AM Transmitter Circuit Diagram Using 741 Op-amp

AM transmitter is a circuit which can transmit message signal to modulated signal. This circuit is designed with limited power and the required power supply of the transmitter circuit is 9 Volt.

The circuit has three parts that is an audio amplifier, radio frequency oscillator and modulator circuit. The frequency oscillator is built with 741 Op-amp and related components. The carrier signal frequency and its amplitude can be varied using variable resistor accordance with V_R1 and V_R2  respectively.  C₁ and C₂ are the main components to generate the carrier frequency.

Another part of the circuit is an audio amplifier circuit. The audio amplifier is built with 741 Op-amp and related components. A microphone is used to convert the voice signal to the audio signal which is feed to the op-amp’s inverting terminal. This audio signal is amplified by the op-amp. The amplified audio signal is filtered using the capacitor C₇. This output is feed to the modulator circuit.

The main part of the AM transmitter is modulator circuit which is built with the transistor BC109. The carrier signal is feed to the base of transistor and the message / audio signal is also feed to the emitter of the transistor.

Here the required modulated signal is taken from the collector of the transistor which is feed to the output antenna.

Design a Temperature Indicator

This circuit is a temperature indicator circuit or differential instrumentation amplifier using a transducer bridge. This circuit is calibrate in degrees Celsius or Fahrenheit. In the circuit used buffer in  and  points for exact voltage of  and  points. Because gain is always 1 of buffer circuit. Then the output voltage of buffers is input voltage of differential amplifier. 

The differential amplifier is difference voltage of  and  points using 741 Op Amp. When temperature is increased then resistance  is also decreased and output voltage  is decreases and when temperature is decreased then resistance  is also increased and output voltage  is increases.

The temperature indicator is a circuit that indicates of temperature in degrees Celsius or Fahrenheit. The temperature is inversely proportional to the resistance or transducer.

  The Fig.01 simplified a temperature indicator circuit using a transducer bridge. A restive transducer whose resistance changes as a function of some physical energy or temperature is connected in one arm of the bridge with a small circle around it and is denoted by , where  is the resistance of the transducer and  is the change in resistance.

Fig.01 - Temperature indicator

In the circuit used as the transducer in the bridge circuit is a thermistor and replaced output voltmeter to temperature indicating meter. Then temperature indicating meter is calibrate in degrees Celsius or Fahrenheit. 

The bridge can be balanced at a desired reference condition, for instance 25⁰C. As the temperature varies from its reference value, the resistance of the thermistor changes and the bridge become unbalanced. This unbalance bridge in turn produces the meter movement.