Concept of Decibels for Gain and Loss

1. Alexander Graham Bell was a Scottish-born scientist, inventor,
engineer, and innovator who is credited with patenting the first
practical telephone and founding the American Telephone and
Telegraph (AT & T)Company in 1885

2. https://www.youtube.com/watch?v=4lJ6Pwb15JY

3. Learning Objectives
• Understand the concept of gain and loss in
electronic circuits
• Learn the concept of decibels (dB)
• Determine voltage gain and loss in dB

4. Voltage Gain of amplifier
• Voltage gain of amplifier is Av:
Av= (
𝑉 𝑜𝑢𝑡
𝑉𝑖𝑛
)
Where 𝑉 𝑜𝑢𝑡 is the output voltage and 𝑉𝑖𝑛 is the
input voltage
• Calculate the gain of amplifier if the output
voltage is 1V and input voltage is 25mV
• Calculate the gain of amplifier if the output
voltage is 1V and input voltage is 1mV

5. What are decibels dB ???
• Decibels are easy way of representing the gain
(amplification) and loss (attenuation) of
electronic circuits, specially amplifier.
• Decibels are mainly used for measuring power
level and sound levels.

6. Power gain in dB
• Power gain in decibel is given as:
Ap (dB) = 10 𝐥𝐨𝐠 𝟏𝟎 (
𝑷 𝒐𝒖𝒕
𝑷𝒊𝒏
)
Where Pout is the output power and Pin the input
power.
• Calculate power gain in dB if output power is 40 W and
input power is 20W.
• Calculate power gain in dB if output power is 20 W and
input power is 40W.
• Positive value means gain and negative value means
loss.

7. Voltage gain in decibel
• Voltage gain in decibel is given as:
Av (dB) = 20 log 𝟏𝟎 (
𝑉 𝑜𝑢𝑡
𝑉𝑖𝑛
)
• Deci- (symbol d) is a decimal unit prefix in the
metric system denoting a factor of one tenth.
Decibel =
1
10
Bel
• Bel is a unit originally designed for measuring
losses of telephone cables.

8. Point out the differences between the scales.

9. Advantages of Logarithmic scale
• It accommodates more number of points/data.
• The scale is not linear.
• Our ears responds to the sound in logarithmic
way, using a decibel scale gives a more
meaningful representation of audio levels.
• In many stages of amplifier or antnenna, if gain
is in dB (or log), it can be simply added.

10. Problems
• Calculate the decibel gain of an amplifier if the
output voltage is 100mv and input voltage is
1mV.
• An amplifier provides an output voltage of 1V
for an input of 1mV. Express the voltage gain of
the amplifier in decibels.
• Differentiated:
• An amplifier provides power gain of 33dB. What
output power would be produced if an input of
2mW is applied ?
http://www.bcae1.com/sig2nois.htm

11. Decibels with reference
• Often decibel are expressed as absolute
power in reference with one milliwatt
written as dBm.
• It is useful in radio, microwave and fibre
optic communication.
dBm = 10 *log10
𝑃𝑜𝑢𝑡
1𝑚𝑊

12. Loss (Attenuation)
• Av=(
𝑉 𝑜𝑢𝑡
𝑉𝑖𝑛
) if Vout > Vin then there’s gain
• If Vout < Vin, then there’s a loss.
• Attenuation is defined as:
Av=( 𝑽𝒊𝒏
𝑽 𝒐𝒖𝒕
)
• Therefore loss in dB can be expressed as:
Av(dB) = 20 𝐥𝐨𝐠 𝟏𝟎(
𝑽𝒊𝒏
𝑽 𝒐𝒖𝒕
)

13. • A transmitter transmits 150w of signal through
a length of co-axial transmission line. The
power measured at the output end of the line
is only 112 watts. What is line loss in dB ?
• Determine the attenuation if the output is 1mV
and input is 20mV.
http://www.bcae1.com/sig2nois.htm

15. A typical conversation occurs at 60 dB – not loud enough to cause damage.

16. Plenary
• Summarize at least three points you have
learned from today’s session about loss and
gain.
• Write down one doubt that still persists in your
mind.
• How can you use this knowledge in your real
life ???