# RF Linear Power Amplifiers

Power amplifiers(PA) are broadly classified into two categories, linear PAs and switch-mode PAs. In linear power amplifiers the active device works as a transconductor, whereas in switching mode power amplifiers it act as a switch.

Below are the assumptions made to simplify the following analysis:
– active device is ideal VCCS or ideal switch
– input is single tone

### Introduction

The schematic of a linear power amplifier is shown in Figure 1. The same amplifier works in class-A, class-AB, class-B or class-C mode depending on conduction angle of the active device.

For , MOS transistor MO behave as a linear transconductor. Inductor L1 acts as DC short. Capacitor C1 blocks DC current flowing to load and acts as short to ac component. and form the tank circuit whose resonant frequency . The tank circuit is required to suppress harmonics in the output. The nonlinearity of active device generate harmonics, which are suppressed by high Q tank circuit. Figure 1. Linear RF Power Amplifier

### Class-A Power Amplifier

In class-A amplifier the active device conducts for full cycle. Therefore conduction angle ( ) = 360 degrees

Under queiscent operating conditions, ; ; Due to voltage signal at the gate input, the drain current is voltage across load, drain to source voltage of , For the transistor to operate in saturation region, .
Therefore peak current is  Maximum output voltage, = (1) Power drawn from source or DC biasing power,

(2) Maximum drain efficiency,

(3) ### Class-B Power Amplifier

In Class-B mode the active device ( ) acts as a transconductor with conduction angle of 180 degrees.

Under quiescent conditions, , and .

Due to small signal at the input of the transistor, the drain current is given by

(4) By Fourier series,

(5) The DC part of drain current flows only through inductor , where as fundamental and higher order harmonic current find the low impedance path through capacitor . The resonant tank offers low impedance path or appears as short to harmonics and high impedance to fundamental component. Therefore only fundamental flow through load resistor .

Swing limits As the drain current increases and device enters into cutoff.
To keep the device in saturation, .
The current is maximum under this condition, and is given by  Max drain efficiency, Power drawn from source under maximum output power, Maximum drain efficiency,

(6) ### Class-C Power Amplifier

Conduction angle : From the Figure, Transistor drain current,

(7) By Fourier series, DC and fundamental components are given by

(8) Swing limits
Voltage at the drain Maximum fundamental current is limited by voltage swing at drain Maximum average output power, Power drawn from source at max output power, Max. Drain efficiency, The drain efficiency is maximum, when .

 Class-A Class-AB Class-B Class-C Conduction Angle( )    Drain Efficiency 50% 50% – 78.5% 78.5% 78.5% – 100% Linearity Best Fair Good Poor

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