Amplifier inductor sharing for inductive peaking

The present disclosure relates generally to an amplifier, and more particularly to inductive peaking of the amplifier.

Inductive peaking is used in many amplifier applications, e.g., broadband amplifiers, to increase the bandwidth. The increased bandwidth is achieved by inserting inductors and utilizing the increase in inductor impedance with frequency to compensate for the effects of decreasing gain with frequency. However, inductors occupy a relatively large area of an integrated circuit chip.

Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an exemplary amplifier sharing inductors among multiple stages for inductive peaking according to some embodiments;

FIG. 2 is an exemplary schematic circuit diagram of the amplifier in FIG. 1 according to some embodiments;

FIGS. 3A-3B are plots of gain vs. frequency of an exemplary 3-stage amplifier similar to FIG. 2 and another exemplary 3-stage amplifier not sharing inductors for different inductance values according to some embodiments;

FIG. 4 is a plot of impulse response vs. time of an exemplary 3-stage amplifier similar to FIG. 2 and another exemplary 3-stage amplifier not sharing inductors for different inductance values according to some embodiments;

FIG. 5 is a flowchart of an exemplary method of sharing inductors for inductive peaking for an amplifier in FIG. 2 according to some embodiments; and

FIG. 6 is a plot of inductor area vs. number of stages of the exemplary amplifier in FIG. 2 having Ln=L/N, another exemplary amplifier sharing inductors with inductance L for inductive peaking, and yet another exemplary amplifier not sharing inductors (having separate inductors for inductive peaking) according to some embodiments.

The making and using of various embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use, and do not limit the scope of the disclosure.

FIG. 1 is a schematic diagram of an exemplary amplifier sharing inductors among multiple stages for inductive peaking according to some embodiments. An amplifier 100 has N stages, e.g., a first stage 102 , a second stage 104 , . . . , an N-th stage 106 . N is an integer number greater than 1. An amplifier input signal Vi is amplified going through each stage providing a respective output, e.g., Vo 1 after the first stage 102 , Vo 2 after the second stage 104 , . . . , Vo(n−1) after the (N−1) stage (not shown), and Von after the Nth stage.

Each stage has an output loading capacitance C shown as dotted lines. The output loading capacitance C is not a separate physical element added to the amplifier 100 , but rather a capacitance observed at each output node of each stage, originating mostly from the next (following) stage (or circuit). In other embodiments, a separate physical capacitor is added to the amplifier 100 at the output node of a stage. The N stages of the amplifier 100 share inductors Ln in the same current phase for inductive peaking. The amplifier 100 includes 2 inductors Ln in this example. In other embodiments, the number of inductors Ln is greater than 2.

FIG. 2 is an exemplary schematic circuit diagram of the amplifier in FIG. 1 according to some embodiments. An amplifier 200 includes a first stage amplifier 202 , a second stage 204 , a third stage 206 , . . . , an N-th stage 208 . N is an integer number greater than 1. The first stage 202 has differential inputs Vip and Vin. The first stage 202 outputs Vop 1 and Von 1 are inputs to the second stage 204 . The second stage 204 outputs Vop 2 and Von 2 are inputs to the third stage 206 , etc. And the (N−1) stage (not shown) outputs Vop(n−1) and Von(n−1) are inputs to the N-th stage 208 . The N-th stage 208 has outputs Vopn and Vonn.

Each stage has two transistors (e.g., T 11 and T 12 for the first stage 202 , T 21 and T 22 for the second stage 204 , . . . , etc.) receiving input signals (e.g., Vin and Vip for the first stage 202 , Vop 1 and Von 1 for the second stage 204 , . . . , etc.). The two transistors of each stage are coupled to two resistors R, input nodes (e.g., nodes for Vip and Vin), and a current source I. The resistors R of each stage are coupled to respective output nodes (e.g., nodes for Vop 1 and Von 1 ). The N stages of the amplifier 200 share two inductors Ln, where each inductor Ln is coupled to a respective resistor R (one of the two resistors) in each stage for inductive peaking of the amplifier 200 as shown in FIG. 2 .

