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 Post subject: Single transistor AM TX
PostPosted: Nov Sun 21, 2021 2:51 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 811
Transistor is 2n3904. Collector resistor is 1K. A 2n7000 biased at a few ma would be needed to amplify its output. Frequency was around 1MHz? 10uH RFC was an axial inductor. The sound was really really good. From experiments in 2008..


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 Post subject: Re: Single transistor AM TX
PostPosted: Nov Tue 23, 2021 1:36 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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Single transistor oscillator but it's now emitter modulated instead of base or collector modulation as changing voltage cause FM modulation of the carrier and this induces distortion in the audio. L1 P= 220uH axial inductor. Feed back coil S is a few turns of thin enamelled wire on top of 220uH. Use the minimum number of turns for reliable oscillation. I think 5 to 10T should be enough. Select R1 for DC Power in of <100mW. A 50pf trimmer should be used for fine tuning the frequency. It's in parallel with a NP0 (Try 33pF for 1 - 1.5 MHz coverage) ceramic capacitor.

Select R2 for 4V to 5V on the collector of second transistor.

9V operation is preferred. Use a battery or 7809 voltage regulator to filter out power supply ripples. A 10K pot on the audio input will prevent over modulation.

A 9 Ft. Wire antenna should cover a small house or apartment.

This transmitter will perform a lot better than expected as emitter modulation has been used here. Both transistors are 2n3906. Fidelity should be very good.

Simple design with low power drain. LM386 eliminated.


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 Post subject: Re: Single transistor AM TX
PostPosted: Nov Wed 24, 2021 3:12 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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Output should not be more than a few milliwatt.
The 470ohm biasing resistor upsets the balance of the diode modulator and letting the carrier pass through enabling AM transmission. L1, C1, C2 and VC1 = values selected for 1 - 1.6 MHz coverage.


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 Post subject: Re: Single transistor AM TX
PostPosted: Nov Fri 26, 2021 2:39 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 811
Not a bad idea. CD4013 driving 4066 for 530-1600 KHz AM TX.
Operate LM386 at a bit lower V+ (9V) and 12V for the two chips. The output 100n capacitor charges and discharges, alternating between audio and ground developing an AM envelope.
3MHz CLK or 2Fout needed for 1.5MHz output. Signal is buffered by a 2n3904 emitter follower, DC in = 65mW , & part15 compatible..100% modulation is easy with switching modulators. NFB might be added to eliminate any distortion for FM like fidelity, probably not required.


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 Post subject: Re: Single transistor AM TX
PostPosted: Nov Sun 28, 2021 11:16 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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Final out: BS170 running Class-E at approximately 90% efficiency.
Power Output: 80mW with +/- 100% modulation capability. 450mW PEP with 120% + mod and 90% - mod.
Modulation: Highly efficient transformer-less Class-G modulator.
Digital tuning: Arduino Nano + Si5351 + OLED display(1 kHz steps MW band) .
Reverse Voltage Protection.
AGC: Effective AGC with front panel LED AGC and overmodulation indicator.

I really need to design such a rig. Arduino would automatically select & diode switch coils / capacitors for output matching network based on frequency & even antenna length.

It would be an attractive design, provided how simple I keep the circuit. Including so many features in a simple circuit is a bit tricky but let's see. Would you guys buy the kit for $100 ?


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 Post subject: Re: Single transistor AM TX
PostPosted: Nov Mon 29, 2021 7:26 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1621
I built a number of single transistor AM TX's in the 1970's. I used the high voltage germanium video output transistor, the AF118. (sadly most of these are affected by tin whisker disease now) The Japanese 2SA358 is a good replacement, or a silicon transistor.

They worked pretty well and I could get a signal outside the house and a few doors down the street. However, although they seemed appealing based on the simplicity of them, it was never possible to eliminate the frequency modulation which also occurs, resulting in some distortion at the receiver. I crystal controlled some too, helped a little, but didn't eliminate the issue. So I concluded way back then it was better to have an independent stable oscillator and a separate modulator & output stage.


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 Post subject: Re: Single transistor AM TX
PostPosted: Nov Mon 29, 2021 3:40 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 811
Hi Acorn...

I tried to modulate the + supply to a LC tuned colpitts osc. The distortion was horrible. Putting a crystal solved this problem. Which xtal oscillator did you modulate? The colpitts give 50% modulation. Adding RFC in emitter ups it to 100% + and 90 to 95 % on negative.

I think you have seen the circuit, have you? I don't have a picture of the modulated signal. It sounded nice on 7.2MHz.
Now only 1MHz ceramic resonator is on sale and all the 1MHz crystals have vanished. That's the limitation. The esign was simple, whatever distortion was there could hv been minimized from 5% to 0.3% with NFB to LM386.


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Last edited by Dare4444 on Nov Mon 29, 2021 10:26 pm, edited 1 time in total.
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 Post subject: Re: Single transistor AM TX
PostPosted: Nov Mon 29, 2021 4:00 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 811
What about a high level diode mixer directly putting out 60mW of carrier. Most part15 TX give 50-75mW output. So field strength should be the same. Add + bias to unbalance. If 240mW is PEP then VFO drive needed would be 2x of that or 480mW. VFO AGC should keep it fixed as to not exceed the legal field strength. Diodes mixer generate perfect AM, the analogue warmth feeling is nice :)


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