An LTE transmitter using a class-A/B power mixer

Abstract

TXFor an LTE transceiver it is quite challenging to reduce power and area while preserving performance. For large emitted signals the TX dominates power consumption but in the past this situation was sufficiently infrequent not to affect energy consumption. In recent times the statistical distribution of the TX power has shifted upward due to the use of data-intensive communications and the introduction of multi-gain power amplifiers. Therefore to extend battery life in fourth generation terminals, TX consumption at high power (>-10dBm) should be reduced. A second challenge of an FDD LTE TX is noise and distortion emission in the RX band since the TX-to-RX distance, relative to the channel bandwidth, can be much smaller than in previous standards [1].A typical RF TX includes DAC, baseband filter, upconverter and pre-power amplifier (PPA). In some cases the PPA is not used and incorporated in the power mixer. This solution draws more current but reduces noise as shown in equation 1 that expresses the mixer signal-to-noise ratio (SNR) as a function of its gm stage bias current (Ibias), for Class-A operation and a given transistor overdrive voltage Vov.(1) From equation (1), the SNR ratio varies as follows. First it increases with bias current. Second it deteriorates with signal Peak-to-Average Ratio (PAR). Third it improves with transistor overdrive. A Class-A power mixer requires a minimum bias current to deliver its target power, which decreases as the output swing is increased. If this current is higher than that required to achieve the target noise in the RX band (for a given duplexer), the use of a PPA becomes feasible. However other considerations may discourage such a choice. First the PPA degrades Counter Intermodulation (CIM) and the Adjacent-Channel Leakage Ratio (ACLR). Second lowering noise emission allows the use of a lower selectivity duplexer which may result in cost savings and/or reduced losses [2].To reduce the bias current of a Class-A power mixer through an increase of its output swing, while limiting overall power consumption the authors in [3] have biased the mixer from 2.7V and the rest of the TX from 1.5V. This requires two high-efficiency switch-mode supplies and stresses the technology. A better solution is to use a Class-A/B power mixer that saves power and reduces noise [4]. The latter because the gm stage current noise is proportional to its average transconductance, which is equal to half of its peak value in Class A and much less in Class A/B especially for high PAR signals.Class A/B operation is achieved by placing in front of the mixer a differential voltage-to-current converter (V-to-I) whose output transistors are biased with a current much smaller than the peak signal current. A conceptual schematic of the V-to-I converter is shown in Fig. 19.7.1. The circuit performs three main functions. First it converts (via Rin) the voltage signal at the filter output into two differential currents that are sourced and sunk by the two push-pull output stages. For good linearity both differen...

A low out-of-band noise LTE transmitter with current-mode approach

“…The output signal provided by a current-steering DAC is buffered, scaled up and sent to a current-driven third order filter that provides the class A/B output current to the power mixer. Compared to the solution reported in [4], the wideband V-I converter at the end of the TX chain is transformed in a current-driven filtering stage.…”

“…This block derives from the V-I converter ( Fig. 2-a) proposed in [4]. The converted current is absorbed by the OTA output stage and fed to the mixer in a way that corresponds to an N-only class A/B transconductor [4].…”

“…In the first case, the most promising approach operates in the voltage-mode and is implemented with a quadrature passive up-converter driven by a filtering stage, followed by pre-power amplifier (PPA) [1], [2]. In the second case, the most efficient solution is based on a current power mixer, where the up-converter has also the role of a pre-power amplifier [3][4][5].…”

“…However, since the BB section generally still operates in voltage-mode, an additional linear V-I conversion stage is needed to drive the current-mode mixer [4], [5]. This makes voltage and current-mode solutions comparable in terms of power/area cost.…”

“…1). The transmitter is an evolution of the one reported in [4], where the filter used to minimize outof-band noise and to remove DAC replicas is merged with the mirror stage that drives the class A/B power mixer. The result is a TX chain with high efficiency and lower RX-band noise (especially for narrow spacing) compared to the state of the art of 4G cellular transmitters.…”