Recently, I found this circuit on the Talking Electronics web page:
I was interested in the modified circuit below, where the feedback capacitor is replaced with short circuit and the 8R speaker with resistor of 8 ohms.
I simulated the circuit with falstad in two situations:
- When the circuit was running without feedback, I added the short circuit.
- The short circuit was present at the beginning of the simulation.
Without the feedback, the circuit is an amplifier, with current amplification of about 100 * 100 = 10 000 A/A. In the first scenario, when I add the short circuit while the circuit is running, the circuit continues working as before, but the voltage on the 8 ohm resistor has lower value. Maybe the addition of the short circuit removes some of the charge in the base of the NPN bjt, so it turns a little bit off, which in turn increases it's collector voltage, so the base current of the PNP bjt has lower value, affecting the collector current of the PNP and finally this decreases the voltage on the 8 ohm resistor. In the second scenario, both transistors are cutoff. Maybe this has something to do with the parasitic capacitance of the BJTs and their turn-on time.
I am kindly asking if someone can explain me how and why the circuit behaves this way.