Bài giảng Electronics circuits for communications - Chapter 2: Basic concepts in RF design - Đinh Quốc Hùng
CHAPTER 2:
BASIC CONCEPTS IN RF DESIGN
Gain, Attenuation, and Decibels
Gain
• Gain means amplification. It is the ratio of a
circuit’s output to its input.
Gain, Attenuation, and Decibels
Gain
• Most amplifiers are also power amplifiers, so the
same procedure can be used to calculate power
gain AP where Pin is the power input and Pout
is the power output.
Power gain (Ap) = Pout / Pin
• Example:
The power output of an amplifier is 6 watts (W). The
power gain is 80. What is the input power?
– Ap = Pout / Pin -> Pin = Pout / Ap
Pin = 6 / 80 = 0.075 W = 75 mW
Gain, Attenuation, and Decibels
Gain
• An amplifier is cascaded when two or more stages are
connected together.
• The overall gain is the product of the individual circuit
gains.
• Example
Three cascaded amplifiers have power gains of 5, 2, and
17.The input power is 40 mW. What is the output power?
– Ap = A1 × A2 × A3 = 5 × 2 × 17 = 170
– Ap = Pout / Pin ->Pout = ApPin
– Pout = 170 (40 × 10-3) = 6.8W
Gain, Attenuation, and Decibels
Attenuation
• Attenuation refers to a loss introduced by a
circuit or component. If the output signal is
lower in amplitude than the input, the circuit
has loss or attenuation.
• The letter A is used to represent attenuation
Attenuation A = output/input = Vout/Vin
• Circuits that introduce attenuation have a gain
that is less than 1.
• With cascaded circuits, the total attenuation is
the product of the individual attenuations.
Gain, Attenuation, and Decibels
Attenuation
Gain, Attenuation, and Decibels
Attenuation
Gain, Attenuation, and Decibels
Decibels
• The decibel (dB) is a unit of measure used to
express the gain or loss of a circuit.
– The decibel was originally created to express hearing
response.
– A decibel is one-tenth of a bel
• When gain and attenuation are both converted
into decibels, the overall gain or attenuation of a
circuit can be computed by adding individual
gains or attenuations, expressed in decibels
Gain, Attenuation, and Decibels
Decibels
• Decibel Calculations:
Voltage Gain or Attenuation
– dB = 20 log Vout/ Vin
Current Gain or Attenuation
– dB = 20 log Iout/ Iin
Power Gain or Attenuation
– dB = 10 log Pout/ Pin
Gain, Attenuation, and Decibels
Decibels
• Example:
– An amplifier has an input of 3 mV and an output
of 5 V. What is the gain in decibels?
dB = 20 log 5/0.003 = 20 log 1666.67= 20 (3.22)
= 64.4
– A filter has a power input of 50 mW and an
output of 2 mW. What is the gain or attenuation?
dB = 10 log (2/50)= 10 log (0.04)= 10 (−1.398)
=−13.98
Gain, Attenuation, and Decibels
Decibels
• dBm and dBc:
– When a decibel value is computed by comparing
a power value to 1 mW, the result is a value
called the dBm. This is a useful reference value.
– The value dBc is a decibel gain attenuation figure
where the reference is the carrier.
• Example dBm:
The output of a 1W amplifier expressed in dBm
Gain, Attenuation, and Decibels
Decibels
• Example dBc:
If the spurious signal is 1 mW compared to the 10 W carrier,
the dBc is:
Gain, Attenuation, and Decibels
Quiz:
• A power amplifier has an input of 90mW
across 10k ohm. The output is 7.8V
across an 8 ohm speaker. What is the
power gain in decibels?
Reactive Components
• Virtually all communications equipment
contains tuned circuits, made up of inductors
and capacitors that resonate at specific
frequencies.
Reactive Components
Capacitors
• Example:
What is the capacitive reactance of a 100-pF capacitor at 2 MHz?
– Xc = 1/2πfC = 1/6.28 (2 ×106) (100 × 10−12 )
= 796.2 Ω.
What a capacitor looks like at high frequencies
Reactive Components
Inductors
• Example:
What is the inductive reactance of a 40-μH coil at
18 MHz?
– XL = 6.28 (18 × 106) (40 × 10-6 )= 4522 Ω
Types of inductors:
(a) Heavy selfsupporting wire
coil.
(b) Inductor made as copper
pattern. (c) Insulating form.
(d) Toroidal inductor.
(e) Ferrite bead inductor.
(f ) Chip inductor.
Reactive Components
Inductors
Reactive Components
Resistors
• At low frequencies, a standard resistor offers
nearly pure resistance.
• At high frequencies, a resistor’s leads have
inductance.
• A resistor’s lead inductance and stray
capacitance cause the resistor to act like a
complex RLC circuit.
• Tiny resistor chips used in surface mount circuits
minimize inductance and stray capacitance.
• Film resistors minimize thermal effect noise.
Reactive Components
Resistors
Equivalent circuit of a resistor at high (radio) frequencies.
Skin Effect
• Skin effect is the tendency of electrons
flowing in a conductor to flow near and on
the outer surface of the conductor
frequencies in the VHF, UHF, and microwave
regions.
• This process increases the resistance of the
conductor and greatly affects the
performance of the circuit.
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