Darlington pair with weird resistor in between

01:14:20

Any body knows why there is a resistor connecting between emitter of 1st transistor and base of 2nd transistor? One thing I noticed is that with the resistor, the emitter of the 2nd transistor will be lower then without the resistor. Any other reasons?

published 9 years ago

rich11292000

9 years ago

Lots of reasons, to much gain. Can simply be 2 transistors and 1 resistor on on a circuit board. Can be one physical transistor with this as a equivalent circuit.

rich11292000

9 years ago

Need more info to narrow it down. Where did you find the circuit? Was there description or discussion?

BillyT

9 years ago

rich11292000 I think the resistor is just to limit the base current, it is possible for the driver transistor to be incorrectly biased and thus over drive the output transistor.

vesau2045

9 years ago

This is where I found the circuit: http://bestengineeringprojects.com/sound-detector-circuit-using-op-amp-741/

vesau2045

9 years ago

It is from the last part of the sound detector circuit. I assumed that resistance of the piezo buzzer was 1kOhms.

slbm1996

9 years ago

They usually have around 5k beta forward so they use it to limit the gain of the pair

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