing down the transition time.

One way to circumvent this shortcoming is to provide an auxiliary path for the signal to activate the bias transistors during the transitions, turning it into active pull-down element, that sinks or sources an extra amount of current from or into the output node, thus speeding up the transitions.

This principle has been described in <CIT> and employed for a single-ended EAM driver. The same principle is also applied for a differential MZM driver in a paper by <NPL>).

One shortcoming of the foregoing implementations is that the load capacitance seen by the predriver stage (hereinafter also referred to as preamplifier) is increased by the series capacitance of the coupling capacitor and the input capacitance of the pull-down device, therefore it is not possible to improve the output transition times without
impairing the bandwidth of the predriver. This is especially true when low power and high voltage swings are required.

It is possible to include additional buffers between the predriver and the output stage, but this would cause an increase in the overall current consumption. Besides, as shown in <CIT>, the supplementary buffers demand a higher supply voltage to prevent the saturation of the pull-down element, which turns into extra power consumption.

US Patent Publication <CIT> discloses a CMOS low-noise wide-band amplifier (LNA). The LNA can include a Gm doubler, a source follower, and a coupling circuit that couples a differential input to the Gm doubler with the source follower for achieving high linearity over a wide frequency range at a low supply voltage. The coupling circuit can capacitively couple a differential input to a gate of the source follower. The gate can be biased to a supply voltage through variable resistors. A cross coupler can be included in a push-pull buffer for additional gain and for allowing the source follower to drive a low impedance load at low power.

The Publication "<NPL>) discloses a modulator driver for photonic-integration based on SiGe-material.

An objective of the present invention is to provide an improved electrical amplifier.

A further objective of the present invention is to provide an improved method of amplifying electrical signals.

A further objective of the present invention is to provide an improved electro-optical device.

An embodiment of the present invention relates to an electrical amplifier according to claim <NUM>.

The positive feedback loop of the amplifier may decrease the transition times at output ports of the amplifier. Therefore, the amplifier may be used as a modulator driver circuit (e.g. for electro-optical devices such as MZM modulators) with improved switching speed and power consumption. Although the proposed amplifier is specifically designed for a SiPh MZM, it could of course be used with other types of optical modulators.

The emitter of the emitter-follower unit of the second output unit preferably forms a second output port of the electrical amplifier.

The base of the bias transistor of the first and/or second output unit is preferably also connected to a common voltage source via a resistor.

The first output port of the differential preamplifier is preferably connected to a base of the emitter-follower unit of the first output unit.

The second output port of the differential preamplifier is preferably connected to a base of the emitter-follower unit of the second output unit.

The first output unit preferably comprises an inner series circuit and an outer series circuit.

The outer series circuit of the first output unit preferably comprises said emitter-follower unit of the first output unit, hereinafter referred to as the outer emitter-follower unit, and said bias transistor of the first output unit, hereinafter referred to as the outer bias transistor. The inner bias transistor is preferably connected in series with the inner emitter-follower unit.

The emitter of the emitter-follower unit of the second output unit is preferably connected to a base of the inner bias transistor of the first output unit through the second capacitor of the positive feedback loop.

The emitter of the inner emitter-follower unit of the first output unit is preferably connected to the base of the bias transistor of the second output unit through the first capacitor of the positive feedback loop.

The first output port of the differential preamplifier is preferably connected to both a base of the inner emitter-follower unit and a base of the outer emitter-follower unit.

The inner series circuit of the first output unit preferably comprises an impedance connected to the emitter of the inner bias transistor. The impedance preferably comprises a parallel resonant circuit having a resistance in parallel with a first auxiliary capacitor.

The second output unit preferably comprises an impedance connected to an emitter of the bias transistor of the second output unit. The impedance of the second output unit preferably comprises a parallel resonant circuit having a resistance in parallel with a second auxiliary capacitor.

The second output unit preferably comprises a second inner series circuit and a second outer series circuit, wherein the second inner and outer series circuits are connected in parallel.

