power amplifier design
The power amplifier is part of the radio-frequency transmitter, and is used to amplify the signal being transmitted to an antenna so that it can be received at the desired distance. A numerical or analog information is processed at low frequency, and converted to an appropriate sinusoidal waveform combined with a modulation. The high frequency converter transforms the low frequency signal flow to a high frequency signal fhigh. The shape of fhigh is identical to flow except that the frequency is one or two orders of magnitude higher. Details about this circuit are provided later in this chapter. The amplitude of fhigh is usually small (10-100mV). A power amplifier is required to multiply the amplitude of the signal in order to transmit enough power to the emitting antenna.
Antenna Model
We can consider an antenna as a load that, in the ideal case, will be a pure resistance. The antenna resistance Ra accounts for the power absorbed by the antenna and appearing at the termination point of the power amplifier. This power is mainly radiated by the antenna. Most mobile phone antennas are resonant monopole [Macnamara] for which the antenna resistance Ra varies from 20 Ω (Ground plane width w=0) to 40 Ω (Infinite ground plane width). The monopole radiates mainly on X and Y directions (Figure 12-14). The length of the antenna is often chosen close from λ/4, where λ is the wavelength of the emitted signal. That length corresponds to a maximum in the sinusoidal wave. From an electrical point of view, we can modelize the antenna as a pure resistive load. The value of 50ohm is commonly used for Ra in simulations, as most equipments are “50 ohm adapted”.
Specific Power Unit
The level of output power in mobile phones ranges approximately from 10mW to 1 Watt. The usual unit for qualifying the power is the dBm, meaning “dB milliwatt”. The correspondence between the Watts and the dBm is given below. A 1Watt amplifier has an output power Pout of 30dBm (Equation 12-4).
Power Amplifier Principles
Most CMOS power amplifiers are based on a single MOS device, loaded with a “Radio-Frequency Choke” inductor LRFC, as shown in figure 12-16. The power is delivered to the load RL, which is often fixed to 50Ohm. This load is for example the antenna monopole, which can be assimilated to a radiation resistance, as described in the previous section. The resonance effect is obtained between LRFC and CL. The formulation for resonance is given below.
For example, a power amplifier designed for Bluetooth operation should resonate around 2.4GHz. If we assume that the inductance has a value of 3nH, the corresponding capacitor is around 1.5pF.
Power Amplifier MOS
The MOS devices used in power amplifier designs have very huge current capabilities, to be able to deliver strong power on the load. This leads to very unusual constraints to the width of the transistor: devices with a width larger than 1000µm are commonly implemented [Hella]. The radio frequency choke inductor has a resonant effect which induces and important voltage swing of node Vout. Consequently, high voltage MOS devices are used to handle large overvoltage. A MOS device with a very large width is not drawn directly, but is obtained by connecting medium size MOS devices in parallel. In Microwind, we generate easily multiple-finger MOS devices, thanks to the MOS generator command (Figure 12-17). The high voltage option is selected, and the number of fingers is fixed to 10.
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