Transistor based amplitude demodulation

[TecHNooB]

A good explanation in terms of the fundamentals would help. Specifically, what is happening to the transistor as the signal comes in, what region of operation it goes into, and how it ultimately demodulates a signal.

Or

Explain to me what Q3 is doing in this diagram.

http://www.zen22142.zen.co.uk/Circuits/rf/amrec.html

 

[bobsmith1492]

Hm, my boss just explained this to me the other day when I was trying to debug why a microphone input was picking up the local AM radio station!

Basically the input is too slow to pass the high-frequency carrier signal so it ends up amplifying just the audio signal that rides on the carrier.

 

[TecHNooB]

Originally posted by: bobsmith1492
Hm, my boss just explained this to me the other day when I was trying to debug why a microphone input was picking up the local AM radio station!

Basically the input is too slow to pass the high-frequency carrier signal so it ends up amplifying just the audio signal that rides on the carrier.

So it has to do with the physical limitations of the switching speed of the transistor? If that's the case for any generic transistor, that's a lot simpler than the diode/low-pass filter method. No need to choose an RC value that works

 

[bobdole369]

No no no, I think its a bit more than that, I think its the bias doing the demodulating.

Yup,

http://www.vintage-radio.com/r...stor_if-rf-stages.html

The secondary of the final IF transformer is connected directly to the base-emitter junction of the transistor. There is no other bias applied to the transistor. The IF signal is sufficient to bias the transistor on the positive half-cycles, so the transistor is operating in class B mode. The transistor also amplifies the detected signal and AGC voltage.

It's not the physical freq limit of the transistor - most transistors can switch up to at least 30mhz WAY past MF (300-3000khz) - It's because its biased at Class B.

 

[esun]

If you understand how diode demodulation works, then this should be very straightforward to understand.

Like a diode, a BJT turns off when V_BE is less than some turn-on voltage (probably around 0.62 V for the transistors in the given circuit). Thus, the base of Q3 is biased right around that voltage. When the base of Q3 dips, Q3 turns off, and when it increases, Q3 amplifies the signal. Thus, it performs envelope detection much like a diode. It just so happens it also amplifies since it is a BJT and not a diode.

To be honest, I don't see where the filtering occurs. There may be RF components at the output or I may just be missing something.

 

[Paperdoc]

I am not an electronics person, so here I'm a total noob. My grasp is that Q3 simply combines the two functions of an older diode detector with additional amplification. A diode demodulator is really simple. Hey, the original diode detector was the "cat's whisker and crystal" in a crystal radio. It was simply a wire touching a quartz crystal surface and making a poor connection. It passes current only in the forward direction, so only half of the full signal envelope gets through it, and we end up with a voltage that is the original audio signal amplitude-modulated on the positive half of the waveform only. It ranges, then, from zero (or more precisely, the diode's turn-on voltage) to some upper max, say 4 v. The diode plus its associated resistors and capacitors establish an RC network that functions as a low-pass filter that passes only frequencies below the filter roll-off point. But this filter does NOT need a sharp roll-off curve because of the frequency range involved. Even if you we are using the full audible spectrum to to 20 KHz in the original signal, that is modulating in an AM radio an IF carrier of 455 KHz, which is 22x the highest audio frequency to be passed. So even a sloppy filter design is going to work here. As it happens, for commercial AM radio, the audio bandwidth is less - max 5 KHz transmitted by regulation - for the sake of limiting bandwidth of the modulated carrier and making space available for more carrier frequencies in the AM band.

 

[TecHNooB]

Thanks for the explanation guys. I just wanted to make sure there wasnt something magical going on that only the analog genie would know about