The picture above is a circuit for a hearing aid, so we need sound amplification.
BC547 is NPN, BC557 is PNP. How is the amplification happening, why is it connected so?
Edit: I found a circuit for sziklai pair amplification.
Looking at my original yellow circuit I can't understand why the rightmost BC547 is connected as it is. I see that the output is obtained from the emitters of the pair in sziklai pair ckt. Why is the BC547(rightmost) added to emitters of the sziklai pair and just inserted in between?
Best Answer
Given this circuit HAS ONLY 1,500 milliVolts to work with, this is an implementation that deserves some understanding.
The 3 transistors implement a super_beta amplifier.
There is very little Miller Effect, so the frequency response does not suffer.
Notice the voltage divider that sets the bias: 220K to rail, 330K to ground. Thus, ignoring base current (which gets bootstrapped anyway, thru the 10Kohm DC feedback), the Vbase is 60% of 1.5v, or 0.9 volts.
Given Lo collector current thru first transistor, assume 0.5 volts Vbe, thus need a total of 0.4 volts across 10K ohm (which is 100 microAmps per volt) and that 3.3 ohm ouput emitter resistor. Most of the 0.4 volts will be across the 3.3 ohms, thus 30 milliAmps Icollector of the 3rd transistor, as first pass bias thinking.
However --- these 3 transistors have very low Vce, thus beta MAY be defined by the shape of the I_V curve where the Early Effect (the flattening) has not kicked in.
If we assume a beta of 5 !!! for each transistor, then we need substantial current thru the 10Kohm, to support that large current thru the 3.3 ohm.
Interesting design, to function well on 1.5 volts, with very few components.