GAIN USING VARACTOR DIODES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims the benefit of U.S. Non-Provisional Application No. 16/378,461, filed April 8, 2019, entitled“Amplifier with Adjustable High-Frequency Gain using Varactor Diodes”, and claims the benefit of U.S. Provisional Application No. 62/683,964, filed June 12, 2018, naming Suhas Rattan, entitled“Amplifier with Adjustable High-Frequency Gain Using Varactor Diodes”, all of which are hereby incorporated herein by reference in their entirety for all purposes.

BRIEF DESCRIPTION

[0002] An amplifier circuit incorporating configurable frequency compensation is described, suitable for use as a continuous-time linear equalizer (CTLE) for communications receiver input signals. Elements of the design facilitate compact circuit layout with the configurable DAC elements closely integrated with the analog devices they control.

BRIEF DESCRIPTION OF FIGURES

[0003] FIG. 1 is a circuit diagram of a CTLE amplifier utilizing NMOS transistors in accordance with some embodiments.

[0004] FIG. 2 is a circuit diagram illustrating how multiple instances of an embodiment such as that of FIG. 1 may be combined to provide increased configurability.

[0005] FIG. 3A is a circuit diagram of a digital-to-analog converter utilizing PMOS transistors suitable for use in producing analog control signals, in accordance with some embodiments.

[0006] FIG. 3B is a circuit diagram including transmission gate implementations of the transistors shown in FIG. 3 A.

[0007] FIG. 4 shows an alternative embodiment of the circuit of FIG. 1 utilizing PMOS transistors. [0008] FIG. 5 is a frequency spectrum of an adjustable high frequency peaking CTLE, in accordance with some embodiments.

[0009] FIG. 6 is a frequency spectrum illustrating CTLE gain without and with a fixed capacitor connected in parallel to varactor capacitors, in accordance with some embodiments.

[0010] FIG. 7 is a block diagram of a digital-to-analog converter (DAC) circuit generating control signals for setting an equalization range of an Amplifier, in accordance with some embodiments.

[0011] FIG. 8 is a diagram illustrating multiple equalization ranges for different coding schemes, in accordance with some embodiments.

DETAILED DESCRIPTION

[0012] Continuous-time Linear Equalization (CTLE) circuits are well known in the art. One common design is based on a conventional differential amplifier circuit utilizing a matched transistor pair having individual source loads but common drain connections to a fixed current sink. Splitting the current sink into two, one for each transistor drain, allows the drains to be cross- coupled with a frequency-dependent impedance such as a parallel RC network, modifying the essentially flat gain-vs-frequency characteristic of the basic differential amplifier into one having distinctly different low- and high-frequency gains.

[0013] In communications system receivers, such a CTLE circuit is typically configured to provide increased high-frequency gain, to equalize or compensate for the inevitable high frequency loss of most communications media. In some embodiments, careful configuration of amplitude and equalization functions is performed to facilitate accurate signal detection and/or clock recovery by subsequent circuits. In some embodiments, a CTLE circuit in which both the gain characteristics and the frequency break points of such frequency-dependent compensation may be adjusted or configured.

[0014] FIG. 1 shows one embodiment of a configurable CTLE circuit utilizing NMOS transistors as gain elements. An equivalent embodiment utilizing PMOS transistors is shown as FIG. 4. Without implying limitation, reference will be made to the circuit of FIG. 1 in the following descriptions. [0015] It should be noted that these embodiments are intended for use in an integrated circuit environment requiring extremely high frequency signals to be processed with minimal power consumption. The available power rails Vdd and Vss may typically provide one volt or less of operating voltage, thus microampere current flows imply path impedances of many thousands to millions of ohms. As resistances of these magnitudes may require substantial surface area in some integrated circuit processes, active circuit elements such as transistors may be preferable to passive element embodiments.

