CommunicationElectronics Principles & Applications Third Edition Chapter 6 Radio Transmitters ©2001 Glencoe/McGraw-Hill Louis E. Frenzel

TRANSMITTER COMPONENTS Carrier signal source, usually a crystal oscillator or a frequency synthesizer Multiple RF amplifiers to increase power level High power final amplifier Impedance matching circuits to ensure maximum power transfer Modulator

Carrier oscillator Audio amplifier Driver AM TRANSMITTER WITH HIGH-LEVEL MODULATION Speech processing Final power amplifier Buffer Driver Modulation amplifier

Sideband filter Microphone Audio amplifier Linear driver amplifier Antenna TYPICAL SSB TRANSMITTER Balanced modulator Speech processing Carrier oscillator Linear power amplifier LO Mixer Buffer Tuned circuit

Phase modulator Microphone Audio amplifier Driver TYPICAL FM TRANSMITTER USING INDIRECT FM Speech processing Carrier oscillator Final power amplifier Buffer Frequency multipliers

CLASSES OF AMPLIFIERS Class A –Conducts continuously –Linear, lowest distortion –Least efficient (<50%) Class B –Conducts for 180º of input –More efficient than class A –Push pull circuit reduces distortion

CLASSES OF AMPLIFIERS (continued) Class C –Conducts for less than 180º –Highly efficient –Generates distortion and harmonics Class D and E –Switching amplifiers –Introduce distortion and harmonics –Most efficient (>90%) –Use BJT and enhancement mode MOSFETS

Class A Operation

Class B Operation

+50 V 3 nF 0.47  F 10  H 410 pF 20  300 pF 2.7 nH 3 nF 20 nH 0.1  F 16:1 9:1 MRF KW PUSH-PULL RF POWER AMPLIFIER RF in RF out Note: gate bias circuits not shown

Negative feedback provides stability and wideband performance. Power MOSFETs provide a power gain of 11 to 14 dB. Toroid input and output transformers provide impedance matching, push-pull operation, and broad-band performance (10 to 90 MHz). Bias Gate bias circuits maintain Class B operation for good efficiency and minimum distortion.

TUBES VERSUS TRANSISTORS IN POWER AMPLIFIERS Individual RF transistors can achieve power levels up to about 500 watts. Using a push pull circuit and/or paralleling transistors produces power levels over 1 kW. For power levels over 1 kW, the outputs of several transistor amplifiers can be combined. For power levels over about 2 kW, tube amplifiers are easier to implement and cheaper.

Crossover Distortion If we look at the diagram for the input and output waveform, we can see some distortion at the cross over where one transistor stops conducting and the other one starts to conduct. A small base-emitter voltage is needed to turn on a transistor QN actually only conducts when vin > 0.7 V QP actually only conducts when vin < -0.7 V When 0.7 > vin > -0.7, nothing conducts and the output is zero. i.e. the input-output relationship is not at all linear.

Class C Amplifier

IMPEDANCE MATCHING NETWORKS Impedance matching networks are used between amplifier stages and between the final amplifier and the antenna load to ensure maximum power transfer. Impedance matching networks consist or inductors and capacitors combined in , T or L configurations.

TRANSFORMERS AND BALUNS Transformers are widely used between amplifier stages for impedance matching. A balun is a special type of transformer that is also used for impedance matching between stages.

SPEECH PROCESSING CIRCUITS Special circuits are used to modify and enhance the voice signal from the microphone. Speech processing includes filtering, level- clipping or compressing to minimize bandwidth. In modern wireless equipment, speech processing is performed by a digital signal processor (DSP).