I. INTRODUCTION

Cellular networks have evolved from 4G long-term evolution to 5G new radio (NR) to increase wireless data transmission. A single-chip RF transceiver must be able to simultaneously support 5G new radio (NR) and the legacy protocols of 2G/3G/4G cellular standards. This is because small form factor and low power consumption are the most important factors for increasing the competitiveness of mobile platforms ^[1,2]. Transceivers typically have many input/output (I/O) pins to support multiple bands, multiple standards, carrier aggregations, and diversity paths. Therefore, the CMOS low noise amplifiers (LNAs) with a single-ended input must be used to reduce the number of I/O pins. CMOS broadband balun-LNAs or balun-low noise transconductance amplifiers (LNTAs) can be an optimal solution because they improve the carrier-to-noise ratio of the overall receiver (RX). Consequently, the number of I/O pins required for the transceiver is reduced ^[3-9]. In addition, the balun-LNA/balun-LNTA can eliminate the passive balun implemented by large transformers, reducing the die size of the transceiver. Specifically, the RF filtering balun-LNA/LNTA can enhance the blocker-tolerance and linearity of the entire RX chain by filtering transmitter (TX) leakage signals and OB blockers. Hence, it can relax the strict input-referred second-order intercept point requirement. In recent decades, many RF-filtering LNAs with Q-enhanced LC filters ^[9-11], active cancellation techniques ^[12], and N-path filters ^{[8, 13-16]} have been studied to improve blocker tolerance of the RX.

In this paper, a sub-6 GHz noise-cancelling balun-LNTA with an embedded dual-band Q-enhanced LC notch filter is proposed for 5G NR cellular applications. Section II presents the detailed circuit design of the proposed balun-LNTA. The simulation results with layout parasitic extraction are elaborated in Section III. The section also presents the comparison of the proposed balun-LNTA with state-of-the-art balun-LNAs/LNTAs. Finally, the conclusions of the study are summarized in Section IV.

II. CIRCUIT DESIGN OF NOISE-CANCELLING BALUN-LNTA WITH DUAL-BAND LC NOTCH FILTER

The proposed balun-LNTA with a dual-band Q-enhanced LC notch filter is shown in Fig. 1. The LNTA performs voltage-to-current conversion. Moreover, it provides current outputs to the input of the following current-mode double-balanced passive mixers driven by 25% duty-cycle local oscillator signals. It can reduce noise by adopting a noise-cancelling 1:5 common-gate (CG)-common-source (CS) LNA topology with local feedback g$_{m}$-boosting and modified current-bleeding techniques ^[7,9]. It also enables RF notch filtering to remove out-of-band blockers and TX leakage signals using a dual-band Q-enhanced LC notch filter with a band-switchable differential inductor.

Fig. 1. Proposed noise-cancelling balun-LNTA with a dual-band LC notch filter.

B. Dual-band Q-enhanced LC Notch Filter with Band-switchable Differential Inductor

The proposed dual-band Q-enhanced third-order LC notch filter is shown in Fig. 2. It has a negative-resistance (R) circuit to improve the Q-factor of the notch filter with the proposed band-switchable differential inductor. With this proposed inductor, the notch filter can support both low-band (LB) and mid-band (MB) frequencies in the 5G NR frequency-division duplexing (FDD) system.

Fig. 2. Proposed dual-band Q-enhanced LC notch filter with band-switchable differential inductor.}

The layout of the proposed band-switchable differential inductor with a center tap is shown in Fig. 3. By switching the LB on and MB off, the entire differential inductor for LB operation can be utilized. By switching the LB on and LB off, some of the inductors for MB operation are disabled, thus reducing inductance. The metal width, metal spacing, and outer radius of the inductor were 15, 5, and 200 ${\mu}$m, respectively. The distance between the differential inductor and switches (SW$_{LB}$ and SW$_{MB}$) is 60 ${\mu}$m. The inductance and Q-factor of the designed band-switchable differential inductor were completely simulated including the routing line to switches through electromagnetic simulation using the Cadence EMX tool. As shown in Fig. 4, the inductance and Q-factor at LB and MB are approximately 8.96 nH/10 and 3.26 nH/12, respectively.

Fig. 3. Proposed band-switchable differential inductor with a center tap.}

Fig. 4. Simulated inductances and Q-factors of the band-switchable differential inductor at LB and MB.