The second inner series circuit preferably comprises a second inner emitter-follower unit, as well as a second inner bias transistor. The second inner bias transistor is preferably connected in series with the second inner emitter-follower unit.

The second outer series circuit preferably comprises a second outer emitter-follower unit and a second outer bias transistor connected in series with the second outer emitter-follower unit.

The emitter of the second inner emitter-follower unit is preferably connected to both a base of the first inner bias transistor and a base of the first outer bias transistor through the second capacitor of the positive feedback loop. The emitter of the first inner emitter-follower unit is preferably connected to both a base of the second inner bias transistor and a base of the second outer bias transistor through the first capacitor of the positive feedback loop. The first output port of the differential preamplifier is preferably connected to both a base of the first inner emitter-follower unit and a base of the first outer emitter-follower unit.

The second output port of the differential preamplifier is preferably connected to both a base of the second inner emitter-follower unit and a base of the second outer emitter-follower unit.

The emitter of the second outer emitter-follower unit preferably forms a second output port of the electrical amplifier.

The base of the first inner bias transistor is preferably connected to a common voltage source via a first resistor.

The base of the second inner bias transistor is preferably connected to the same or another common voltage source via a second resistor.

The emitter of the first inner bias transistor is preferably connected to a reference potential through a first impedance.

The emitter of the second inner bias transistor is preferably connected to the same or another reference potential through a second impedance.

Preferably, all of the emitter-follower units are or comprise npn-bipolar transistors.

Preferably, all of the bias transistors are npn-bipolar transistors.

A further embodiment of the present invention relates to a method of amplifying an electrical signal according to claim <NUM>.

A further embodiment of the present invention relates to an electro-optical device according to claim <NUM>.

In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which.

The preferred embodiments of the present invention will be best understood by reference to the drawings. It will be readily understood that the present invention, as generally described and illustrated in the figures herein, could vary in a wide range. Thus, the following more detailed description of the exemplary embodiments of the present invention, as represented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention.

<FIG> shows a first exemplary embodiment of an electro-optical device <NUM> and an electrical amplifier <NUM> according to the present invention. The electrical amplifier <NUM> is connected to a supply voltage Vc, a reference potential V0, and two loads MD1 and MD2. The loads MD1 and MD2 may be diodes of a Mach-Zehnder interferometric modulator (MZM) e.g. based on SiPh. The diodes are preferably assigned to opposite interferometer arms of the MZM in order to increase their influence on the optical signal generated by the Mach-Zehnder interferometric modulator.

The electrical amplifier <NUM> comprises a differential preamplifier PA. In the exemplary embodiment of <FIG>, the differential preamplifier PA comprises resistors R1 and R2, transistors (preferably npn-bipolar transistors) T1 and T2 and a current source providing a constant current I.

Input ports Pi1 and Pi2 of the differential preamplifier PA allow inputting of a differential input signal ± Vin/<NUM>. In response, the differential preamplifier PA generates a first pre-amplified signal V1 at a first output port P1 of the differential preamplifier PA, and a second pre-amplified signal V2 at a second output port P2 of the differential preamplifier PA.

The electrical amplifier <NUM> further comprises a first output unit <NUM> that is connected to the first output port P1 of the differential preamplifier PA, and a second output unit <NUM> that is connected to the second output port P2 of the differential preamplifier PA.

The first output unit <NUM> comprises an emitter-follower unit <NUM> and a bias transistor <NUM> that is connected in series with the emitter-follower unit <NUM>. The emitter-follower unit <NUM> and the bias transistor <NUM> may be bipolar transistors (preferably npn-transistors) or may comprise bipolar transistors (preferably npn-transistors). The base of the bias transistor <NUM> of the first output unit <NUM> is connected to a constant common voltage source Vc1 via a resistor Rb1.

