Power Amplifier Using Envelope Following Technique

1. Power Amplifier Using Envelope Following Technique מגיש: יניב מרוז RF Power Amplifiers

2. Introduction • With the explosive growth of RF portable devices and their increasing functional densities (data, voice, video), efficient power-saving techniques are intrinsic in prolonging battery lifetime. • Consequently, energy-efficient RF power amplifiers are key components in mobile battery-operated systems. RF Power Amplifiers

3. Introduction • These communications employ digital modulation such as quadrature phase shift keying (QPSK). • The modulation format along with the baseband filtering confines most of the signal energy to the desired transmit frequency band, thereby allowing efficient usage of available spectrum. • However, an undesirable consequence of filtering the pulses to confine spectral energy, is to impart a time varying amplitude dependence on the modulated RF signal. RF Power Amplifiers

4. Introduction • Hence, the RF power amplifier contained in the transmitter must faithfully reproduce both the time varying amplitude and phase characteristics of the signal. • Since these applications utilize battery powered mobile radios, maximizing power amplifier efficiency at all critical power levels is crucial to achieving extended battery operation. RF Power Amplifiers

5. Introduction • General power amplifiers efficiency improves by operating the amplifier near gain compression. • Doing so causes the envelope of the signal to be distorted (compressed in amplitude) which results in spectral regrowth. • There is an inherent trade-off between amplifier efficiency and spectral linearity in designing linear power amplifiers. RF Power Amplifiers

6. Introduction • As the power amplifier is operated in back-off away from gain compression, the efficiency drops rapidly. • There is a need for a linear power amplifier which efficiently amplifies time varying amplitude modulated signals over wide dynamic ranges. • One promising method is the envelope following technique. RF Power Amplifiers

7. Envelope Following Technique • The envelope following technique combines a high efficiency envelope amplifier with a highly efficient, but non-linear RF amplifier, to form a highly efficient linear RF amplifier. • Operationally, we deploy a drain-bias voltage (Vdd) to the the RF amplifier, so it is in or near gain compression. which results in high efficiency operation. • The overall amplifier is efficient if both the envelope and RF amplifiers are efficient. RF Power Amplifiers

8. Envelope Following Technique. • Ein(t) describes envelope properties of the modulation • θin(t) describes phase characteristics of the signal RF Power Amplifiers

9. Envelope Following Technique • The class-S modulator is similar in form to a buck dc-dc converter. where output pulses are produced where the width or duty cycle of the pulse is proportional to the input voltage. • The low pass filter functions to produce an average value of the pulse signal. Since the detected envelope signal is time varying, the drain supply bias is also time varying. RF Power Amplifiers

10. Envelope Following Technique • The DC power (into RF PA) is the product of the average drain supply voltage Vdd(t) and average supply current Idd(t). • Compared to non-drain supply modulated RF amplifiers operating at a fixed supply voltage Vdc. Vdd(t) can be significantly less than Vdc especially when the modulated RF signal exhibits a high dynamic range. • Since Idd(t)*Vdd(t) < Idd(t)*Vdc, the envelope following amplifier is much more efficient under power back-off for the same reason. • This can be especially important for battery life in mobile radios that function for significant periods of time at reduced transmit power levels. RF Power Amplifiers

11. Envelope Following Technique. • To achieve spectral linearity, it is imperative that the amplifier faithfully reproduces at it's output the time varying amplitude and phase characteristics of the modulated RF input signal. RF Power Amplifiers

12. Factors for minimizing spectral distortion • bandwidth of the class-S modulator. • time delay differences between the RF and envelope signal paths (due principally to the group delay associated with the low pass filter). • am-am and am-pm distortions in the RF amplifier. • developing the proper functional relationship between Vdd(t) and Ein(t) to satisfy the gain G. RF Power Amplifiers

13. Using the Feedback • An error signal is developed “e(t) = Ein(t) - Eout(t)” and fed back to the class-S modulator. RF Power Amplifiers

14. Proper drain supply • Developing the proper drain supply signal Vdd(t) is critical for minimizing spectral distortion. (Note that the gain G is a function of both Vdd and Ein). • For example, measured load pull data illustrating gain, input envelope voltage, and supply voltage for a small HEMT device at particular bias and source/load conditions. RF Power Amplifiers

15. DC-DC Converter • The class-S modulator is integrating the pulse width modulation (PWM) circuitry along with large NMOS and PMOS power FET devices for switching large currents at high switching speeds on a single chip. • PWM develops pulses whose width or duty cycle is proportional to the input voltage. RF Power Amplifiers

16. DC-DC Converter • PWM is accomplished by first generating a triangle waveform and then comparing that waveform to the input envelope voltage. • The PWM signal is amplified using a totem pole arrangement of large P- and N-MOSFET devices. RF Power Amplifiers

17. DC-DC Converter • Operating the class-S modulator at high switching speeds offers several advantages: • Increases the bandwidth of the class-S modulator, thereby improving the linearity of the RF amplifier. • Provides better suppression of switchingfrequency components by the low pass filter. • The value and size of the L/C filter network are dramatically reduced. RF Power Amplifiers

18. DC-DC Converter RF Power Amplifiers

19. RF Power Amp. Output vs. Input RF Power Amplifiers

20. RF Power Amplifiers