Bài giảng Mạch điện tử thông tin - Chapter 2: RF Power Amplifiers

Chapter 2:

RF Power Amplifiers

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

1

Definitions (1)

 Efficiency: Efficiency is a crucial parameter for RF power amplifiers.

It is important when the available input power is limited, such as in

battery-powered portable or mobile equipment. It is also important for

high-power equipment where the cost of the electric power over the

lifetime of the equipment and the cost of the cooling systems can be

significant compared to the purchase price of the equipment.

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

2

Definitions (2)

Collector efficiency: Collector efficiency is a term more appropriate for

amplifiers using bipolar transistors (BJTs), although it is often used for any

RF power amplifiers. Some authors prefer to use plate efficiency for

amplifiers using vacuum tubes or drain efficiency for amplifiers using

MOSFETs or, simply refer to it as efficiency. Collector efficiency is defined

as

where P₀is the RF output power (dissipated into the load) and P_dc= V_dcI_dcis

the input power supplied by the dc supply to the collector (or drain /plate)

circuit of the power amplifier. P₀usually includes both the RF fundamental

power and the harmonics power. In many applications, harmonic suppression

filters are included in the output-matching network. Because the harmonic

power is negligible, the RF fundamental power is a very good approximation

for P₀.

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

3

Class C RF Power Amplifier (1)

 Conductance angle: The portion of the RF cycle the device spends in its

active region is the conduction angle and is denoted by 2θ_C. Based on

the conduction angle, the amplifiers are generally classified as:

• Class A amplifiers, if 2θ_C= 360°. The

active device is in its active region during

the entire RF cycle.

• Class AB amplifiers, if 180° < 2θ_C< 360°.

• Class B amplifiers, if 2θ_C= 180°.

• Class C amplifiers, if 2θ_C< 180°.

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

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Class C RF Power Amplifier (2)

 Basic circuit of single-ended class A, AB, B, or C amplifier:

This is a single-ended circuit, and the transistor operates in the common

emitter (CE) configuration (common-base configurations are also possible).

Variations among practical circuits operating in different classes may occur

in the base-bias or drive circuits. The collector circuit includes an RF choke

(RFC) that provides a DC input current, I_dc, a DC blocking capacitor, C_d

(short-circuit at the operating frequency and its harmonics), the load resistor,

R, and a parallel resonant LC circuit tuned to the operating frequency ₀.

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

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Class C RF Power Amplifier (3)

The DC component of the collector current i(θ) flows through the RFC and

then through the DC-power supply. The variable component of i(θ) flows

through DC-blocking capacitor C_dand through the parallel RLC tuned

circuit. The tuned circuit provides a zero impedance path to ground for the

harmonic currents contained in i(θ) and only the fundamental component

of i(θ) flows through the load resistance. As a result, the output voltage is a

sinusoidal waveform. This requires the use of a parallel resonant circuit (or

an equivalent band-or low-pass filter).

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

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Class C RF Power Amplifier (4)

The collector current is a periodical waveform described by

Its Fourier analysis results:

where

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Class C RF Power Amplifier (5)

Fourier series coefficients _nversus the conduction angle (_c)

Dept. of Telecomm. Eng.

Faculty of EEE

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Class C RF Power Amplifier (6)

Due to the ideal tuned circuit, the output current (flowing through the load

resistance R) is sinusoidal and its amplitude is given by

As a result, the output voltage is also sinusoidal, with the amplitude

V₀= RI₀. The collector voltage is

The DC input power P_dcand the collector efficiency  are

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

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Class C RF Power Amplifier (7)

The maximum theoretical collector efficiency (obtained for V₀= V_dc) varies

with the conduction angle as

If V₀= V_dc, the peak collector voltage is v_max= 2 V_dcand the peak collector

current is given by

The output power P₀is

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

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Class C RF Power Amplifier (8)

 DC bias: The conduction angle in a Class C amplifier is controlled by a

DC-bias voltage V_Bapplied to the base, and an amplitude V_bof the signal

across the base-emitter junction.

For - _c<  < _c, the transistor is in its

active region. Consequently, the voltage

across its base-emitter junction is

V_BE(on) 0.7 and

This equation allows calculation of the

required bias voltage, V_B, in the base

circuit.

Dept. of Telecomm. Eng.

