building an am receiver

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amriths04

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bc107 simple receiver

hello friends,
i just wanted to get your advice before i can try this out.
i intend to build a simple am receiver in a "BREAD BOARD".

my idea is to first have an
1)antenna(about 1m)
2)tuned circuit(L || C(var)).
3)this output is given to a double stage (bc 107 bjts), with gain=100*100=10000 so that it is amplified well.
4)this amplified output is then given to an envelope detector.(using a germanium diode and R and C combination).
5)this output is then sent to another single stage amplifier.
6)the output is taken from speakers.

my doubt is,whether my logic is correct? is there any practical problem in the above steps? or any other alternative?

thanking you in advance,
amrith.s
 

Your concept is usuall for beginners level. Pay attention on impedance matching between LC tank and following amplifier. Bandwidth and selectivity of the receiver depends on Q factor of the LC tank. You can find some examples of such receiver on internet.
 

This is a TRF (tuned rf) type receiver which was popular in the 1920s.
 

amriths04 said:
hello friends,
...
3)this output is given to a double stage (bc 107 bjts), with gain=100*100=10000 so that it is amplified well.
...
amrith.s
Click to expand...

could be hard to achieve.
 

sorry borber,
i cannot understand how to match the impedance between the tuned circuit and the amplifier. would you kindly recommend me a site which deals with impedance matching between stages?

thank you,
amrith.s


Added after 2 minutes:

hello artem,
do you mean i should not use bc 107 or that i should not use bjt itself?
then what other device i can use for the rf amplifier stage? can i use a high frequency opamp? please advise.

thank you,
amrith.s


Added after 2 minutes:

this is the circuit i designed.
please advise.

thank you,
amrith.s
 

You are using a power diode for the detector. The capacitance may be too high. Try using a small signal diode like the 1N918 silicon type or the 1N5711 Schottky type.
 

You are using strange power supply connection on you circuit, please check again.

You don't use a capasitor on small signal common emitter amplifier
 

Your design is totally wrong.
LC parallel tank connected in series with antenna is bandstop filter. It must be connected as bandpass filter.
Amplifier is wrong, it's gain is about 0dB or one, not 10000 what you expect. This is because transistors are saturated. The ratio of collector to emitter resistors is correct but 330k collector resistor is too large. Start design with assumption of collector current of 1-2mA. Transistors should operate in A class. And at the end calculate bias resistors.
Visit:
**broken link removed**
 

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    amriths04

    Points: 2
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ya, i understood that.
can you just say whether the below design would be right. pardon me, i am still a beginner..

let Vcc=10v,
let ic=1mA,
let Rc=5.6k,
thus, Vc=10-5.6=4.4v,

gain=100,
thus, Re=0.056k,
thus,Ve=0.056v,
and Vb=0.656v.

so,biasing resistors, Ra(top one)=93.4k and Rb=6.6k.
using another stage with similar configurations will make it a double stage with gain 100*100. will this work?
and how to change that LC ckt into a bandpass arrangement?

thankyou,
amrith.s


Added after 2 minutes:

ya budhy, i got what you meant. it was a mistake which i commited while drawing it in pspice.
which capacitor you mean to say?

amrith.s

budhy said:
You are using strange power supply connection on you circuit, please check again.

You don't use a capasitor on small signal common emitter amplifier
Click to expand...
 

Your amplifier will have gain of about 30. Try to get some info here:
**broken link removed**

Are you using pspice only for drawing schematics? It can simulate your circuit and that is the main purpouse of spice programs.
 

but i am having a student version of pspice. i donno how to use it for transient analyses. do you have any tutorial on pspice?


 

Pspice shurelly have all basic simulations like AC and transient. I do not use pspice and can not help you directly. Try to use Help or search internet for pspice manual. You will find a lot of informations.
Multisim or Electronics Workbench is more suitable for beginners I think.
 

Your receiver needs a means to change the gain at the input of the audio amplifier.
 

do you mean i need to use an agc in there or just a voltage divider with a nominal gain would do?

thank you,

flatulent said:
Your receiver needs a means to change the gain at the input of the audio amplifier.
Click to expand...
 

Between the detector and the audio amplifier have a variable resistor with one end grounded. The far end goes to the detector and the variable tap goes to the audio amplifier.

Some other things to watch out for is oscillation of the two stage RF amplifier. This is an issue of careful layout and power supply filtering.

You may also want to have a switch between two antennas, one very long and one very short. That way local stations will not clip in the RF stage.
 

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