The transfer function of each stage (e.g., 202 , 204 , 206 , 208 ) is given by the following equation:

V output V input = g m ⁡ ( R + sL LCs 2 + RCs + 1 ) = - g m ⁢ R ⁢ s + 2 ⁢ ζω s 2 + 2 ⁢ ξ ⁢ ⁢ ω ⁢ ⁢ s + ω 2 ⁢ ω 2 ⁢ ξ , Equation ⁢ ⁢ ( 1 )

where L is the inductance of Ln (for a single stage), C is the capacitance of an output node (of the single stage), R is the resistance (for a single stage), a damping factor

ξ = R / 2 ⁢ C L ,

and gm is a transconductance of one transistor (of the single stage), e.g., T 11 or T 12 .

For example, Vinput is the difference between the two inputs Vip and Vin, and Voutput is the difference between the two outputs Vop 1 and Von 1 in the first stage 202 . For the amplifier 200 , the damping factor and elements values such as resistance and capacitance are selected in order to have a stable output signal. In one example, the damping factor is selected as

ξ = 2 2 ,

which makes the inductance L=R 2 C/2. More details regarding the damping factor are provided as described below. The amplifier 200 can be used in various applications, e.g., broadband operational amplifier, limiting amplifier, trans-impedance amplifier, etc.

FIGS. 3A-3B are plots of gain vs. frequency of an exemplary 3-stage amplifier similar to FIG. 2 and another exemplary 3-stage amplifier not sharing inductors for different inductance values according to some embodiments. In FIG. 3A , a waveform 302 represents gain vs. frequency for the exemplary 3-stage amplifier similar to FIG. 2 having an inductance of Ln greater than L/3, where L is the value calculated for a single stage based on the relationship

ξ = R / 2 ⁢ C L ,

with the damping factor

Another waveform 304 represents gain vs. frequency for another exemplary 3-stage amplifier not sharing inductors (each stage has separate inductors). The waveform 302 indicates that having an inductance of Ln greater than L/3 shared among multiple stages result in unstable gain vs. frequency plot due to the damping factor less than

2

2

.

In FIG. 3B , a waveform 306 represents gain vs. frequency for the exemplary 3-stage amplifier having an inductance of Ln less than L/3, where L is the value calculated as described above, corresponding to the damping factor of

2 2 .

Another waveform 308 represents gain vs. frequency for another exemplary 3-stage amplifier not sharing inductors (each stage has separate inductors). The waveform 306 indicates that having an inductance of Ln less than L/3 shared among multiple stages result in reduced bandwidth due to the damping factor greater than

2

2

.

The amplifier 200 sharing inductance of Ln=L/N has a similar bandwidth as an amplifier not sharing inductors (each stage has separate inductors). For example, for an exemplary 2-stage amplifier sharing two L/2 inductors, the BW is about 21 GHz, while an exemplary amplifier not sharing inductors (using 4 inductors) has BW about 19.5 GHz. Also, for an exemplary 3-stage amplifier sharing two L/3 inductors, the BW is about 16.9 GHz, while an exemplary amplifier not sharing inductors (using 6 inductors) has BW about 16.7 GHz.

FIG. 4 is a plot of impulse response vs. time of an exemplary 3-stage amplifier similar to FIG. 2 and another exemplary 3-stage amplifier not sharing inductors for different inductance values according to some embodiments. A waveform 402 represents an impulse response vs. time for the exemplary 3-stage amplifier similar to FIG. 2 having an inductance Ln greater than L/3, where L is the value corresponding to the damping factor of

2 2 .

The waveform 402 shows that it takes a relatively longer time for the impulse response to stabilize.

A waveform 404 represents an impulse response vs. time for the exemplary 3-stage amplifier in FIG. 2 having an inductance Ln equal to L/3, where L is the value calculated as described above, corresponding to the damping factor of

2 2 .

A waveform 406 represents an impulse response vs. time for another exemplary 3-stage amplifier not sharing inductors (each stage has separate inductors). The waveforms 404 and 406 show similar impulse responses.

A waveform 408 represents an impulse response vs. time for the exemplary 3-stage amplifier in FIG. 2 having an inductance Ln less than L/3, where L is the value corresponding to the damping factor of

2 2 .

