AM

1. AM Generation
Prof. Dr. G.Aarthi,
Associate Professor, SENSE

2. Types of AM modulator
• Amplitude modulators - AM wave generating circuits
Amplitude
Modulators
Nonlinear
Square law
Balanced
Linear
Low level
Product
Transistor
• Based on power level

3. Low level modulators
• Generation takes place at low power level
• Modulation takes place prior to the output element
• Linear amplifiers are preferred
• Advantages:
• Less modulating power is required
• Disadvantages:
• Extremely inefficient due to the usage of linear amplifiers

4. High level modulators
• Generation takes place at higher power level
• Modulation takes place at the output element
• Class C amplifiers are preferred
• Advantages:
• Extremely efficient due to the usage of Class C amplifiers
• Disadvantages:
• High modulating power is required

5. Linear modulator
• The Devices are operated in the linear region of its
transfer characteristics.
• The relation between the amplitude of the modulating
signal and the resulting depth of modulation in linear.
• Switching modulator
• Transistor modulator

6. Nonlinear modulator
• Devices used in this modulation are operated in the
non linear region of its characteristics.
• Uses non linear property of diode, BJT and FET
• Simple diode can be used as a nonlinear modulator
• Undesired frequency terms are filtered using BPF
• Square law modulator
• Balanced modulator
• Product modulator

7. Square law modulator
Let the modulating and carrier signals be denoted as Vm (t) and Vc (t) respectively.
These two signals are applied as inputs to the summer (adder) block. This
summer block produces an output, which is the addition of the modulating and
the carrier signal.

8. Square law modulator
• Semiconductor diodes and transistors are commonly used
• Filtering done using single or double tuned filter (BPF)

9. Square law modulator
• The input to the nonlinear device is given as
V1(t) Vm (t) Vc (t)      (1)
V1(t) Vm cosmt Vc cosct      (2)
• The input-output relation for a nonlinear device is given as

10. Square law modulator
• The LC circuit acts as a bandpass filter and is tuned to the
frequency of fc and its bandwidth is equal to 2fm
• Therefore it allows ωc, ωc+ωm, ωc-ωm

11. Square law modulator
• The filter circuit is tuned to the frequency of fc and its bandwidth is
equal to 2fm
• Therefore it allows ωc, ωc+ωm, ωc-ωm
• The output of the BPF is given as
V0 (t)  aVc cosct  bVmVc[cos(cm)t  cos(cm)t]     (6)

12. Non linear Modulators-Drawbacks
• Heavy filtering is required to remove the unwanted terms present
in the output of the modulators.
• The output power level is also very low.
• Hence a substantial linear amplification is necessary to bring the
power upto the desired level.

13. Switching modulator

14. Switching modulator
• Simple diode can be used as an AM switching modulator
• Consider the diode is ideal and carrier is stronger
message signal
than the
• Diode is forward biased – Positive half cycle of the carrier [c(t)>0]
• Diode is reverse biased – Negative half cycle of the carrier [c(t)<0]

15. Switching modulator
• The transfer characteristic of the diode is approximated by a
piecewise linear time varying relationship
• The input voltage is given as
V1(t) Vc cosct  m(t)      (1)

16. Switching modulator
0;c(t)  0
2
      (2)


• The transfer characteristic of the diode is approximated by a
piecewise linear time varying relationship
• The input voltage is given as
V1(t) Vc cosct  m(t)      (1)
• The resulting voltage is given as
V1(t);c(t)  0
V (t) 

17. Switching modulator
0;c(t)  0
2
      (2)


• The transfer characteristic of the diode is approximated by a
piecewise linear time varying relationship
• The input voltage is given as
V1(t) Vc cosct  m(t)      (1)
• The resulting voltage is given as
V1(t);c(t)  0
V (t) 
• Mathematically, Equation (2) is expressed as
V2 (t)  [Vc cosct  m(t)]gp (t)      (3)
• Where gp(t) is a periodic pulse train

18. Switching modulator
• Representing gp(t) by its Fourier series we have
• Sub Eq.(5) in Eq.(3), the filter output equation is given as
• The unwanted components are removed by the BPF with the mid-
band frequency fc and bandwidth 2W
cos[2
 

n1 2n1
1 2 (1)n1
2  c
f t(2n 1)]     (4)
p
g (t) 
2
V 4
0
V (t) 
c
c
c
V
cos t[1 m(t)]     (6)
c
p c
g (t) 
1

2
cos t 
2
cos3 t ....   (5)
2  3
Vc
Amplitude Sensitivity k 
4