5. 5 CASCADED AMPLIFIERS
™ Amplifiers FF cascaded together to provide stage
FF by FF stage power amplifier.
) For two power amplifier of FF G1 Û G2 FF
cascaded
) the total power gain
GT = G1 x G 2
™ In general
) If n amplifiers of G1 Û G2 Û Gn FF cascaded
) the total gain
GT = G1 x G2 x ..... x Gn
™ In dB
GT (dB) = G1 (dB) + G2 (dB) + ..... + Gn (dB)
Example (5. 5.1)
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™ Three amplifiers of 2, 5, and 10 gain power are
connected in series.
™ What is the total gain ?
GT = 2 x 5 x 10 = 100
Example (5. 5. 2)
™ Repeat in dB
GT (dB) = 3 (dB) + 7 (dB) + 10 (dB) = 20 dB
5. 6 GAIN AND INSERTION LOSS COMBINED
™ When using IL and G in practical applications FF all
the calculations will be FF in dB .
amplifier
Pin
P
out
cable
Pm
٢
™ In the fig FF Pin applied to one end of transmission
line of certain IL
™ Pin FF attenuated FF when pass through the cable
) amplified by an amplifier of a given fixed gain.
What is the output power ?
 Pin = 10 dBm (10 mW) and the total IL = 1 dB for
the line
 Pm (the signal coming out of the cable) FF Pout
from the cable and Pin to the amplifier.
Pm (dBm) = 10 dBm - 1 dB = 9 dBm
From B
Pout (dBm) = 9 (dBm) + 20 (dB) = 29 dBm
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 All the above steps of finding Pout FF can be
summarize as :
Pout (dBm) = Pin (dBm) - IL + G (dB)
OR
Pout = Pin x G / IL
Where power in any consistent unit Û G Û IL are
dimensionless.
Example ( 5.6.1)
 The signal power leaving a generator is 3 dBm.
 It is connected to an amplifier by a cable 4 ft long.
 The attenuation per foot of cable is 0.2 dB/ft
 The gain of the amplifier is 30 dB .
What is the output power ?
Solution:
) The total attenuation of the cable FF 4ft long.
IL = 4 x 0.2 dB = 0.8 dB.
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) The output power of the amplifier is
Pout (dBm) = 3 dBm - 0.8 dB + 30 dB = 32.2 dBm
Example (5.6.2)
G = 20 dB
G3 = 10 dB
G = 15 dB
1
2
1
2
cable 2
1 ft
cable 1
0.3 ft
3
cable 3
5 ft
Pout
 In the Fig. FF three amplifiers connected in series
by cables.
 The gain and lengths FF of the individual
components are shown
If Pin = - 5 dBm
Calculate Pout
Solution
Pout (dBm) = -5 dBm – 0.3 dB + 20 dB – 1 dB + 15 dB – 5
dB + 10 dB = 33.7 dBm
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