Faculty of EEE

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Class C RF Power Amplifier (9)

Simple bias circuits for BJTs

Simple bias circuits for MOSFETs

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Class C RF Power Amplifier (10)

 Practical considerations: The effects of V_saton the performance of

Class C amplifiers are determined as

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Class C Frequency Multipliers (1)

 Frequency multipliers are often used to multiply the frequency of the

master oscillator or to increase the modulation index in the case of phase

or frequency modulation.

The Class C frequency multiplier has the same schematic as the Class C

power amplifier and operates in much the same way. The only difference

is that the collector resonant circuit is tuned to the desired harmonic,

suppressing all other harmonics.

Assuming that the parallel LC output circuit is ideal, tuned to the nth

harmonic, a sinusoidal output voltage is obtained:

The output power is given by

Dept. of Telecomm. Eng.

Faculty of EEE

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Class C Frequency Multipliers (2)

The DC power is

The collector efficiency is

The collector efficiency is highest if V₀= V_dc

Dept. of Telecomm. Eng.

Faculty of EEE

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2

Class C Frequency Multipliers (3)

 The variation of the maximum collector efficiency _maxwith the

conduction angle _C, for a Class C amplifier (n = 1), a doubler (n = 2),

and a tripler (n = 3), is shown

Note that the collector efficiency decreases as the multiplying order n

increases. Also note that a Class B circuit (_C= 90°) cannot be used as a

frequency tripler, because a half-wave sinusoidal waveform does not

contain the third harmonic

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

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Class C Frequency Multipliers (4)

 Optimum performance of frequency multipliers is obtained for

a. frequency doubler: _C= 60°, _max= 63.23%

b. frequency tripler: _C= 39.86°, _max= 63.01%

 As multiplication factor n increases, the output power (and also the

power gain of the stage), the collector efficiency, and the power

output capability decrease. On the other hand, if n increases, it

becomes more difficult to filter out adjacent harmonics n - 1 and n + 1

because they lie closer to the desired harmonic, and the relative

bandwidth becomes narrower. As a result, Class C frequency multipliers

are not recommended for use at high power levels or for a multiplication

factor exceeding n = 3.

Dept. of Telecomm. Eng.

Faculty of EEE

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Class D RF Power Amplifiers (1)

 Class D amplifier is a switching-mode amplifier that uses two active

devices driven in a way that they are alternately switched ON and OFF.

The active devices form a two-pole switch that defines either a rectangular

voltage or rectangular current waveform at the input of a load circuit. The

load circuit contains a band- or low-pass filter that removes the harmonics

of the rectangular waveform and results in a sinusoidal output.

The load circuit can be a series or parallel resonant circuit tuned to the

switching frequency. In practical applications, this circuit can be replaced

by narrowband pi or T-matching circuits, or by band- or low-pass filters

(in wideband amplifiers).

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

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Class D RF Power Amplifiers (2)

 Complementary Voltage Switching (CVS) Circuit

Input transformer T₁applies the drive signal to the bases of Q₁and Q₂in

opposite polarities. If the drive is sufficient for the transistors to act as

switches, Q₁and Q₂switch alternately between cut-off (OFF state) and

saturation (ON state). The transistor pair forms a two-pole switch that

connects the series-tuned circuit alternately to ground and V_dc.

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

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Class D RF Power Amplifiers (3)

The analysis below is based on the following assumptions:

. The series resonant circuit, tuned to the switching frequency, f, is

ideal, resulting in a sinusoidal load current. The CVS circuit requires

a series-tuned circuit or an equivalent (that imposes a sinusoidal

current), such as a T-network. A parallel-tuned circuit (or an

equivalent, such as a pi-network) cannot be used in the CVS circuit.

. The active devices act as ideal switches: zero saturation voltage,

zero saturation resistance, and infinite OFF resistance. The

switching action is instantaneous and lossless.

. The active devices have null output capacitance.

. All components are ideal. (The possible parasitic resistances of L

and C can be included in the load resistance R; the possible parasitic

reactance of the load can be included in either L or C).

Dept. of Telecomm. Eng.

Faculty of EEE

CSD2020

DHT, HCMUT

7

Bài giảng Mạch điện tử thông tin - Chapter 2: RF Power Amplifiers - Đỗ Hồng Tuấn