AM modulator circuit

[Walid]

Hi ,

pleaze look at the following circuit:
**broken link removed**

can anyone please explain how the transistor works here as a mixer ? based on the resistors given i don't think the transistor is biased in the saturation(nonlinear) mode, it's in the active linear region , i have calculated VCE for Vcc=12v and hfe=180 and it
gave VCE > 4v . So how does this work as a mixer ? please help.

 

[Nigel Goodwin]

Hi ,

pleaze look at the following circuit:
**broken link removed**

can anyone please explain how the transistor works here as a mixer ? based on the resistors given i don't think the transistor is biased in the saturation(nonlinear) mode, it's in the active linear region , i have calculated VCE for Vcc=12v and hfe=180 and it
gave VCE > 4v . So how does this work as a mixer ? please help.

It doesn't work as a mixer, it works as an amplitude modulator - completely different things.

 

[Walid]

i think AM modulation is mixing between two signals, s it should work as a mixer i think. If not then what is the difference ?

 

[ericgibbs]

i think AM modulation is mixing between two signals, s it should work as a mixer i think. If not then what is the difference ?

hi,
Check these links,
Amplitude Modulation, AM :: Electronics and Radio Today
Introduction to Mixers

 

[JimB]

I would argue that an RF mixer and an Amplitude Modulator are one of the same thing.

Consider two frequencies F1 and F2 (where F1 > F2) as the input to a mixer, the outputs will be F1, F2, F1+F2 and F1-F2. Filtering on the output of the mixer will select the required component, (F1+F2) or (F1-F2) and suppress the others.

In an amplitude modulator, let the carrier be F2 and the modulating signal be F1, the outputs are F1, F2, F1+F2 and F1-F2.
The filtering on the output of the modulator is tuned to the carrier frequency F1, but as the side frequencies F1+F2 and F1-F2 are close to the carrier and well within the passband of the filter the outputs are (F1-F2), F1, (F1+F2).
We could rewrite this as (Fc-Fm), Fc, (Fc+Fm) where Fc is the carrier frequency, and Fm is the modulation frequency.

Going back to the OPs original question, one of the signal sources is in series with the emitter and so will change the "bias" of the transistor resulting in AM modulation.
I also see that the AM waveform is somewhat distorted.

As an afterthought, if the filter on the outout of the mixer has a narrow enough passband, it can select just the (Fc+Fm) or the (Fc-Fm) resulting in a single sideband signal.
(But lets not go there otherwise AG will go off on his hobby about quacking ducks! ;-) )


JimB

 

[audioguru]

if the output of the mixer has a narrow enough passband ..... resulting in a single sideband signal.
(But lets not go there otherwise AG will go off on his hobby about quacking ducks! ;-) )

Correct.
The single-sideband radios that I have heard have such a narrow bandwidth that people talking sound like quacking ducks.
Broadcast band AM radios sound almost as bad.

 

[Nigel Goodwin]

I would argue that an RF mixer and an Amplitude Modulator are one of the same thing.

I wouldn't, amplitude modulation is linear, mixing isn't - any non-linear circuit is a mixer.