Literature Survey

The BDAs can be classified into full-duplex and half-duplex, which are applied in FDD and TDD system, respectively.

The full-duplex BDA amplifies the signal in two opposite directions simultaneously.

However, the stability is the most critical concern due to the gain loop forming the positive feedback. In order to avoid the oscillation, the isolation network is necessary.

The circulator is the ideal functional block for this purpose. Conventional circulator is realized by the ferromagnetism material, which has the properties of low loss and high power capability. However, it is large in size and incapable to be integrated on chip. For the full-duplex application, the quasi-circulator is sufficient, thus the active circulator is

developed in recent years [9]-[11]. However, the active circulators suffer from high loss, low power capability, and large layout size.

To avoid the drawbacks of the circulator, another topology built by the reflection-type amplifier is proposed [12]. It contains two reflection-reflection-type amplifiers and a 3-dB 90º coupler, as shown in Fig. 1.5. The coupler helps the incident signal to be coherently combined at the output port. Ideally the gain of the BDA is equal to the reflection-type amplifier. However, the gain and isolation are sensitive to the imbalance of the two reflection-type amplifiers. Besides, the gain and bandwidth are restricted by the single-stage transistor and the 90º hybrid, respectively.

Fig. 1.5. Full-duplex BDA built by the reflection-type amplifiers [12].

The half-duplex BDA amplifies the signal in only one direction at a time, and the direction of amplification can be switched as desired. The switched function is easy to realize by the single-pole double-throw (SPDT) switches, which offers the pass-through in the active path and the isolation in the disable path. These BDAs are built by a pair of SPDT switches and two opposite-direction amplifiers [13]-[15], as shown in Fig. 1.6.

Another switching method is switching the input and output port, and hence the amplifier can be reused in two-direction amplification [16], as shown in Fig. 1.7. However, the switches suffer from a non-neglected loss in high operation frequency. For instance, at 60 GHz, the state-of-the-art CMOS SPDT switches are with at least 2-dB insertion loss [17], [18]. In brief, the 2-dB insertion loss rises up the noise figure of receiver around 2 dB and degrades the efficiency of transmitter around 37 %. In other words, the phased array needs to add extra elements that is more than a half of the original quantity to compensate the degradation. For this reasons, using the switches to realize the bidirectional function is not very practical.

SPDT

Amp

Amp

SPDT

Fig. 1.6. Conventional BDA.

SPST

Amp

SPST

SPST

SPST

Fig. 1.7. Component-reused BDA.

To improve the drawbacks of the switch-switching topology, the “switchless”

topology is proposed. That is, the designs absorb the switches into the amplifiers and optimize the junction matching network. Compared with the switch-switching topology, the loss can be reduced to improve the power efficiency and the noise performance.

Besides, owing to the integrated functional blocks, the chip area and cost can be reduced.

In the previous works, the switchless BDAs can be classified into 4 categories:

common-gate (CG), common-source (CS) or common-emitter (CE), bidirectional distributed amplifier (BDDA), and bidirectional constructive wave amplifier (BCWA).

The CG BDA [19]-[21] uses the identity of drain and source terminal in the transistor.

While the dc bias of drain and source are interchanged, the direction of amplification can be switched, as shown in Fig. 1.8. The CG BDA takes the advantages of simple design and natural property of symmetry. However, the performance of gain, noise, and power handling are poor due to the CG stages.

The CS BDA [20], [22] and CE BDA [4], [23]-[25] are realized on FET and HBT process, respectively. As shown in Fig. 1.9, two of CS-based or CE-based amplifiers are used and placed in opposite directions, which are similar to the switch-switching topology.

The difference is the SPDT switches are replaced by the passive networks, or called

“junction network”. It offers the function as the SPDT switches, but the insertion loss of the pass-through arm is reduced due to the absence of transistors. On the other hand, although the junction networks will not offer more isolation at the isolated path, the stability is confirmed by turning off the unused amplifier.

The BDDA [26]-[28] uses two anti-parallel gain cells to replace the single gain cell in the conventional distributed amplifier (DA). The topology is two individual DAs in opposite directions, and shares the passive components as shown in Fig. 1.10. The same feature of broadband is as the conventional DA; however, both drawbacks are low gain and poor power efficiency. Besides, in each direction of amplification, the turned-off gain cells contribute the extra capacitance, thus results in a lower cut-off frequency.

The BCWA [29], [30] is realized by the constructive-wave concept [31], the feedback amplifiers are built by the quarter-wave transmission line with cascode amplifiers as shown in Fig. 1.11. In each amplification mode, the inactive amplifiers are tuning off, and the feedback path can be reused for the two directions of amplification by tuning off the inactive amplifiers. However, the gain of the BCWA is affected by the loss of feedback quarter-wave transmission line [31]. Thus, the high performance active device (0.12-μm SiGe process in [29]) or low-loss substrate (45-nm CMOS SOI process in [30]) is needed to compensate the passive loss. Besides, due to the quarter-wave transmission line, the gain cell occupies more area than other types of BDA.

Fig. 1.8. CG BDA [20].

Fig. 1.9. CS/CE BDA [4].

Fig. 1.10. Bidirectional DA [26].

Fig. 1.11. BCWA [29].

Table 1.1 listed the performance comparison of the cited previous works. From this table, the CS-type has the better gain performance, which is based on the un-equal signal path in each direction of amplification. The amplifiers can be optimized for noise and power applications while operated in receiving (Rx) and transmitting (Tx) mode, respectively. For the above reasons, in this thesis, the research is focused on the CS-type switchless BDA and two CS-type switchless BDAs are proposed by using different processes and topologies.

Table 1.1. Previous reported MMW switchless bidirectional amplifiers.

Ref. Process Topology Frequency (GHz)

*Inference from the measured IP1dB with the gain in Tx Mode.

**Including the IF amplifiers.

***Total transceiver area.