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| Explanation of transistor configuration in FM transmitter |
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| d4n13l:
Hi, I'm starting as an electronics hobbiest, right now I'm trying to adapt this circuit to a 5V input supply as an exercise. I have questions about the last part of the circuit. What I understand is that Q2 together with C4, L1 and C5 make the carrier wave, however I'm unsure about how Q2 is operating. Initially I thought that Q2 is in a common emitter configuration, where you get the input from C2 to the base and output from the collector. However I've seen on comments over the web that it is in a common base configuration, where I get that C5 would provide the input, but in all tutorials I've seen for that configuration the base needs to be grounded which I don't see here, also I don't know what C3 is doing. If someone can help me clarify this, it would be much appreciated. |
| mikerj:
Common base configurations are often shown with the base grounded when the circuit is explained in books or websites, but that doesn't have to be the case. The transistor needs to be biased to the required operating point and in the simple explanatory circuits this is achieved with a voltage source applied across Base/Emitter If the base was grounded (0v) in a real circuit, this would require a negative voltage on the emitter to bias the transistor, in a single rail circuit the base and emitter have to operate above 0v. Most of these FM bugs are pretty terrible designs. In the circuit shown the transistor is biased using a single resistor to the supply which will give poor biasing performance, with the operating point having dependence on transistor parameters, temperature and battery voltage, but usually works well enough. |
| iMo:
Q2 is an oscillator (Collpits). C5 creates a positive feedback from collector (hot side of the LC tank) to emitter such it oscillates. C4 and L1 tunes to a TX carrier frequency (ie. 100MHz). The R7 isolates the high TX freq from ground (sometimes a few uH choke is used instead of the R7). C3 "grounds" the Q2's base for high frequencies (+9V and GND is the "same" when talking "grounding", I would add a 10nF||10uF decoupling capacitors from +9V to GND to make the grounding better), but allows the audio freqs (ie. 300Hz-5kHz) to enter the Q2s base. That causes the FM and AM modulation of the Q2 by the audio frequencies coming from Q1 via C2 (Q1 is an audio freq amplifier with gain aprox 100). The R6 sets the Q2 operating (collector) current such a) it oscillates, b) you get certain power out of the LC tank. In most countries (double-check your legislation) the output power of these "bug transmitters" is limited to a couple of mW as the signal they create is a crap. Also transmitting any signal per se could be considered illegal in many countries, unless you own a special permission. Larger powers are always illegal with these kind of transmitters. |
| d4n13l:
Hi, thanks for the answers, that helped understanding the circuit, I have a more practical question now. I'm having trouble getting it to work on a breadboard, I tried to make it as compact as possible and use as little wire as possible, but would the fact that is on the breadbord be enough for it not to work? |
| mikerj:
High frequencies and breadboards just don't mix, the stray capacitance's will be of similar magnitude as the feedback cap in this circuit. It's a simple enough circuit that you could build it freeform (i.e. just form the leads and solder together) or build it on a small piece of PCB, "Manhattan style". |
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