The notch frequency of an LC notch filter can be tuned to the strong out-of-band blockers or TX leakage signals using 5-bit digitally controlled capacitor arrays (C$_{N1}$ and C$_{N2}$). The overall transconductance gain of LNTA with an embedded LC notch filter can be expressed as

(4)

$ G_{mNotch}=G_{m}\frac{G_{m}Z_{N}}{\left(1+G_{m}Z_{N}\right)} $

where Z$_{N}$ is the equivalent impedance observed in the LC notch filter. It is given to the following:

(5)

$Z_{N}=\frac{1+s^{2}L_{N}(\frac{C_{N1}}{2}+C_{N2})}{\frac{sC_{N1}}{2}(1+s^{2}L_{N}C_{N2})}=\frac{1+s^{2}/\omega _{N}^{2}}{\frac{sC_{N1}}{2}(1+s^{2}/\omega _{P}^{2})}$,

where f$_{P}$ and f$_{N}$ are the pole and notch frequencies, respectively. In the passband frequency, G$_{mNotch}$ becomes G$_{m}$ and approaches zero near f$_{N}$. As known from (5), the third-order LC notch filter has a pole around the RX band, so that a flat frequency characteristic can be obtained without droop around the RX band ^[11]. It prevents the gain and NF degradation at the RX band caused by the notch filtering. The finite Q-factor of L$_{N}$ and the switched-on resistance of the capacitor arrays of C$_{N1}$ and C$_{N2}$ reduce the overall Q-factor of the LC notch filter, consequently degrading notch selectivity. To improve the notch depth, the negative-R circuit shown in Fig. 2 is used ^[10].

III. SIMULATION RESULTS

The proposed low-noise broadband balun-LNTA with an embedded Q-enhanced third-order LC notch filter for sub-6 GHz 5G NR cellular applications was designed in a 65-nm CMOS process. The layout of the balun-LNTA is illustrated in Fig. 5. The total active die area without the bond pads is 0.55 mm$^{2}$. To verify the performance of the designed balun-LNTA, post-layout simulations were performed.

Fig. 5. Layout of the designed balun-LNTA.

The simulated S$_{11}$ is shown in Fig. 6. The balun-LNTA has an S$_{11}$ value of less than ${-}$10 dB (over the frequency range 50 MHz-6 GHz). Fig. 7 shows the simulated transconductance gain of the balun-LNTA versus RF frequencies with and without the negative-R circuit. The LNTA obtains a maximum transconductance gain of 44 mS at 500 MHz and 3-dB bandwidth of 6.24 GHz. The notch frequency can be adjusted to the TX frequency or strong interferer frequency of the operating band by tuning the inductance and capacitance of the third-order LC notch filter. Because of the negative-R circuit, the simulated notch depth can exceed 15 dB throughout the operating frequencies of the sub-6-GHz 5G NR. The simulated gain and phase mismatches of differential output currents are described in Fig. 8. In the entire frequency range, the gain and phase mismatches are less than 0.5 dB and 2$^{\circ}$, respectively. The simulated NF of the balun LNTA with the loading effect of the input impedance of the following current-mode double-balanced passive mixer is shown in Fig. 9. The figure indicates that the simulated NF is less than 4.8 dB at a maximum frequency of 6 GHz, and the minimum NF is 2.38 dB. The simulated in-band input-referred third-order intercept point (IIP3) is depicted in Fig. 10. The two-tone test conditions for the IIP3 are f$_{1}$ = f$_{RXLO}$ + 1 MHz, f$_{2}$ = f$_{RXLO}$ + 1.1 MHz, and p$_{f1}$ = p$_{f2}$ = ${-}$40 dBm, where f$_{RXLO}$ is the RX LO frequency. The simulated in-band IIP3 exceeds ${-}$6.1 dBm throughout the band.

Fig. 6. Simulated S$_{11}$.

Fig. 7. Simulated frequency response of the notch filtered LNTA with and without the negative-R circuit.

Fig. 8. Simulated gain and phase mismatches.

Fig. 9. Simulated NF.

Fig. 10. Simulated in-band IIP3.

Table 1 summarizes and compares the performances of the designed balun-LNTA with previous state-of-the-art balun-LNAs. The proposed balun-LNTA provides a broadband frequency response with a minimum NF of 2.38 dB and a transconductance gain of 44 mS with balanced differential output currents. It is the first balun-LNTA with an embedded dual-band LC notch filter that can sufficiently reject TX leakage signals and out-of-band blockers.

IV. CONCLUSION

A broadband low-noise noise-cancelling balun-LNTA with an embedded dual-band Q-enhanced third-order LC notch filter is designed to implement a blocker-tolerant receiver for 5G NR sub-6 GHz cellular applications. To design the dual-band LC notch filter, a band-switchable differential inductor with a center tap was proposed. The proposed balun-LNTA can reject unwanted blockers and strong TX-leakage signals in the 5G NR FDD bands, thus relaxing the linearity requirement of the following mixer. It also achieves low NF and broadband characteristics by employing a 1:5 CG-CS balun-LNTA topology with local feedback g$_{m}$-boosting and current bleeding techniques. Based on the simulation results, it obtains a minimum NF of 2.38 dB, a maximum transconductance gain of 44 mS and a 3-dB bandwidth of 6.24 GHz. Moreover, its blocker rejection ratio for the LB and MB of the 5G NR exceeds 15 dB.