The second output unit <NUM> comprises an emitter-follower unit <NUM> and a bias transistor <NUM> that is connected in series with the emitter-follower unit <NUM>. The emitter-follower unit <NUM> and the bias transistor <NUM> may be bipolar transistors (preferably npn-transistors) or may comprise bipolar transistors (preferably npn-transistors). The base of the bias transistor <NUM> of the second output unit <NUM> is connected to a constant common voltage source Vc2 via a resistor Rb2. The constant common voltage source Vc2 may be the same as the constant common source voltage Vc1. The resistors Rb1 and Rb2 may be identical.

The first output port P1 of the differential preamplifier PA is connected to a base of the emitter-follower unit <NUM> of the first output unit <NUM>. The second output port P2 of the differential preamplifier PA is connected to a base of the emitter-follower unit <NUM> of the first output unit <NUM>.

The first output unit <NUM> and the second output unit <NUM> are electrically arranged in parallel relative to each other as well as parallel to the preamplifier PA.

In the exemplary embodiment of <FIG>, the emitter of the emitter-follower unit <NUM> of the first output unit <NUM> forms a first output port Poutl of the electrical amplifier <NUM>. The emitter of the emitter-follower unit <NUM> of the second output unit <NUM> forms a second output port Pout2 of the electrical amplifier <NUM>. The electrical amplifier <NUM> is therefore dual-ended and allows feeding the two loads MD1 and MD2 with inverse inputs signals.

The electrical amplifier <NUM> further comprises a positive feedback loop <NUM> that couples the first output unit <NUM> and the second output unit <NUM> and comprises a first capacitor C1 and a second capacitor C2. In the exemplary embodiment of <FIG>, the positive feedback loop <NUM> consists of the capacitors C1 and C2 and the bias transistors <NUM> and <NUM>.

The first capacitor C1 of the positive feedback loop <NUM> connects an emitter of the emitter-follower unit <NUM> of the first output unit <NUM> and a base of the bias transistor <NUM> of the second output unit <NUM>.

The second capacitor C2 of the positive feedback loop <NUM> connects an emitter of the emitter-follower unit <NUM> of the second output unit <NUM> and base of the bias transistor <NUM> of the first output unit <NUM>.

In the exemplary embodiment of <FIG>, the differential pair of transistors T1 and T2 together with the collector resistors R1 and R2 provide the voltage swing necessary to generate an electro-optical effect in the loads MD1 and MD2. The two emitter-follower units <NUM> and <NUM> offer a low-impedance output and therefore comparably large output currents compared to the output currents of the preamplifier PA alone.

As described above, each of the emitters of the emitter-follower units <NUM> and <NUM> are coupled to the bases of the bias transistors <NUM> and <NUM>, respectively, via the capacitors C1 and C2, thereby forming a differential pseudo push-pull arrangement. This differential pseudo push-pull arrangement creates a positive feedback loop that significantly speeds up the transition time. Indeed, when the output port Voutl goes high, the base voltage of the bias transistors <NUM> will raise, speeding up the discharging of the output port Vout2. Consequently, the base voltage at the bias transistors <NUM> will lower faster; which, in turn, causes more current to flow into the load MD1, so that the output port Voutl will go high even faster, and so on. The loop <NUM> is self-sustaining only during signal transitions and until the voltage at the bases of the bias transistors <NUM> and <NUM> is back to its steady-state value.

Since the coupling capacitors C1 and C2 are connected to the output ports Voutl and Vout2, they have no influence on the bandwidth of the preamplifier PA, so that the preamplifier PA can be optimized for low power consumption, i.e. having low bias current and high output resistance.

In order to visualize the effect of the positive feedback loop <NUM> of <FIG>, <FIG> shows an embodiment without positive feedback loop. In <FIG>, the coupling capacitors C1 and C2 are coupled to the preamplifier PA and can therefore not form a positive feedback loop. The positive effect on the speed of the signal transitions, as explained in connection with <FIG>, is therefore not present in the embodiment of <FIG>. The embodiment of <FIG> is similar to prior art.

