# AM Transmitter A simple One with explanation

Circuit Diagram

Working Principle

The improved Micropower AM Transmitter circuit has four sections as follows:

1. Crystal Oscillator Section

2. Audio Amplifier Section

3. Differential Amplifier Section

4. Antenna Matching Circuit

Each section is briefly described next.

Crystal Oscillator

A piezoelectric crystal such as quartz, exhibits electromechanical-resonance characteristics that are very stable and highly selective. The circuit symbol of crystal and its equivalent circuit is shown below.

The circuit of fig.(b) has two resonant frequencies.

A) Series resonant frequency ( fs )

B) Parallel resonant frequency ( fp )

The mathematical formulas given below

fs = ?s /2? = 1/(2??(LCs))

fp = (?p /2?)=(1/2?) ?( (Cp+ Cs) / LCpCs)

It is found that ?p > ?s as Cp >> CS. The reactance of the crystal is inductive over very narrow frequency band ?s & ?p . For a given crystal this frequency band is well defined .Thus we may use the crystal to replace the inductor of the Colpitts Oscillator.

In this circuit a Colpitts Oscillator is incorporated with crystal oscillator. The resulting circuit will oscillate at the resonance frequency of the crystal inductance L with the series equivalent of CS and Cp + {C1 C2 / (C1 + C2)}.

C1 & C2 are the 120 pF & 560 pF capacitors.

Since Cs is much smaller than the three other capacitance, it will be dominant and

?0 ~ 1 /?(LCs) = ?s

This oscillator circuit produces the carrier frequency of 1.8 MHz. We get this carrier frequency at the base of the Q1 transistor.

Audio Amplifier Section

In this section the audio signal is fed through the 10?F capacitor. Then this audio signal is fed to the base of the Q2 transistor. By the help of DC biasing, the transistor Q2 is kept in active region. This Q2 transistor amplifies the input audio signal & we get the amplified audio signal at the collector of the Q2. This collector point is connected to the common emitter point of the Differential Amplifier.

Differential Amplifier Section

In this section, transistors Q3 and Q4 comprise an emitter coupled differential amplifier. Here Q3 and Q4 are matched transistors forming a symmetric circuit. If we remove the audio signal then Q2 and corresponding biasing circuit will act as a constant current source. In DC bias condition if base voltages of Q3 & Q4 are equal (VCM) and if a constant current source is connected to the emitter of Q3 & Q4 then due to symmetry the current will divide equally between Q4 & Q3 and the common emitter point voltage will be almost VCM – VBE or (VCM – 0.7)V and the voltages of two collectors will be equal and the difference in voltage between collector is zero.

The gain of the differential amplifier is

Ad = -gm Rc [gm transconductance of Q1 & Q2 ]

The single ended gain is : Ad = – (1/2)gm Rc

The base of the Q4 is kept at a certain fixed voltage by proper dc biasing and the carrier signal of about 1.8MHz is given at the base of Q3 and the amplified output of the audio signal is fed to the common emitter point. Due to the varying emitter current at the audio rate, the transconductance varies at the audio rate affecting the gain. As a result, the amplitude of the carrier is varied according to the audio input signal and we get the amplified amplitude modulated wave at the output collector, which is fed to the matching circuit.

Antenna Matching Circuit

Antenna matching is very essential otherwise it may cause distortion in the transmitted signal. In this circuit the 0.1 ?F capacitor blocks the DC component and along with the variable capacitor ( 10 – 100 pF) it works as a matching circuit for the antenna. Here the 0.1 ?F capacitor is connected in series while variable capacitor is connected in parallel.

Applications:

1. A continuous-loop tape could give sales information to passing cars. Place a sign that says, “tune to xxxAM for information,” next to the house or car that is for sale.

v Transmit special seasonal music at Christmas or Halloween to enhance your decorations. (Use a similar sign.)

2. Transmit a cassette player or other audio source to the car radio for better sound.

3. Make a pair of toy AM band two-way radios by adding inexpensive AM radios. Or talk between cars on a trip using the car radio for reception.

4.  Build a fully functional radio station for the kids – complete with vu meters, slide faders, and an “on the air” light.