1) AM transmitters can use either low-level or high-level modulation. Low-level modulation occurs prior to power amplification, while high-level modulation occurs at the final output stage. 2) Low-level modulation is less efficient due to the use of linear amplifiers but requires less modulating power. High-level modulation is more efficient due to the use of Class C amplifiers but requires a high-power audio amplifier. 3) The key stages of an AM transmitter include an oscillator, buffer, modulator, and power amplifier(s) to boost the signal strength for transmission.

1. AM Transmitters
Prof.Dr.G.Aarthi,Associate Professor,VIT

2. AM Transmitter
AM
Transmitter
Low level High level
• A transmitter not only performs modulation
• Raises the power level of the modulated signal

3. • The RF Oscillator produces the carrier signal.
• The RF oscillator is stabilized in order to maintain the frequency deviation within the
prescribed limit.
• The carrier frequency is equal to the transmitter frequency.
• Crystal provides the best way to obtain the described frequency with good stability.
Low level AM transmitter

4. • The buffer is a class A amplifier which isolates the oscillator from the
rest of high power circuit and amplifies it to an adequate level.
• The modulating signal is obtained from source such as a microphone
and applied to a buffer processing unit.
Low level AM transmitter

5. • The amplified modulating signal is applied to the modulator along with
the carrier.
• At the output of the modulator we get the AM wave.
• The AM signal is then amplified using a chain of linear amplifier(class
A,AB or B) to raise the power level.
Low level AM transmitter

6. • The linear amplifiers are used in order to avoid the wave form
distortion in AM wave.
• The amplitude modulated signal is then transmitted using transmitted
antenna.
• The transistorized modulator circuits can be used for low level
modulator due to the low power which is to be handled.
• Overall efficiency is much lower compared to high level modulation.
Low level AM transmitter

7. • The AF modulating signal is passed through an audio processing unit
before applying it to the modulator.
• This block carries out some form of speech processing in the form of
filtering and amplitude control.
• The weak signals amplified automatically with a higher gain and strong
signals are amplified with smaller gain.
• This will bring all the signals to a sufficient level.
Low level AM transmitter

8. Low level AM transmitter
• Generation takes place at low power level
• Modulation takes place prior to the output element
• Linear amplifiers are preferred
• Less modulating power is required
• Extremely inefficient due to the usage of linear amplifiers
• Used in remote control,wireless intercom and walky
talkie

9. High level AM transmitter
• Many of the AM transmitters use the high level modulation technique.
• The crystal oscillator produces the required carrier signal.
• The class A amplifier following the oscillator acts as a buffer which isolates the
oscillator from the high power circuit.
• The output of this class A amplifier is applied to a class C power amplifier which
raises the power level of the carrier to an intermediately high value.

10. High level AM transmitter
• The modulation takes place in the last class C RF amplifier.
• The modulator output is AM wave which can be directly transmitted.
• The collector modulated transistorized circuit or plate modulated
vacuum tube modulator is used as modulator stage.
• The advantage of high level modulation is its high efficiency due to
the use of highly efficient class C amplifiers.
• The disadvantage is that a large AF power amplifier is needed to
raise the modulating signal to the adequate power level.

11. High level AM transmitter
• Generation takes place at higher power level
• Modulation takes place at the output element
• Class C amplifiers are preferred
• Extremely efficient due to the usage of Class C amplifiers
• High modulating power is required
• Used in high power broadcast transmitters