[0016] In FIG. 1, inductive load composed of inductors Lo and Li shunt peaking at high frequency, allowing a high frequency peaking of more than lOdB. The corresponding load impedances are provided by resistor arrays generating load impedances RLO and RLI . AS shown, each resistor array may receive a multi-bit thermometer code load resistance control input Ri<n-l :0>, and may selectively connect successive resistors in the parallel network to set a load resistance RL. In some embodiments, adjusting the load resistance may be beneficial to adjust the common mode of the subsequent processing stage, e.g., a variable gain amplifier. Alternatively, load resistance may be adjusted to tweak the magnitude of the high frequency peaking and the location of the second pole of the response, described in more detail below with respect to FIG. 5. As shown in FIG. 1, circuit slice 100 includes transistors 112 and 122 are a matched differential pair, accepting input signals Vin+ and Vin- and producing output signals Vout- and Vout+, respectively. Cascoded transistors 111 and 121, may be configured to receive a gate voltage of Vcasc, and may provide isolation of the input from the output while inherently helping to reduce the input miller capacitance seen at the input. Identical current sources 113 and 123 set the allowable current flow, and may be adjusted to adjust the magnitude of the high frequency peaking, as described below. The source R _s resistance provided by transistor 131 and the capacitance provided by the combination of fixed capacitor 132 and varactor capacitors 133 and 134 determine the frequency-dependent gain characteristics of the equalizer.

[0017] In some embodiments, voltage Vsw is configurable, allowing the impedance of transistor 131 to be adjusted. In other embodiments voltage Vsw is fixed, with that voltage and the physical transistor channel dimensions determining the resulting impedance. [0018] In a further embodiment, voltage Vsw may be set to one of two different predetermined values (i.e. a binary selection,) as subsequently described. In one such embodiment, when Vsw causes transistor 131 to be switched“on” (e.g., low impedance), circuit 100 is configured into a first or“flat” operational mode in which the frequency domain zero produced by capacitors 132, 133, 134 is minimized, and the DC equalization is less relative to the peak equalization. Conversely, when Vsw causes transistor 131 to be switched“off’ or high impedance, that impedance along with capacitances 132, 133, 134 result in circuit 100 being configured into a second or high frequency“peaking” operational mode, increasing the DC equalization relative to the peaking equalization.

[0019] FIG. 5 illustrates a frequency response of a CTLE providing high-frequency peaking, in accordance with some embodiments. FIG. 5 includes 4 points of interest, first zero A (w’ _/cro ), first pole B (wpo), peak frequency C (rvp _eak ), and second pole D (w _pl ). The values for each point of interest are given in Eqns. 1-4 below:

(Eqn. 1)

[0020] where R _s is the source resistance provided by transistor 131, and C _s is the source capacitance, which may be equal C _Dom + C _vc o _/i , where C _DOHI is the dominant capacitance of fixed capacitor 132 and C _vc o _/i is the capacitance of one of varactor diodes 133 or 134. Here and in the following description, the notation“0/1” may correspond to the value of one element‘O’ or‘G in the set of (0 1 }, as may be common in half circuit analysis. In at least one embodiment, C _DOHI may have a capacitance of approximately lOOfF in total (e.g., from 20 slices of 100 each having C _DOHI = 5fF), while the varactor diodes 133 and 134 have capacitances ranging approximately from 80- 400fF (e.g., 20 slices of 100 having varactors capacitances ranging from 4fF-20fF based on a 0- 800mV Vctrl). Depending on application, the capacitances of the fixed capacitor and varactors may be designed accordingly, e.g., a large fixed capacitance with varactors configured to fine tune via smaller capacitances, or alternatively may be broken up into separate capacitance elements.

[0021] The first pole wpO may be calculated as follows:

(Eqn. 2)

[0022] The peak frequency w'p _eak ay be calculated as follows:

(Eqn. 3)

[0023] and finally the second pole w _v\ may be determined as follows:

[0024] In some embodiments, capacitances 133 and 134 are provided by voltage-variable capacitors, which may include varactor or other P-N junction diodes, and the voltage-dependent body capacitance of MOS transistor devices whose channels change and are also non-linear and can be a function of time, depending on the manufacturing process used. Charge density in active devices changes over time and is much more noticeable in small channel length devices. As shown, varactors 133 and 134 are connected back-to-back, to minimize the unwanted modulation of the resulting capacitance by signal voltage, with analog control voltage Vctrl used to adjust the overall capacitance. Use of varactors significantly reduces area occupancy on a chip as compared to e.g., a switched capacitor bank. Furthermore, the varactors increase bandwidth and speed, as they do not introduce as much parasitic capacitance as conventional capacitor arrays introduce. Incorporating fixed capacitor 132 in parallel with the variable capacitance of 133 and 134 further reduces these unwanted signal distortion effects due to the varactors described above by reducing the adjustable range to a suitable amount. FIG. 6 includes two frequency spectrums illustrating CTLE gain with respect to frequency. The top graph illustrates tuning the frequency of the first pole by tuning the varactors 133 and 134 without fixed capacitor 132 connected in parallel, and the bottom graph illustrates tuning of the frequency of the first pole by tuning the varactors 133 and 134 with fixed capacitor 132 connected in parallel. As shown, omitting the fixed capacitor 132 results in high non-linearity between tuning steps of the varactor, while including the fixed capacitor 132 makes the curves much more linear. Furthermore, it should be noted that the peak remains more constant when including the fixed capacitor 132. The zero contribution is more constant with fixed capacitor 132 and more towards an expected 20dB/dec. Without the fixed capacitor 132, the zero changes slope with respect to frequency. [0025] As an added benefit, the capacitance of fixed capacitor 132 may allow use of a MOS ladder DAC, e.g., the MOS ladder DAC of FIG. 3A, which may result in further reduction in power and chip area compared to e.g., a resistor ladder DAC. While MOS ladder DACs may suffer from differential non-linearity (DNL) and integral non-linearity (INL), the fixed capacitor 132 helps eliminate such effects similar to how fixed capacitor eliminates the non-linearity of the varactor diodes. Furthermore, the fixed capacitor may be split into two parts. A source degeneration cap may be used instead of large (or multiple) varactors, which take up a lot of area. In such cases, half the capacitance may be handled by the varactor while the other half may be incorporated using a metal-insulator-metal (MIM) capacitor or a metal over metal (MOM) capacitor thus, the MIM/MOM capacitors may be vertically stacked in the layers, where the bottom silicon layer contains the varactors while the top metal layers contain the MIM/MOM capacitors. Such capacitors may be useful in applications where a large capacitance is used to move the zero to a smaller frequency.

[0026] FIG. 2 shows a collection of parallel slices of identical CTLE circuits 100 to take advantage of the previously-described binary mode selection by control voltage Vsw. In some embodiments, the parallel slices 100 may each be connected to a common load inductance (e.g., inductors Lo and Li) and load impedance (e.g., load impedances RLO and RLI). Without implying limitation, eight slices of circuit 100 are shown, each controlled by a single binary value taken from Vsw<7:0>, which may be considered as a single variable encoded as a“thermometer” or unary count value, from 0 (all low) to 7 (all high). With all instances of transistor 131 controlled by Vsw“open”, each instance of circuit 100 will be in its first or“high frequency” operational mode, thus the resulting DC equalization relative to the peak will be at a maximum, and DC gain will be at a minimum. As increasing numbers of Vsw values are set to selectively“short” transistors 131, increasing numbers of 100 instances will enter their second or“wideband” operational mode, resulting in increasing DC gain and thus DC equalization relative to the peak will start to decrease. Finally, with all instances of Vsw“shorted”, all instances of 100 will be in their second operational mode, and DC gain will be at a maximum, and DC equalization relative to the peak will be at a minimum. The use of such a parallel-slice configuration to control the DC equalization relative to the peak may reduce wideband noises, e.g., thermal noise, and may introduce less parasitic capacitance. [0027] In some embodiments, NMOS transistors 131 may be configured to operate in between the “high frequency” and“wideband” operational modes by providing source impedance control signals Vsw<7:0> as inputs to transistors 131 to cause transistors 131 to operate in the linear region. FIG. 7 is a block diagram of a DAC 705 configured to provide multiple voltages of a range of voltages as the source impedance control signals Vsw<7:0>. In FIG. 7, voltages from OmV to lOOOmV are provided in 200mV increments using a resistor-ladder DAC 710, however such numbers should be considered non-limiting, and a DAC 705 may be designed to have any set of values. Further, the DAC 705 should not be considered to be limited to the R-ladder DAC 710 shown; other types of DACs known to those of skill in the art may be used as well.

[0028] As one will note, increasing the gate voltage of an NMOS transistor, e.g., via a source impedance control signal Vsw<0> provided to NMOS transistor 131 in FIG. 1, will decrease the channel resistance of transistor 131, thus decreasing the equalization, and vice versa with regards to decreasing the gate voltage to decrease the channel resistance of NMOS transistor 131. Conversely, decreasing the voltage provided to a PMOS transistor, e.g., transistor 431 in FIG. 4, will decrease the channel resistance of PMOS transistor 431, decreasing the equalization, and vice versa with regards to increasing the gate voltage to increase channel resistance of PMOS transistor 431. In such embodiments, the value of the voltage output by the DAC to the transistors 131/431 acts as a“coarse” adjustment to set an equalization range, while the number of enabled parallel slices 100 acts as a“fine” adjustment to finely tune the equalization within the set equalization range.

