AM receiver circuit qestion .. ?

[wiwi]

plz take a look at the following link :

**broken link removed**

the last 3 lines in the first paragragh say :

"however the input signal levels at RF are tiny, typically 50uV appearing across the coil being amplified by Q2 and being about 5mV RF across the 2k2 load resistor ".

my question is a 5 mV signal is too low to forward bias the b-e junction of Q3 , so how would Q3 demodulate such a too low signal ? plz explain . thanks alot .

 

[FvM]

my question is a 5 mV signal is too low to forward bias the b-e junction of Q3 , so how would Q3 demodulate such a too low signal ?

Supposed the 5 mV number is correct. The BE junction is already forward biased in the circuit. So what happens is, that the 5 mV RF is applied to the non-linear (exponential) characteristic of the diode. It causes a small change in average diode current, so in fact, a small part of the signal will be demodulated.

 

[wiwi]

so can this low signal be applied to any CE amplifier and be demodulated ? let's say a voltage divider CE amlifier with high gain , will it demodulate and amplify the 5 mV AM signal ?

 

[FvM]

What do you mean with "voltage divider" CE amplifier? If you refer to an amplifier with emitter resistor, it will only have a low demodulator effect.

Baiscally, I wanted to explain the effect, not exactly predict the behavior of different circuit variants. In my view, RF demodulation of transistor amplifiers is mainly an unwanted effect, because it is an entrance for interferences.

 

[chuckey]

Reading the text it says that Q3 is the demodulator.
Frank