Analysis and compensation methodology of the AM-AM and AM-PM distortion of cascode class-E power amplifiers are presented. A physical-based model is proposed to illustrate that the nonlinear capacitance and transconductance cause the AM-AM and AM-PM distortion when modulating the supply voltage of the PA. A novel methodology that can reduce the distortion is also proposed. By degenerating common-gate transistor into a resistor, the constant equivalent impedance is obtained so that the AM-AM and AM-PM distortion is compensated. An experimental prototype of 2.6 GHz cascode class-E power amplifier with the AM-AM and AM-PM compensation has been integrated in a 0.18
To increase the data rate, recent wireless communication systems allow the carried information encoded in both amplitude and phase of the RF signal and therefore a linear power amplifier (PA) is required. The linearity achieved in such amplifiers by operating below their maximum output power has the drawback of low efficiency and hence reduced battery lifetime. The switching class-E amplifier is a potential candidate for power amplification in wireless transceivers [
For highly integrated transceivers, many researches have been focused on the switching class-E PA [
After identifying the main sources of the distortion in a cascode class-E PA in Section
In class-E PA, the voltage-current separation for efficiency enhancement is obtained by using a tuned LC network. The switch is closed at the instant where both the switch voltage and its first derivative are zero. The requirement of a zero first derivation makes the amplifier less sensitive to component variations. This leads to the well-known class-E conditions as stated by N. O. Sokal and A. D. Sokal [
Figure
Schematic of common-source class-E PA.
Regarding gate-oxide breakdown which occurs, if high voltages drop across the gate oxide, deserves particular care in a conventional common-source class-E PA [
Schematic of CMOS cascode class-E PA.
An additional device (
In general, from the reliability standpoint, the bias voltage
The AM-AM distortion is the difference between the supply voltage and the envelope of the RF output voltage. Such a difference is caused by a nonlinear relationship between the supply voltage and the envelope of the RF output signal. The AM-AM distortion in the RF PA itself can be kept low if it is always operated as a switching amplifier. In other words, the supply voltage of the PA driver stage will be kept high to ensure the switching nature of the amplifier. Besides the AM-AM, the AM-PM distortion will also be presented in the circuit. This distortion is an unwanted phase modulation of the RF output carrier due to the modulation of the supply voltage.
When modulating the PA supply, different operating regions of the transistor
Equivalent model of the transistor
In small supply
Theoretical results of
To illustrate the AM-AM and AM-PM distortion, the impedance
Theoretical and simulated result of
AM-AM and AM-PM distortion.
The result reveals that the relationship of the envelope of the RF output voltage against the supply
Since various impedances will cause the nonlinear effect, a constant
Equivalent schematic of the PA when the PA with AM-AM and AM-PM compensation.
In Figure
Simulated
The results of the PA with and without the AM-AM and AM-PM compensation are compared in Figure
Compared AM-AM and AM-PM distortion.
One more advantage of the transistor
Figure
Compared DC current and drain efficiency.
Gain and output power against supply voltage.
A fully integrated cascode class-E power amplifier with a self-biased control circuit for compensating the AM-AM and AM-PM distortion has been implemented in a 0.18
Detailed schematic of proposed PA.
The self-biased control circuit tends to produce a voltage, the sum of a DC voltage and an analogue voltage. It is implemented by a voltage adder, including an operating amplifier with several resistors. Selecting the ratio of resistors, the expected output voltage is obtained. Besides, the operating amplifier has the unit-gain frequency of 100 MHz for achieving a correct output voltage of the self-biased control circuit during supply modulation. The control circuit is just to produce a gate biasing voltage of
Fabricated prototypes have been measured by the probe testing. The chip photomicrograph is reported in Figure
Die photo.
Measured AM-AM and AM-PM effect.
Measured output power (
The output power increases proportionally to
Measured output power and power gain versus input power.
Measured output power and PAE versus frequency.
The design proposed here shows that the AM-AM and AM-PM distortion of the cascode class-E PA has been compensated by a self-biased control circuit without dissipating more power. However, because of the concern of an allowed current capacity on the top metal layer, the aspect sizes of the transistors for efficiency optimization are difficult to achieve. Therefore, the equivalent series resistances of the transistor are large when the transistor is on, so that there is reduction on the drain voltage waveform and the output power. Due to the reduction of the output power, the efficiency of the amplifier is also reduced.
A system cosimulation platform of envelope elimination and restoration (EER) architecture shown in Figure
System cosimulation platform of EER.
Histogram of the envelope voltage.
Obtained constellations (a) without compensation and (b) with compensation.
All of results have been summarized on Table
Performance summary.
Simulation | Cosimulation | Measurement | |
(AM-PM) | (AM-PM) | ||
18° | EVM = | N/A | |
(0.5 V–1.8 V) | |||
3° | EVM = | 5° | |
(0.5 V–1.8 V) | (0.7 V–1.8 V) |
A cascode class-E PA with a self-biased circuit has been proposed in this paper to achieve the reduction of the AM-AM and AM-PM distortion. We carefully investigated the distortion in cascode class-E PAs, finding that an equivalent triode-resistor by the common-gate transistor is a way to cancel the nonlinear capacitance and the transconductance for the reduction of the AM-AM and AM-PM distortion. A simplified equivalent model is introduced to illustrate the mechanisms of the distortion. Furthermore, a design methodology for compensating this distortion has been identified and a circuit solution has also been proposed. The prototype of 2.6 GHz class-E amplifier, realized in a 0.18
This work was conducted by the Trans-Wireless Technology Laboratory (TWT Lab.) and sponsored jointly by the Ministry of Education and the National Science Council, Taiwan. The authors would like to thank Dr. K. W. Huang of National Nano-Device Laboratory (NDL), Taiwan, for chip testing.