# amplitude modulation with transistor

Discussion in 'The Projects Forum' started by CiaranM, Aug 11, 2012.

1. ### CiaranM Thread Starter New Member

Jul 15, 2012
25
2
hi! I've got a question regarding transistors. When using it as a switch, is it possible to only output the collector signal?
I put together a 555 astable with a frequency of a few kilohertz and outputted it to a 2N3904's base. I routed music audio to the collector, and the emitter was routed to my speakers. I could hear AM ('ring modulation'; whatever you wanna call it) but I could also hear the 555's output wave. Is it possible to hear only the music audio instead of having a square wave too? Do I have to make the output of the 555 equal to ~0.7V, perhaps? If so, would a potential divider work?
Thanks.

Note:
I thought of having the 555 outputted to an inductor, which would be placed by a reed switch. Would an inductor's magnetic field be strong enough to close a reed switch?

2. ### wayneh Expert

Sep 9, 2010
15,873
5,960
Why? What are you trying to do? So far, I'd say just remove the 555 circuit and you'll get better sound.

3. ### CiaranM Thread Starter New Member

Jul 15, 2012
25
2
Its for modulating the amplitude of an audio signal. When the 555 output is high, the transistor switches 'on' and passes the audio signal out. When the 555 output is low, the transistor switches 'off' and the audio signal is blocked. A square wave alternates the transistor's state, which creates an AM effect on the audio signal.
If I remove the 555 then there won't be any AM, which would be counterproductive since that's what I'm trying to get

4. ### vk6zgo Active Member

Jul 21, 2012
677
85
What you are doing is mixing two frequencies which are close together,rather than performing Amplitude Modulation.

Say the square wave fundamental frequency is around 2kHz.
Mixing products from the fundamental & the audio,will extend from several Hz,up to around 12 to 16kHz,depending on the bandwidth of the original audio.
Most of these products will appear in the same frequency range as the original audio,as will the square wave,causing noise & distortion to the audio,if listened to directly.

The Square wave,however,has many harmonics of the fundamental, some of which may appear at the lower part of the MF Broadcast band.
These will appear as carriers,with upper & lower sidebands containing both the original audio,& the noise & distortion referred to above.
In terms of what you are trying to achieve it will be a failure!

An Amplitude Modulated signal must use a sine wave signal for the carrier,
and its frequency must be considerably higher than the highest audio component of the modulating frequency.

I would suggest that instead of blundering around "learning by doing" with crud like 555 timers,you instead read up on how AM is really done.

If,on the other hand ,you are turning the audio on & off at 2kHz,to obtain some special effect,you are not Amplitude Modulating the audio,you are simply turning it on & off at an audio rate,so you will not be able to separate it from the audio.
You could possibly make a balanced modulator which would balance out the 555 signal,just give you the products & the audio.
It may still sound like the square wave is present, no matter what you do,because of the nature of the mixing products mentioned above.

Last edited: Aug 12, 2012
5. ### CiaranM Thread Starter New Member

Jul 15, 2012
25
2
ah well that's a pain. Its a simple idea, but it won't work. I think I'll use optocoupling instead of a transistor.

6. ### EarlAnderson Member

Nov 13, 2011
166
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What are you trying to do exactly. Are you trying to make a AM radio transmitter, or a "tremelo" effect.

7. ### CiaranM Thread Starter New Member

Jul 15, 2012
25
2
I was trying to make a tremolo/'ring mod' that could have its speed altered. I've made the circuit with an optocoupler and it works great, so everything's OK

8. ### Austin Clark Active Member

Dec 28, 2011
412
47
I'm not sure why everyone beat around the bush on this issue, (although, yes, you weren't totally clear, I too was confused at the 555 timer, it seemed like you just wanted a sort of amplifier).

The solution is simple. Instead of connecting your audio output to the collector, your signal to the base, and your speakers to the emitter, and then from that to ground.

Put your audio output directly to your speakers, and then from your speakers to the collector, keep the base the same, and put your emitter directly to ground.
That way, when the transistor is triggered, current is allowed to flow from your audio input through your speakers, but when the transistor isn't triggered, the speakers have nowhere to route the current.

But yeah, I figured you'd be interested for future reference.

9. ### Ron H AAC Fanatic!

Apr 14, 2005
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Transistors don't do a good job of switching AC.

10. ### Austin Clark Active Member

Dec 28, 2011
412
47
Ah, right, because beta changes with collector voltage,right?

11. ### Ron H AAC Fanatic!

Apr 14, 2005
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Well, yes, but that's not the issue. The collector voltage needs to be near zero when the transistor is on (saturated). The problem is, the current will change direction (AC), so the roles of the collector and emitter will reverse. Reverse beta on a bipolar transistor is typically very low (less than 10), but it depends on the type and the individual transistor selected. In any case, it will generally be too low to allow the reverse current to flow.

Austin Clark likes this.
12. ### t06afre AAC Fanatic!

May 11, 2009
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This a very cheap analog switch http://www.fairchildsemi.com/ds/CD/CD4016BC.pdf However I think you need some kind of zero crossing detector/logic in order to avoid "clicking" then switching on and off. Maxim also have a line of analog switches

13. ### Ron H AAC Fanatic!

Apr 14, 2005
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400Ω typical on resistance is not going to turn on a loudspeaker.

14. ### t06afre AAC Fanatic!

May 11, 2009
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The OP used a 2N3904. Which is a typical low power transistor. Hence I was sure the switching was meant to be done before the power amp stage

15. ### Ron H AAC Fanatic!

Apr 14, 2005
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That is definitely the way to do it, but the entire thread has been about a switch in series with a speaker. I got caught up in the details of that.