Radio frequency circuit

Radio-Frequency Circuit
2.1 Introduction
2.2 High-Frequency Effects
2.3 Radio-Frequency Amplifiers
2.4 Radio-Frequency Oscillators
2.5 Mixers
2.6 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 Multipliers
2. 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.
• Frequencies
Frequency 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, since
the 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
• 𝐴 𝑣 =
−(𝑅 𝑐 ll 𝑅 𝐿)
𝑟′ 𝑒
Where: 𝐴 𝑣= voltage gain as a ratio: 𝐴 𝑣= 𝑣 𝑜 𝑣𝑖.
𝑅 𝑐 ll 𝑅 𝐿= parallel combination of the collector resistance and the load resistance.
𝑟′ 𝑒= ac emitter resistance of the transistor.
• 𝑟′ 𝑒 =
26 𝑚𝑉
𝐼 𝐸
Where: 𝑟′ 𝑒= emitter resistance in ohms.
𝐼 𝐸= dc emitter current in amperes.
• Resonant frequency
• 𝑓𝑜 =
1
2𝜋 𝐿1 𝐶1
Where ∶ 𝑓𝑜= resonant frequency in hertz
𝐿1=primary inductance in henrys
𝐶1=primary capacitance in farad
• Bandwidth
• B=
𝑓𝑜
𝑄
Where: B = bandwidth
𝑓𝑜=resonant frequency
Q =loaded Q at resonance

A RF amplifier has the circuit shown in Figure 2.6. Find the
(a) Operating frequency
(b) Bandwidth
Assume 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 C
Conduction 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

Radio frequency circuit

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Radio frequency circuit