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Why this circuit doesn't work , and how to simulate it?

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ashour

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Hi

my professor gave us a circuit to create as an assignment , he wants us just to learn the technical part before we gets into real science , so i don't understand how it works i just have the diagram.

i connected it on a bread board but it didn't do anything! just nothing i think there is an error in the values of resistors or maybe i got wrong transistors he didn't mention what model to use just npn or pnp so i got help from a pre-junior friend and told me about some models but i'm not sure if they are correct.

the circuit is supposed to amplify the sound produced from the mic , but i get nothing.

A second question , how would i simulate such circuit on multisim i can't find the transistor models which is pnp bc160 and npn c945 in its database also i can't find a speaker or a mic?

note:
mic is capacitor type (didn't find any other type at the shop)
resistances are 3Ohms and 2 Watts
diodes are pn junctions up to one ampere
speaker is of 8 ohms and 0.5 watts of power

Thanks
 

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Cant work as drawn. No simulator needed. I'll let you discover the screw up.

To simulate a mic, use a voltage source that makes the same output as the real mic.
To simulate an audio (non-critical) circuit with a transistor you don't have in the library, look up the data sheet for the transistor, and find a similiar one that is in the library.
A 1N4148 or 1N4007 will work for almost any Silicon Diode.
For a first order approximation, use a 8 resistor in place of the speaker. If you can measure some physical parameters of the speaker and its enclosure, you can make a more complex model that accounts for resonances, speaker box tuning, etc, but just get the basic sim working first...
 
Hint, reverse the C-E connections on PNP.

You need base bias. Let you figure how to do it.
 
well i forgot to mention that the other lead of the mic is grounded but still after reversing the pnp as u said its not working i tried simulating it by just putting an 8 ohm resistance and a DC voltage as a mic but it just shows very poor voltages across the resistance (speaker)
 
I simulated it and got this..!
[/ATTACH]
 

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View attachment test2.pdf
this is what i got when i changed the diodes biases..
the diode is forward biased to npn transistor on positive half cycle..
i think that's the error
 
How do u get such a pdf? (sorry for noobish question) i just put voltmeters in the circuit and double click them to know the results , but i don't detailed graphs like that u use multisim too?

i'm gonna try the circuit as you said now - but in real - hope it works
 
Producer tag of pdf strongly indicate use of pdf printer software :)
 
How do u get such a pdf? (sorry for noobish question) i just put voltmeters in the circuit and double click them to know the results , but i don't detailed graphs like that u use multisim too?

i'm gonna try the circuit as you said now - but in real - hope it works

hey... it only a pdf printer software.. you can download one from 100s available..
i used the microsim simulator PSpice.
 
Just on the surface of it, I'd either tie the collectors together or the emitters as an emitter follower but not collector to emitter.

Secondly, the diodes are a problem, they will Block the forward biasing of the Transistors. Even if you turn them around to fwd bias the transistors they will attenuate the mic signal by .6V and that's a lot. Replace the diodes with 1K resistors to limit the base current drive to prevent saturation.The nature of a bipolar transistor is it will amplify the waveform that forward biases it. Base Diodes are not a requirement unless you are playing with RF detectors.

The ground is the common junction between the dual pwr supplies.
 
There are a number of things wrong with the circuit.
The supply voltages are wrong and incorrectly drawn.
The lower transistor is incorrectly drawn and connected incorrectly,
The 3R resistors will reduce the output to the speaker considerably, "The mic input will not have sufficient drive to drive the speaker via the two transistors in push-pull and the speaker goes to earth - but we don't know how the battery is connected.
The speaker is directly coupled by we don't know how the battery is connected and this may not be correct - it may need to be AC coupled.
On top of this the bases are not biased and the purposes of the diodes are not utilized.
The purpose of the diodes is to reduce cross-over distortion but the circuit does not provide this feature.
 
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@magnatro i didn't mean how you write the pdf :D i mean how you output those graphs from multisim
its called transient analysis.. it in every p spice program..
 
I think that there is a fatal problem with this circuit it amplifies signals but it doesn't have any pre-amplifier part .

so someone have a sugesstion about some other circuit even using ICs that takes input from mic and outputs it to a speaker?
 
Use an LM324 Opamp as a preamp and an LM386 (1.25W) or LM380 (2.5 - 5W) to drive your speakers. Google for circuits...lots of those around.
 
The two transistors including the PNP that is connected backwards are turned off all the time so they will not do anything.
A mic has an output level of only 0.005V. It needs to have a preamp with a voltage gain of 100 to 200.
 
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