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Selecting Opamps and transistors

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kodiak

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Greetings friends! I've recently decided to design an AM transmitter as my first eletronic project, with a Wien's Bridge and the signal attached to a Common Emitter Amplifier (with a NPN). The Wien's bridge that I designed runs in the simulators with a frequency of 769kHz and its output goes from -9V to 9V. The whole circuit runs well in the simulators. I pretend to buy the components soon, but, I'm a complete amateur in this area and I don't really know how to choose a suitable Opamp ( I guess those 741 that I have access can't handle this frequency and range) and transistor. And so, would you have any suggestion of which components should I buy, and most of all, what should I take in consideration to decide? (I live in a small city, and then I'm not sure if the eletronics shop would have several different opamps, is there any guide with the most common ones?)
Thank you!
 

Audioguru

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The 741 opamp was introduced about 46 years ago and its spec's are horrible today. Look at its datasheet, it has trouble above 9kHz and its input and output voltages do not work within many volts from the positive or negative supply.
Why use an opamp at RF frequencies? Why use a Wien Bridge at RF frequencies? Why use a simulator that guesses about the spec's for parts?
Instead, use an LC oscillator using a transistor or two.

How will you produce amplitude modulation?
 

kodiak

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Hello, thank you for your reply! I'm taking a very introductory course in eletronics, i had to design a circuit that must contain opamps, diodes and transistors. Those 741 I only mentioned because these are the opamps I can use from the course, anything else I must buy. I read something about the oscilators and decided to make an AM transmitter, it is not meant to be anything perfect or commercialy applyable, just my first project for fun and for the course :) I would also know how to make the modulation with an analog multiplier of opamps, but it would not be interesting and the output signal would be even worse than the one from the circuit I'll do. I'm aware that attaching a signal in the Common Emitter Amplifier will not actually "modulate" it, but the effect is very similar (again, this is not meant to be something perfectly functional, I don't really mind with some noise, recognizing the signal in the radio is enough :) ). The "modulator" might be something like this one described here https://www.zen22142.zen.co.uk/spice/ammod.htm . The circuit seems to run well, and if it does half of what he is doing in the simulations, i'm happy :)
I see that LC should be better, but I really need to use an opamp and transistor at least.
What should I take in consideration to evaluate if an opamp is or not suitable for the circuit?
Thank you
 

betwixt

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I suggest using the transistors in as 1. an LC oscillator and 2. a modulator stage. The op-amp performance is poor by todays standards but if you keep it's gain low it will work as an audio amplifier so it can be used to boost the modulation signal voltage. I certainly wouldn't recommend a Wien bridge circuit at such high frequency and the 741 has absolutely no chance of working in an RF amplifier or oscillator stage.

Brian.
 

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