The waveform 408 indicates that the impulse response is relatively slower (due to reduced bandwidth).

The simulation results shown above in FIGS. 3A , 3 B, and 4 also apply to the amplifier 200 with different number of stages. For example, for N stages, if Ln is greater than L/N, the amplifier gain vs. frequency will be unstable, while if Ln is less than L/N, the amplifier will have reduced bandwidth BW. (N is an integer number greater than 1.) Thus, the amplifier 200 in FIG. 2 has inductors Ln having inductance=L/N to provide stable gain vs. frequency and impulse response having improved bandwidth in some embodiments. For example, Ln=L/2 for a 2-stage amplifier, Ln=L/5 for a 5-stage amplifier, etc.

FIG. 5 is a flowchart of an exemplary method of sharing inductors for inductive peaking for an amplifier in FIG. 2 according to some embodiments. At step 502 , the bandwidth of a single stage is determined with reference to a bandwidth (BW) specification of the amplifier. The bandwidth is determined based on element values such as resistor R coupled to the output node of the single stage and output loading capacitance C of the single stage. For example, if the calculated BW=1/(2πRC), then BW=3.18 GHz when R is 100 Ω and C is 500 fF. If the BW specification of the amplifier is 3 GHz, the calculated BW satisfies the BW specification.

At step 504 , the number of stages N (N>1) is determined based on a gain specification. For example, if the gain specification is 30 dB and each stage gain is 10 dB, N=3 stages. At step 506 , a single stage inductance L is calculated for inductive peaking to have a stable impulse response, e.g., based on a damping factor of about

2 2 .

For example, L=R 2 C/2=2.5 nH to have a damping factor of

2 2 ,

given the above example values of elements R and C.

At step 508 , a shared inductance, e.g., Ln in FIGS. 1 and 2 , for inductive peaking is calculated by dividing the single stage inductance L by the number of stages N. For example, Ln=L/3=0.833 nH, given the above example value of L and a 3-stage amplifier. At step 510 , at least two inductors having the shared inductance Ln are shared among multiple stages for inductive peaking, e.g., as shown in FIG. 2 .

FIG. 6 is a plot of inductor area vs. number of stages of the exemplary amplifier in FIG. 2 with Ln=L/N, another exemplary amplifier sharing inductors with inductance L for inductive peaking, and yet another exemplary amplifier not sharing inductors (having separate inductors for inductive peaking) according to some embodiments.

A line 602 shows the inductor area when an amplifier does not share inductors among multiple stages (having separate inductors for inductive peaking). It shows that the inductor area increases linearly with the number of stages. A line 604 shows the inductor area when an amplifier shares inductors that has the same inductance L of a single stage. A line 606 shows the inductor area for the exemplary amplifier in FIG. 2 with inductance Ln=L/N, which indicates substantial reduction in the inductor area compared to other cases, the difference becoming more significant as the number of stages increases.

According to some embodiments, a method of sharing inductors for inductive peaking of an amplifier having at least two stages includes calculating a single stage inductance of a single stage of the at least two stages for inductive peaking in order to have a stable impulse response. A shared inductance is calculated for inductive peaking by dividing the single stage inductance by a number of stages of the at least two stages. At least two inductors having the shared inductance are shared among the at least two stages for inductive peaking.

According to some embodiments, an amplifier includes at least two stages, wherein the at least two stages are coupled in series and each stage provides a partial amplifier gain of the amplifier. The amplifier includes at least two inductors. The at least two stages share the at least two inductors for inductive peaking. Each inductor of the at least two inductors has an inductance value for inductive peaking equal to the inductance of a single stage divided by the number of stages.

A skilled person in the art will appreciate that there can be many embodiment variations of this disclosure. Although the embodiments and their features have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosed embodiments, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure.

The above method embodiment shows exemplary steps, but they are not necessarily required to be performed in the order shown. Steps may be added, replaced, changed order, and/or eliminated as appropriate, in accordance with the spirit and scope of embodiment of the disclosure. Embodiments that combine different claims and/or different embodiments are within the scope of the disclosure and will be apparent to those skilled in the art after reviewing this disclosure.