I have to analyze the following circuit and understand its function.

As far as I understand the top part is the simple phase-shift oscillator generating output sine wave at circa 145kHz.

The NE555 is an monostable(?) multivibrator that generates square pulse. But what about transistors between them?

It looks like a Darlington circuit but I'm not sure and I also don't know how it works.

Does anyone have any ideas or suggestions?

Some pictures of the simulation in LTspice below:

enter image description here enter image description here

  • \$\begingroup\$ Where did the circuit come from? Doesn't the original source provide some explanations? \$\endgroup\$ Jul 1, 2020 at 17:26
  • \$\begingroup\$ @ElliotAlderson unfortunately no. this is the part of assignment that I received at the university. We should analyze the netwokr(which is actually bigger than I showed here). I could find some info regarding other parts but nothing about the part I am asking about here. \$\endgroup\$
    – Kral
    Jul 1, 2020 at 17:32

1 Answer 1


Basically, Q4 kills off the output from the phase-shift oscillator every time the 555 timer (low frequency oscillator) output goes low. Q4 acts as a short and basically "mutes" the output from Q1 and significantly disrupts the bias point of Q2 to turn it off.

Q4 is helped along the way with a buffer amplifier formed by Q3. It's likely that a single half-decent MOSFET could replace both Q3 and Q4.

It looks like Darlington Circuit



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