Circuit takes in audio from microphone and converts to lights when sound. Please

Question

Circuit takes in audio from microphone and converts to lights when sound. Please explain how this circuit works in detail of the 3 stages include nformation on the input voltage and if its dc/ac or both used, what the capcitor does in each stage, the operating region of each of the 3 transistors(active,off, saturated). Do the currents/voltages increase as they go from one transistor to another or vice versa?

R8, R9, R10, R11 390 Ohms R3 R5 10K 220K R6 1.2K 10K 1011 R2 620K R1 C2 100nF | Q2 R7 270 Ohms C1 100nFC1 R4 20K 2N4401 Four LEDs Q3 Electre 2N4401 2N4401 Mic

Explanation / Answer

let's start with the working region of all the transistors:-

as we can clearly see that all the transistors are in npn connection, so it is quite clear from the connection that the V(CE) or V(C)voltage is same through all the transistors. now to make transistor work in the active region this V(c) should be more than V(b). and by using the beta rule, (i.e. I(c)=beta*I(b) +V(C(0))), it is quite clear that V(c)>V(b) in all the transistors.Hence, all the transistros will work in the active region. (though, we can also find the region by calculating the current through each resistor, but that would make the system complex only)

now coming to the working of all role of capacitors:-

the capacitors being used in the circuit is just to eliminate the value of zero frequency signal from the circuit and to stabilize it when the frequency component occurs in the signal. thats why, during DC input, the capacitor will not allow any current and will make the circuit (across capacitors) work as the open circuit. Also, the capacitor will make the circuit work under the bandpass circuit (if we wish so).

now let's talk about the current and voltage value from one transistor to another:-

let's assume that the rightmost transistro is I and the leftmost is III. Now, while coming from the transistor I to II, the voltage drop across V(c) would be SAME but due to the different resistors placed as R(c), the current will be different in them (current across R(c) of I would be more than II). Similarly, the current across R(c) of III would be most in the transistor III.

Similarly :-

Current I(b) across transistor :- I<II<III

Current I(c) across transistors:- I>II>III

Current I(e) would be same in ac circuit but in DC circuit :- I>II>III

Voltage V(c) would be constant for all.

Voltage V(b) across transistors:- I>II>III

.

THankyou :)

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