I have Found One Circuit Which acts as Switch to Pass 15NV (15 Negative Voltage) to Opamps Negative (-VCC) Supply But I am unable to understand its working . Why Two NPN are used in this Circuit ? What is the Purpose of Capacitor C1 ? R3 is used to limit current or has different purpose ? Please Help me understand the Working of this Circuit and its actual Usage .
It looks to be a
12mA 15mA short circuit protection current limiter on Vee
It acts as a Voltage switch that limits current.
0.6V/47 ohms = 12mA
- actually up to 15mA at cold temp as 0.6 rises 4mV/'C to 0.7V at +25'C rise
- which is a typical BJT Op Amp current limit, since the Q2 base current is very low
- ~15V/12k /hFE=Ib the threshold is only 0.6V at room temp.
For more examples of this limiter, see this for a 12V 1A Zener regulator limited to 1.3A Short circuit output