Explanation of transistor configuration in FM transmitter - Page 1
Forum section: electronics / beginners

author: d4n13l
author_rank: Regular Contributor
timestamp: July 08, 2019, 05:35:21 pm
content: 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.

author: mikerj
author_rank: Super Contributor
timestamp: July 08, 2019, 06:08:01 pm
content: 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.

author: iMo
author_rank: Super Contributor
timestamp: July 08, 2019, 06:58:21 pm
content: 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.

author: d4n13l
author_rank: Regular Contributor
timestamp: July 08, 2019, 08:59:53 pm
content: 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?

author: mikerj
author_rank: Super Contributor
timestamp: July 08, 2019, 09:42:23 pm
content: 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".

author: Audioguru
author_rank: Super Contributor
timestamp: July 08, 2019, 10:56:42 pm
content: I analyzed that horrible FM transmitter circuit more than 14 years ago and fixed its 4 problems:1) Its mic preamp transistor is saturated when the battery is new and it is cutoff when the battery voltage has dropped a little.2) The radio frequency changes as the battery voltage runs down.3) The radio frequency changes when something moves towards or away from its antenna.4) Its sound is muffled when heard on FM radios because it is missing pre-emphasis that all FM radio stations have.My fixes:1) I corrected the biasing of the preamp transistor and added a 5V low dropout voltage regulator IC that still regulates when the battery voltage has dropped to 5.6V.2) The voltage regulator also powers the oscillator transistor to keep its voltage and radio frequency stable.3) I added an RF amplifier/buffer transistor between the oscillator and antenna so that the radio frequency is stable.4) I added pre-emphasis.I agree that it probably will not work on a breadboard.I built mine on stripboard where each copper strip is cut to a short length and the cut strips, parts and a few jumper wires form a pcb. It sounds great and has a distance that is illegal unless its antenna is shortened.

author: d4n13l
author_rank: Regular Contributor
timestamp: July 09, 2019, 12:06:53 am
content: I'll have all that into consideration, thanks for the answers and the new circuit!

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 09, 2019, 01:02:19 am
content: Quote from: imo on July 08, 2019, 06:58:21 pmOk sounds good to me. I was confused by this type of Colpitt's before because it has a series tank circuit from Vcc to Q2Em, and I didn't consider that. Also I forget that C3 is meant to a LowPass filter, for the AM, and to block any of the RF from the Colpitt's.I really have to practice calculating multi-stage transistor circuits with AC analysis. Good circuit to start with

author: amyk
author_rank: Super Contributor
timestamp: July 09, 2019, 02:41:00 am
content: Here's my (very simplified and probably somewhat flawed) explanation of how it oscillates:Ignore Q2 for the time being, and consider only C4, L1, C5, and R7. When power is first applied, C4 and C5 charge through R7 with their tops positive, and meanwhile the current through L1 increases as it builds up its magnetic field; but as C5 charges, the current through it decreases. L1 resists this and its decaying magnetic field continues pushing current up the bottom of C4 and it becomes positive with respect to its top. The current through L1 goes to 0 and then reverses as C4 begins to discharging through it. Thus it should be clear that there will be oscillations, but owing to natural resistance and the energy lost through the radiation of the antenna, they will eventually die out, and a steady state will be reached where there is no more voltage across C4 and L1, and C5 will have charged to the full supply voltage.Now we can see what Q2 adds during each cycle of oscillation: when C4 and L1 are in the "clockwise" direction of the cycle, i.e. C4 is charging from the bottom, the top of C5 is positive and this reaches the emitter, keeping Q2 off or nearly so; but on the other half of the cycle, when the current through C4 and L1 is "anticlockwise", near the end of the cycle the top of C5 is pulled negative and this turns on Q2 by pulling its emitter down, causing more current to flow and thus replenish the energy of oscillation, causing it to continue indefinitely.