Your load of 16 Ohms is too low for your single-transistor amplifier. Its load is 470 Ohms as on the schematic, with your headphones it becomes ~12 Ohms only. This is why you have no audio gain.
You should use at least two transistor stages, and an output audio transformer for 470 to 16 Ohms impedance reduction.
Also, please use 12 kOhms instead of 2.2 k Ohms to feed Vcc to the electret microphone.
Since it's a project for class, I am restricted to only one transistor. I'll try to imcrease the microphone impedance and do the calculations for a higher load.
Thanks.
I'm the student. It's about designing a single stage transistor amplifier, with the output from an electret microphone being the signal to amplify. It should be easy if I knew how much current is good for these headphones, but I can't find a clear explanation on the headphone parameters apart from the impedance. After picking the adequate current, it's just solving the system equations.
After picking the adequate current, it's just solving the system equations.
Here is a rough calculation:
Ic=7 mA >>> transconductance g=Ic/Vt=7/25 and 1/g=3.6.
Gain calculation: A=-RL/(1/g+RE) with RL=470||16=15.5 ohms.
This leads to A=-15.5/(3.6+100)=-0.15.
That means: No surprise because of a very small load resistance as well as signal feedback.
However, if you use a bypass capacitor across RE the gain will rise to A=-15.5/3.6=- 4.43
Is this sufficient?
That allows a signal swing slightly more than 5mA RMS. Since the headphone impedance is much lower than the 470R collector resistor, most of that will go to the headphone.It was calculated for a IcQ=7.9057 [mA].
With a 16 Ohm load, the output power = I squared * R = 5mA*5mA*16R = 0.4mW. That's 4 decibels less than 1mW, so the maximum sound pressure level from the headphone will be 108 - 4 = 104dB. That's very loud.According to the headphones' manufacturer, the impedance is 16 [Ω], and it has a sensitivity rate of 108 [dB/mW].....
My main question is, how do you determine how much current this circuit will need to feed the headphones, when the manufacturer info is quite obscure?
I've no idea how you worked that out but anyway.....AC voltage gain = -0.0678 ans Current gain of -2.8853.
It's easy to model the amplifier's behavior in a spice simulator. I don't know any wwwwa to model the microphone's behavior though.How can I simulate this behavior in Proteus or similar?
I actually though your choice of DC conditions was very good. For example there's a decent amount of voltage across the emitter resister so the circuit won't be very sensitive to changes in temperature or battery voltage.I was doing the design wrongly, since I was fixing the DC conditions with values calculated for the AC part.
Yes.the Power Handling Capacity is 100 [mW]. What does this value mean? Is it the maximum power the headphones can handle without burning?
The specs say "Sensitivity (db): 108 dB/mW" so 1mW will give you 108dB SPL, which is very loud.And what power would be a reasonable choice?
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