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EDIT: I've seen an alert under my question:
Please clarify your specific problem or add additional details to highlight exactly what you need.

Maybe this is due to the fact that I'm not very good to express myself in English, I'm sorry; I'm also a novice about these subjects: so, again, I apologize for my mistakes with the language and for my schematic. So, considering my exposed limits, I underline the part of my question where I stated “However I have some exact questions about” with a list of four question about the problem which I encountered: I asked If i correctly designed the amplification stage and if there is a better way to amplify the output of the oscillator.

Thank you all for your kind understanding.

I start by saying that I'm doing my “experiments” in a building located in a rural location and that, however, local laws allows to use low power rf transmitters.

Well: since I need to transmit an FM signal to a distance of about 30 feet inside such building, and the output signal have to pass through the walls, due to the fact that I don't have the space for a longer antenna, I was thinking to implement an amplifier stage on the output of the transmitter. The following image shows the full schematic:

Schematic

Where the amp stage is still not present, and on the base of Q1 there will be a female jack where to inject the audio signal.

By a search on internet I've found that there are many similar circuits where the amp stage, most of the time is made using a “common emitter” transistor instead of a “emitter follower”, but some of them (few), however, rely on the emitter follower. I choosed the emitter follower configuration because I figured out that the emitter follower has a lower gain (usually almost equal to the input) and so for avoiding to exceed with the output power. I calculated the values of R4, R5 and R6 looking at the BFR36 datasheet and then I've found an online calculator where I inserted these values to get the values of these resistors.

However I have some exact questions about:

1) Is the emitter follower shown in my schematic, correctly designed? I have some doubts about the resistor R4 (maybe the value is too much great?)

2) Are R6 and R5 really necessary, or I can just leave R6 and get rid of R5?

3) Will be able, the amp stage showed in my schematic, to boost however the output signal in terms of strength and distance? I mean: is possible to estimate the power boost which I will get?

4) Can someone suggest me some improvements to the amp stage of my circuit or suggest me a different approach, maybe using a common emitter configuration?

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closed as unclear what you're asking by Olin Lathrop, Daniel Grillo, uint128_t, Asmyldof, PeterJ Apr 7 '16 at 0:01

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ -1 for the rotated schematic. That's the first thing I saw, so I never read the rest and downvoted instead. Not only are you disrespecting those you seek a favor from, but it was the very first impression. As a result, you don't get a chance to make a second impression. Now that you have thumbed your nose at me, my reaction is screw this, and I'm voting to close too. This in part because I don't know what you're asking because I'm not going to tilt my head sideways, and in part to teach you a lesson by not giving you the desired result. Next time show some respsect. \$\endgroup\$ – Olin Lathrop Apr 6 '16 at 13:05
  • \$\begingroup\$ Hi Olin, I am very sorry and I can assure you that wasn't my intention disrespect you and the other people from which i seek an help. In fact I'm replying to you to show you my respect. I'm not very able to draw schematic with the software LTspice IV, so this is the result of my inexperience. I'm very sorry. \$\endgroup\$ – Giov Apr 6 '16 at 13:12
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    \$\begingroup\$ Do you understand that your antenna represents a fixed impedance? And that this implies that you cannot increase the signal power without changing BOTH the voltage and current going into it? An emitter follower is useless in this application, since it has less than unity voltage gain. \$\endgroup\$ – Dave Tweed Apr 6 '16 at 13:48
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An emitter follower doesn't amplify the voltage of a signal, but it amplifier the current driving capability. For an FM transmitter with a small antenna (high impedance), what you really need is increased voltage, not increased current.

There are ways to match impedances between an emitter follower and an antenna, but they require many components and some tuning to get it correct.

You would be better to use a common-emitter amplifier and you can reuse some of the bias circuits for Q1: Tie the base of Q2 to the base of Q1 via 10k, and add 27 pF between Q2's base and Q1's collector. Connect the emitter of Q2 to GND via R4 -- 47 ohm -- bypassed with 10 nF. This will run about 100 mA (same as Q1 -- seems very high for this application). Put an inductor (choke) or 27 ohm resistor between the collector and supply, and connect the antenna to Q2's collector.

Your biasing for Q1 seems high -- generally 470 ohm or even 4.7k should be OK for R1 (10 mA to 1 mA bias).

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  • \$\begingroup\$ Hi jp314, thank you for your reply. However I don't understand a pair of things: why connect the base of Q2 to the base of Q1, instead of to collector of Q1 (where the rf signal come out)? The 27 ohm resistor on the collector of Q2 will be attached only to the antenna? In some other circuits with a common emitter amp, the collector also goes to the positive rail of the power supply. \$\endgroup\$ – Giov Apr 6 '16 at 11:25
  • \$\begingroup\$ edited - cap q2b from q1c \$\endgroup\$ – jp314 Apr 6 '16 at 12:08
  • \$\begingroup\$ I've seen your edits, thank you. So, in facts, the following image shows how should be the new schematic? s18.postimg.org/ufrw7jfvt/Oscillator_and_Amplifier_III.png \$\endgroup\$ – Giov Apr 6 '16 at 12:20
  • \$\begingroup\$ In the previous comment I think i did some error in the schematic: the following should be correct, according to your suggestions: s23.postimg.org/ed60s423f/Oscillator_and_Amplifier_4.png Am I right? \$\endgroup\$ – Giov Apr 6 '16 at 12:37
  • \$\begingroup\$ You need to add 1k-10k between the top of C3 and the junction of the base of Q2 and C6 \$\endgroup\$ – jp314 Apr 6 '16 at 14:45
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R4 R5 R6 are way too high for Q2 to give a sensible current .If this is fixed you will have a current amplifier but not a voltage amplifier.Your proposed short antenna is high impedance at the signal frequency like David tweed said.The radiation resistance will however be low .This is covered in basic antenna theory elsewhere.If you use series inductance to tune out the antenna capacitance you get a low impedance which should be good for the emitter follower.This series inducter is nothing new .It is often called a loading coil and it is a trusted and true way of getting plausible performance out of a short antenna.

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