[0029] FIG. 8 is a frequency response for two equalization ranges, in accordance with some embodiments. As shown, FIG. 8 includes equalization ranges 802 and 805. In the context of using the NMOS embodiment of FIG. 1, the DAC 705 may be configured to output a relatively higher voltage provided as source impedance control signals Vsw<7:0> to NMOS transistors 131 in enabled parallel slices 100 to configure the amplifier in equalization range 802, and DAC 705 may be configured to output a relatively lower voltage provided as control signals to transistors 131 in enabled parallel slices 100 to configure the amplifier in equalization range 805. Conversely, in the PMOS embodiment, the DAC may be configured to output source impedance control signals having a higher respective voltage to PMOS transistors 431 in enabled parallel slices 100 to set the amplifier in equalization range 802, and a lower respective voltage to the enabled parallel slices 100 to set the amplifier in equalization range 805.

[0030] Such embodiments for configuring the amplifier to operate in multiple equalization ranges may accommodate factors including different types of coding schemes, different cable/channel lengths, and/or a combination of various other factors. In one particular example, a non-return to zero (NRZ) coding scheme may be configured to have a max equalization of ~7dB, while an ensemble non-return-to-zero (ENRZ) orthogonal differential vector signaling code scheme may be configured to have a max equalization of ~l0dB. Thus, by selecting an output voltage from the DAC, the correct equalization range may be selected depending on use of ENRZ or NRZ coding schemes, while enabling and disabling parallel slices 100 may finely tune the desired equalization within the selected equalization range.

[0031] This configuration of numbers of essentially parallel amplifier slices into a first or a second operational mode provides direct control over the resulting differential gain between the low- frequency and high-frequency regions of the aggregate system’s gain-vs-frequency curve. Combined with the previously described control of the variable capacitance elements of each amplifier slice, both the amplitude and corner frequency of the high frequency“peaking” may be configured independently. These adjustments may be combined with other control methods, including varying circuit DC current by adjustment of current sources 113 and 123, and modification of effective load impedance by adjustment of RLO and RLI via the parallel resistor networks controlled by R.L<n-i _: o>. Additionally, the use of multiple parallel slices provides an option to independently control the varactor capacitances in each slice individually, increasing granularity at the cost of a multi-bit control signal Vctrl<7:0>, as shown in FIG. 2. However, some embodiments may provide the same control signal Vctrl to every slice.

[0032] Based on Eqns. 1-4 and the descriptions above regarding the varactor diodes for adjusting source capacitance C _s and multiple parallel slice configuration for adjusting source impedance R _s , it follows based on the frequency response of FIG. 5 that the circuits of FIGs. 1 and 2 may have multiple degrees of control. The first zero w _cro may be controlled by controlling the voltage across the varactors using Vctrl, which adjusts the source capacitance C _s . The DC equalization relative to the peak may be controlled by selecting how many parallel slices 100 are switched“off’ or“open” (high frequency mode) vs switched“on” or“shorted” (wideband mode). Such adjustments are illustrated in FIG. 5 by the 8 horizontal lines 505. In 505, each horizontal line may correspond to a various step of the 8 steps of the thermometer code described with respect to FIG. 2. As described in FIG. 2, the lowest horizontal line of 505 (e.g., where the DC gain is at a minimum and thus DC equalization relative to the peak is at a maximum) may correspond to the transistors 131 in every slice being switched“off’ or“open”. Alternatively, every transistor 131 being switched“on” or “shorted” may correspond to the highest horizontal line of 505, indicating that DC gain is at a maximum, and thus DC equalization relative to the peak is at a minimum. Further, the second pole w _pi of the frequency response may be adjusted by adjusting the effective load impedance via load resistors RLO and RLI . In some embodiments, the effective load impedance may be set via tunable resistors (not shown). Having adjustment of the zero and pole allows adjustment of the desired frequency band (width and location) of the high frequency peaking, as well as the gain of the high frequency peaking. Such embodiments are useful in backwards compatibility with some legacy designs, as well as dynamic adjustment in response to changes in channel response. Another degree of control may correspond to adjusting bias current in current sources 113 and 123, which may control the magnitude of the peak by increasing or decreasing gm. Further, the common mode provided to the following signal processing stage may be adjusted by controlling the load impedances RLO and RLI, e.g., via the resistor banks shown in FIG. 1.

