0
\$\begingroup\$

Electronics student here, so i'm printing a IR receiver for a project of my school. I'm trying to understand the filtering part of the circuit.

The signal is received through the BP104 photodiode and then passes in a filtering/amplifying part of the circuit to be decoded then by a LMC567 tone decoder. Here's the schematic:

enter image description here

So far this is what I know :

Q1, Q2, Q3 and Q5 are Darlington pairs, to amplify the signal.

My questions are :

C2 and C3 would be low pass filters ?

What is the purpose of R11/C7, R7/C5 and C11/R14 ?

Any help would be appreciated.

Thanks.

\$\endgroup\$
2
\$\begingroup\$

Q2/Q3 is a Darlington pair configuration (not Q5/Q1).

C3 is a DC-blocking capacitor, so that only the signal goes into Q2 and not the DC bias given by D1/R10.

R14 provides DC bias for Q5, and C11 will short circuit R14 for AC signals, giving a high AC gain for Q5.

C2 is a bootstrap capacitor, providing a higher input impedance at Q2.

Now think of Q2/Q3/Q5/Q1 as a high gain amplifier (like an opamp).

From the output you have a (frequency-dependent) voltage divisor given by R5/C5 and R11/C7. The signal in the midpoint is used as feedback to the amplifier by means of R6.

You can do the math for this voltage divider; I suspect that the overall circuit is a band-pass amplifier.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.