Opamp VHF FM Transmitter

  
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ICs that in the past were far too expensive for the hobbyist tend to be more favourably priced these days. An example of this is the AD8099 from Analog Devices. This opamp is available for only a few pounds. The AD8099 is a very fast opamp (1600 V/ms) and has high-impedance inputs with low input capacitance. The bandwidth of the opamp is so large
Opamp VHF FM Transmitter - schematic

that at 100 MHz it still has a gain of nearly 40. This means that this opamp can be used to create an RC oscillator. The circuit presented here realises that. The circuit has a few striking characteristics. Firstly, unlike normal oscillators that contain transistors this one does not have any inductors. Secondly, there is no need for a varicap diode to do the FM modulation. The opamp is configured as a Schmitt trigger with only a small amount of hysteresis. The output is fed back via an RC circuit. In this way, the trimmer capacitor is continually being charged and discharged when the voltage reaches the hysteresis threshold. The output continually toggles as a consequence. This results in a square wave output voltage. With a 10-pF trimmer capacitor the frequency can be adjusted into the VHF FM broadcast band 88-108 MHz). The frequency of the oscillator is stable enough for this. The output voltage is about 6 Vpp at a power supply voltage of 9 V. The transmitter power amounts to about 50 mW at a load of 50R. This is about 20 times as much as the average oscillator with a transistor. With a short antenna of about 10 cm, the range is more than sufficient to use the circuit in the home as a test transmitter. Because the output signal is not free from harmonics the use of an outdoor antenna is not recommended. This requires an additional filter/adapter at the output (you could use a pi-filter for this). The FM modulation is...



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