Radio-Frequency Circuit

2.1 Introduction2.2 High-Frequency Effects

2.3 Radio-Frequency Amplifiers2.4 Radio-Frequency Oscillators

2.5 Mixers2.6 Frequency Synthesizers

• SIGNALS

• CARRIER Frequencies

• AMPLIFIER and OSCILLATOR Circuit

• Different Design and Construction of the Circuit

• Techniques Used RF Circuit

• Different Devices

Microwave Communication

1. Frequency Multipliers2. Mixers

Audio Frequencies

• Radio frequency (RF) is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals.

• FrequenciesFrequency Wavelength Designation Abbreviation[5]

3 – 30 Hz 104 – 105 km Extremely low frequency

ELF

30 – 300 Hz 103 – 104 km Super low frequency SLF

300 – 3000 Hz 100 – 103 km Ultra low frequency ULF

3 – 30 kHz 10 – 100 km Very low frequency VLF

30 – 300 kHz 1 – 10 km Low frequency LF

300 kHz – 3 MHz

100 m – 1 km Medium frequency MF

3 – 30 MHz 10 – 100 m High frequency HF

30 – 300 MHz 1 – 10 m Very high frequency VHF

300 MHz – 3 GHz

10 cm – 1 m Ultra high frequency UHF

3 – 30 GHz 1 – 10 cm Super high frequency

SHF

30 – 300 GHz 1 mm – 1 cm Extremely high frequency

EHF

300 GHz - 3000 GHz

0.1 mm - 1 mm

Tremendously high frequency

THF

• Bypass Capacitor, that looked like a short ckt at 1 kHz was not longer so simple at 20 MHz.

• Some of these capacitances involved actual components

in the ckt, while others were incidental parts of

components, for example the junction capacitances in

transistors.

• Perhaps an amplifier contains a transformer or other inductive element.

• Reactive effects.

The Effect of Frequency on Device Characteristics

• Equivalent circuit for capacitor Transistor junction capacitances

As the frequency increases into gigahertz range, transit-time effects also become important. Transit-time is the time it takes a charge carrier to cross a device.

• NPN-Transistor• PNP-Transistor

It is the time were the holes exhibit transit-time.

It is the time taken for electrons to cross the base.

High-Frequency Construction Technique

• Inductive Coupling• Toroidal cores• Shielding• Bypassing

Ground plane can provide useful shielding.

• Radio communication

In order to receive radio signals an antenna must be used. However, sincethe antenna will pick up thousands of radio signals at a time, a

radio tuner is necessary to tune in to a particular frequency

(or frequency range).This is typically done via a resonator – in its

simplest form, a circuit with a capacitor and an inductor forming

a tuned circuit. The resonator amplifies oscillations within a

particular frequency band, while reducing oscillations at other

frequencies outside the band.

Narrowband Amplifiers

• (a) Low-frequency RC-coupled amplifier • (b) High-frequency transformer-coupled tuned amplifier

• Narrow range frequency• Also known as Baseband signals is a

signal that occupies the frequency range from 0Hz up to a certain cutoff. It is called the baseband because it occupies the base, or the lowest range of the spectrum.

Figure 2.5 Common-emitter amplifier

• Voltage gain

• 𝐴𝑣 =−(𝑅𝑐 ll 𝑅𝐿)

𝑟′𝑒Where: 𝐴𝑣= voltage gain as a ratio: 𝐴𝑣= 𝑣𝑜 𝑣𝑖.

𝑅𝑐 ll 𝑅𝐿= parallel combination of the collector resistance and the load resistance.

𝑟′𝑒= ac emitter resistance of the transistor.

• 𝑟′𝑒 =26𝑚𝑉

𝐼𝐸Where: 𝑟′𝑒= emitter resistance in ohms.

𝐼𝐸= dc emitter current in amperes.

• Resonant frequency

• 𝑓𝑜 =1

2𝜋 𝐿1𝐶1Where ∶ 𝑓𝑜= resonant frequency in hertz

𝐿1=primary inductance in henrys

𝐶1=primary capacitance in farad

• Bandwidth

• B=𝑓𝑜

𝑄Where: B = bandwidth

𝑓𝑜=resonant frequencyQ =loaded Q at resonance

A RF amplifier has the circuit shown in Figure 2.6. Find the (a) Operating frequency(b) BandwidthAssume that the loaded Q of the transformer primary is 15,

Figure 2.6

• In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the

input and output terminals.

• Although the term Miller effect normally refers to

capacitance, any impedance connected between the

input and another node exhibiting gain can modify the

amplifier input impedance via this effect. These properties of the Miller effect are generalized in the Miller theorem.

• Figure 2.7 Practical common-emitter amplifier w/ tapped primary.

• Figure 2.8 Narrowband RF amplifier w/ tuned input and output.

• Figure 2.9 Common-base amplifier.

Wideband Amplifiers • Also known as broadband signal is a signal which does not occupy the lowest range, but instead a higher range, 1MHz to 3MHz

Are classified according to the portion of the input cycle during w/c the active device conduct current and this is called “conduction angle”.

Amplifying devices operating in class A conduct over the whole of the input cycle. A class-A amplifier is distinguished by the output stage being biased into class A

Class-B amplifiers only amplify half of the input wave cycle, thus creating a large amount of distortion, but their efficiency is greatly improved and is much better than class A. Class-B amplifiers are also favored in battery-operated devices, such as transistor radios.

Class-C amplifiers is in RF transmitters operating at a single fixed carrier frequency, where the distortion is controlled by a tuned load on the amplifier. The input signal is used to switch the active device causing pulses of current to flow through a tuned circuit forming part of the load.

CLASS A B CConduction Angle 360˚ 180˚ <180˚ Maximum Efficiency 50% 78.5% 100%Likely Practical efficiency 25% 60% 75%

A process of cancelling the feedback.

neutralizing capacitor(electronics) Capacitor, usually variable, employed in a radio receiving or transmitting circuit to feed a portion of the signal voltage from the plate circuit of a stage back to the grid circuit.

• Figure 2.13 Neutralized RF amplifier.

• is an electronic circuit that generates an output signal whose output frequency is a harmonic (multiple) of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal.

Figure 2.14 Doubler waveforms

Input signal Output signal Collector current

•THE END!!!