<FIG> shows time-domain simulations depicting the voltage pulse response U(t) and the pull-down current I(t) over time t for the embodiments of <FIG> and <FIG>. It can be seen that the transition time is significantly smaller in the embodiment of <FIG> compared to the embodiment of <FIG>. A further improvement can be achieved by additional measures explained further below in connection with <FIG>.

<FIG> shows a second exemplary embodiment of an electro-optical device <NUM> and an electrical amplifier <NUM> according to the present invention. The electrical amplifier <NUM> of <FIG> is single-ended and provides a single output Voutl, only. The other features of the second embodiment of <FIG> are identical with those of the first embodiment of <FIG>. Therefore, the explanations above with respect to the positive feedback loop <NUM> and its positive effect on the speed (e.g. the transition times of signal edges) are valid for the second exemplary embodiment as well.

One shortcoming of the electrical amplifiers <NUM> of <FIG> and <FIG> is that if the load MD1 and/or MD2 has an inductive component - e.g. because of electrodes, metal interconnections or wire bonds - the positive feedback provided by the positive feedback loop <NUM> may lead to unwanted ringings.

In order to mitigate or circumvent this negative effect, it is possible to split the output transistors in two parts, where one of the two parts is biased and connected to the output load, as shown so far, and the other part has very little current and is connected to the coupling capacitor.

One possible implementation of this split-branch approach is shown in <FIG>.

In the amplifier <NUM> of <FIG>, the first output unit <NUM> comprises a first inner series circuit <NUM> and a first outer series circuit <NUM>. The first inner series circuit <NUM> and the first outer series circuit <NUM> are connected in parallel relative to each other as well as parallel to the preamplifier PA.

The first inner series circuit <NUM> comprises a first inner emitter-follower unit <NUM> and a first inner bias transistor <NUM> connected in series with the first inner emitter-follower unit <NUM>. The first outer series circuit <NUM> comprises a first outer emitter-follower unit <NUM> and a first outer bias transistor <NUM> connected in series with the first outer emitter-follower unit <NUM>.

The second output unit <NUM> comprises a second inner series circuit <NUM> and a second outer series circuit <NUM>, the second inner and outer series circuits <NUM>, <NUM> being connected in parallel.

The second inner series circuit <NUM> comprises a second inner emitter-follower unit <NUM> and a second inner bias transistor <NUM> connected in series with the second inner emitter-follower unit <NUM>.

The second outer series <NUM> circuit comprises a second outer emitter-follower unit <NUM> and a second outer bias transistor <NUM> connected in series with the second outer emitter-follower unit <NUM>.

The emitter of the second inner emitter-follower unit <NUM> is connected to both a base of the first inner bias transistor <NUM> and a base of the first outer bias transistor <NUM> of the the first output unit <NUM> through the second capacitor C2 of a positive feedback loop <NUM>. The emitter of the first inner emitter-follower unit <NUM> is connected to both a base of the second inner bias transistor <NUM> and a base of the second outer bias transistor <NUM> through the first capacitor C1 of the positive feedback loop.

The first output port P1 of the differential preamplifier PA is connected to both a base of the first inner emitter-follower unit <NUM> and a base of the first outer emitter-follower unit <NUM> of the first output unit <NUM>. The second output port P2 of the differential preamplifier PA is connected to both a base of the second inner emitter-follower unit <NUM> and a base of the second outer emitter-follower unit <NUM> of the second output unit <NUM>.

The emitter of the first outer emitter-follower unit <NUM> forms a first output port Poutl of the electrical amplifier <NUM>. The emitter of the second outer emitter-follower unit <NUM> forms a second output port Pout2 of the electrical amplifier <NUM>.

The base of the first inner bias transistor <NUM> is connected to a common voltage source Vc1 via a first resistor Rb1. The base of the second inner bias transistor <NUM> is connected to the same or another common voltage source Vc2 via a second resistor Rb2.

The emitter of the first inner bias transistor <NUM> is connected to the reference potential V0 (e.g. earth potential) through a first impedance Z1. The emitter of the second inner bias transistor <NUM> is connected to the same or another reference potential V0 through a second impedance Z2.

