# Voltage Circuit Switch with BJT

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 .

• The circuit topology is pretty standard, when $Q_1$ is operating in active mode. It sets a collector current that drives $R_2$. However, in this case $Q_1$ is saturated and its collector acts more like a voltage source than a current source. Which might make good sense if the topside of $R_1$ is how you enable/disable this. But you don't explain much of the context. So I can't really tell you much more. \
– jonk
Jan 5, 2019 at 8:06
• @jonk R2 is probably just raises the output to bias it on. Jan 5, 2019 at 8:37
• @jonk The Purpose of this circuit is to pass the -15NV which is generated From Somewhere else and give it to Opamps Negative VCC Supply. Please Check my understanding . When The Base of Q1 is less nagative ( approximately by 0.7) than emitter then Current will start flowing from collector to emitter . due to High Resistance of R2 there will Very Less Collector current so all Emitter Voltage appear across the R2.so Q1 acts like switch here. the purpose of R1 is just to give bias voltage to Q1. Jan 6, 2019 at 18:05
• @Rohan I'm not sure I understand your writing. (My apologies.) I tried. But I'm just not parsing it well. The only question I have is whether or not there is a control signal that is used to enable and disable this supply. If so, it would likely be one that exists (unshown) at one end of $R_1$. Is there a control signal to enable/disable this supply to the opamp? Or not?
– jonk
Jan 6, 2019 at 19:07

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
err 12.77

• actually up to 15mA at cold temp as 0.6 rises 4mV/'C to 0.7V at +25'C rise
• 0.7/47=15mA
• 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

• Can You Please Elaborate its Current Limiting Action. 15mA will Flow through transistor Q2 base ? you want say the base current of Q1 is very Low ? what happens if more than 15 mA flows ? Jan 6, 2019 at 18:23
• Q2 Vbe sees 0.6V and pulls down Q1 Vb to limit current. Current gain in the loop is hFE1*hFE2 Jan 6, 2019 at 18:31