8.3 Power amplifier controllability and stability


8.3 Power amplifier controllability and stability

Implementing power amplifiers in CMOS technology is considered a major step towards realisation of a complete transceiver on-chip. Modern transceivers require means for adjusting the transmitted power over a finite range to further reduce power consumption and improve channel capacity. A low-performance, short-range wireless standard such as Bluetooth requires a high level of integration and low-cost that can only be achieved using CMOS technology together with means of controlling the output power up to 20 dBm. In addition, power amplifiers are typically backed off relative to their peak power and power added efficiency points in order to meet the linearity requirement of the system. The degree of back-off varies depending on the modulation scheme employed: 0 dB for Gaussian-filtered minimum shift keying (GMSK for GSM and DECT), 7 dB for π/4-DQPSK (IS-54 and PHS), 10 dB for QPSK (IS 95) and 12 dB for 16 QAM being typical figures. Thus adding efficient techniques for adjusting the output power level is considered a challenging issue to integrated power amplifiers. Along with controllability of RF PAs, the stability is an important design issue. RF PA oscillation problems can be broadly categorised into two kinds: bias oscillations and RF oscillations.

8.3.1 Power amplifier controllability

Linear power amplifiers can have their power adjusted by variation of biasing or by a dynamic variation of load seen by the output stage (Doherty amplifier [3]). Output power can also be controlled through a variation of the input signal amplitude, realised by having a variable gain amplifier as a preceding stage. However a large dynamic range of output power requires a linear wide dynamic range variable gain amplifier which is usually power consuming and hard to achieve. Also this configuration suffers from a large reduction in efficiency at lower power transmission since the standing bias current at the output stage does not scale with the output power. Also, this technique requires a very linear power amplifier for any kind of signal shaping at the input.

In non-linear power amplifiers, the input to the amplifier provides only timing information. Thus, the out put power cannot be control led through the variation in input signal amplitude as is done in linear or weakly non-linear amplifiers. Instead, output power control can be achieved effectively through a variable supply, implemented for example by a DC–DC converter. The losses in the DC–DC converter might cause the efficiency to drop to reach that of a linear power amplifier case. A new methodology based on switching a combination of power amplifiers in parallel is presented in [22] and represents an extension of the Doherty amplifier to non-linear power amplifiers.

8.3.2 Power amplifier stability issues

RF PA oscillation problems can be broadly categorised into two kinds: bias oscillation and RF oscillation. Bias oscillations that occur at very low frequencies, in the megahertz to VHF range, are caused by inappropriate or unintentional terminations at those frequencies by the bias insertion circuitry, large-value decoupling capacitors being the common cause. These oscillations have little to do with the details of the RF matching circuitry, where the RF blocking and decoupling capacitors render open circuit terminations at lower frequencies. RF oscillations, on the other hand, typically occur either in band or commonly out of band but still quite close to the desired bandwidth on the low-frequency side. These kinds of oscillations are very common in single-ended multistage designs, and their elimination will require modifications of the RF matching topology and element values. Both kinds of instability can be analysed effectively using k-factor analysis [4]. Although k-factor analysis assumes a linear two-port device, it is usually a satisfactory assumption to assume that RF oscillations in a power amplifier will more likely occur when the amplifier is backed off into its linear region, where the k-analysis is valid. In the case of deep class AB or B operation, it is necessary to increase the quiescent current to perform stability analysis with a representative amount of gain. A simple way to test stability of the PA is to run the entire circuit on a linear simulator, sweeping the frequency all the way down to DC.

Higher frequency instability will show up in a k-factor analysis of individual stages. Any single-ended design must show a k-factor greater than unity over the widest frequency sweep, extending from the low-frequency bias circuit range all the way up to the frequency at which the gain rolls off to lower than unity. Designing or modifying a circuit to obtain such a response for the k-factor typically involves some sacrifices in the in-band RF performance through use of resistive elements.




Wireless Communication Circuits and Systems
Wireless Communications Circuits and Systems (IEE Circuits, Devices and Systems Series 16)
ISBN: 0852964439
EAN: 2147483647
Year: 2004
Pages: 100
Authors: Yichuang Sun

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