The impedances Z1 and Z2 may comprise or consist of a parallel resonant circuit having a resistance Re1, Re2 in parallel with a first auxiliary capacitor Ce1, Ce2. The resistances Re1 and Re2 are preferably identical. The first auxiliary capacitors Ce1 and Ce2 are preferably identical.

In other words, in the exemplary embodiment of <FIG>, the outer series circuits <NUM> and <NUM> are each biased via a Widlar current mirror (provided by the inner series circuits <NUM> and <NUM>) in addition to the capacitors C1 and C2 in order to increase the gain of the positive feedback loop <NUM> at higher frequencies. Hence, the positive feedback loop <NUM> acts on the inner push-pull stages (the inner series circuits <NUM> and <NUM>), and at the same time on the output pull-down elements (the outer series circuit <NUM> and <NUM>).

<FIG>, which had been already mentioned above in connection with the embodiments of <FIG> and <FIG>, also shows the effect of the split-branch approach of <FIG>. The time-domain simulations clearly indicate that the transition time is significantly smaller in the embodiment of <FIG> compared to the embodiment of <FIG>.

<FIG> shows a fourth exemplary embodiment of an electro-optical device <NUM> and an electrical amplifier <NUM> according to the present invention. The electrical amplifier <NUM> of <FIG> is single-ended and provides a single output Voutl, only. Therefore, the second outer series circuit <NUM> is not necessary and was discarded. The other features of the fourth embodiment of <FIG> are identical with those of the third embodiment of <FIG>. Therefore, the explanations above with respect to the split-branch approach, the positive feedback loop <NUM>, and their positive effect on the speed (e.g. the transition times of signal edges) are valid for the fourth exemplary embodiment as well.

<FIG> shows a fifth exemplary embodiment of an electro-optical device <NUM> and an electrical amplifier <NUM> according to the present invention. The electro-optical device <NUM> comprises a Mach-Zehnder interferometric modulator <NUM>, e.g. based on SiPh. The output ports Poutl and Pout <NUM> of the electrical amplifier <NUM> are connected with diodes MD1 of MD2 which are assigned to separate parallel interferometer arms <NUM> and <NUM> of the Mach-Zehnder interferometric modulator <NUM>. Since the output signals Voutl and Vout2 at the output ports Poutl and Pout2 of the electrical amplifier <NUM> are inverse, the optical modulation of the optical input signal Sopt is optimal.

Claim 1:
Electrical amplifier (<NUM>) comprising
a differential preamplifier (PA) having a first output port (P1) and a second output port (P2);
a first output unit (<NUM>) connected to the first output port (P1) of the differential preamplifier (PA) and a second output unit (<NUM>) connected to the second output port (P2) of the differential preamplifier (PA), the first and second output units (<NUM>, <NUM>) being electrically arranged in parallel relative to each other; and
a positive feedback loop (<NUM>) that couples the first and second output units (<NUM>, <NUM>) and comprises a first capacitor (C1) and a second capacitor (C2);
wherein each of the first and second output units (<NUM>, <NUM>) comprises an emitter-follower unit (<NUM>, <NUM>) and a bias transistor (<NUM>, <NUM>) that is connected in series with the emitter-follower unit of its output unit;
wherein an emitter of the emitter-follower unit (<NUM>) of the first output unit (<NUM>) is connected to a base of the bias transistor (<NUM>) of the second output unit (<NUM>) through the first capacitor (C1) of the positive feedback loop (<NUM>);
wherein an emitter of the emitter-follower unit (<NUM>) of the second output unit (<NUM>) is connected to a base of the bias transistor (<NUM>) of the first output unit (<NUM>) through the second capacitor (C2) of the positive feedback loop (<NUM>), and
wherein the emitter of the emitter-follower unit (<NUM>) of the first output unit (<NUM>) forms a first output port (Pout1) of the electrical amplifier (<NUM>).