[0033] Adjustment of these various configurable elements may utilize multiple control elements, such as digital-to-analog converters (DACs), that increase system power consumption and layout area. FIG. 3A shows an embodiment of a PMOS-ladder DAC optimized for minimal integrated circuit area and low current utilization. In some embodiments, the PMOS-ladder DAC is well- suited with the PMOS circuit of FIG. 4, as the PMOS-ladder DAC may utilize the same substrate that circuit 100 is disposed on, resulting in significant savings in overall circuit area. As fixed value resistors may occupy significant layout area in some integrated circuit processes, the channel resistance of PMOS transistors are used instead, as seen at 351 through 358. In a practical embodiment, all of 351 - 358 will be identical PMOS transistors, designed with sufficiently narrow channels to provide the desired series resistance or“R” value in a conventional R-2R ladder structure. In such embodiments, the ratio of the width-to-length (W/L) of transistors 351-358 may be twice the W/L ratio of transistor 313, resulting in a halved resistance‘R’, as increasing width with respect to length reduces the resistance. In some embodiments, gate bias Vss is applied to insure each such transistor is in its resistive channel state.

[0034] Each driving element 300 for the ladder, corresponding to a switch-selected voltage source in series with a“R” resistance transistors 351-358, the exception being transistor 313 having resistance 2R, is shown as a MUX composed of two identical MOS transistors 311 and 312, with 311 selecting voltage Vrefh and 312 selecting voltage Vrefl, the desired high and low values for the DAC output range. In some embodiments, transistor 313 will have a channel resistance twice that of transistors 351 - 358. In some embodiments, the Vrefh and Vrefl may range from 700- 900mV. In embodiments utilizing NMOS transistors, the voltage range may be lower, e.g., 0- 200mV, as NMOS transistors are more proficient in passing lower voltages.

[0035] Transistors 311 and 312 are driven by complementary control signals, here shown as derived from binary control input Vc7, one value taken from control word Vc<0:7> and the other the inverted version of VC7, VC 7. In the example of FIG. 3A, eight instances of 310 are used to drive the eight ladder taps, thus the binary control input for each instance is derived from a different bit value in control word Vc<0:7>, with Vc7 controlling the most-significant bit of the ladder, and VcO controlling the least-significant bit. In some embodiments, the values in control word Vc<0:7> are differential, thus for example control value Vc7 is inherently available as both Vc7 and its compliment VC 7 , which may directly control transistors 311 and 312. In other embodiments, each instance of 310 incorporates buffer/inverter gates to provide the appropriate drive signals to 311 and 312 from a single digital control value.

[0036] In one particular integrated circuit embodiment, DAC 300 producing analog output Vctrl was sufficiently compact to be physically collocated with or near the variable capacitance diodes 133 and 134 it controlled, minimizing the introduction of unwanted parasitic loads into analog circuit 100.

[0037] Although PMOS transistors are shown in FIG. 3A, an equivalent embodiment may be produced using NMOS transistors. Such single-transistor embodiments may be useful for implementations outputting a narrow range of voltages near Vss (e.g., 0-200mV) for NMOS designs, and voltages near Vdd (e.g., 700-900mV) for PMOS designs. In the above described embodiments, the varactors may respond particularly well to the control voltages provided near Vdd, however such embodiments should not be considered limiting. Depending on the relative voltages of Vrefh and Vrefl compared to Vdd, Vss, and the particular transistor gate thresholds, NMOS, PMOS, or even hybrid embodiments in which MUX structures 310 are implemented using transmission gates may be desirable. Such an embodiment in which MUX elements 310, and transistor elements 313 and 351-358 are Tx gates is shown in FIG. 3B. FIG. 3B includes a MUX element 310 utilizing Tx gates, where the selection inputs Vc7 and VC 7 control NMOS and PMOS transistors connected to Vrefh, respectively, and control PMOS and NMOS transistors connected to Vrefl, respectively. In transistor circuit 3 l3, an NMOS connected to Vdd and a PMOS connected to Vss are connected in parallel. In such embodiments utilizing Tx gates, the NMOS transistor is proficient in passing lower voltages whereas the PMOS transistor is proficient in passing higher voltages. Such embodiments may increase the voltage range provided at Vrefh and Vrefl. Whereas the PMOS only MOS ladder may receive a range of 700-900mV, the Tx gate version may receive a range of 0-900mV.

[0038] While MOS ladder DACs may provide significant advantages in terms of power savings and chip area savings, it should be noted that other DACs, e.g, R-2R ladders utilizing resistors instead of transistors may be utilized as well to provide the various control signals.