Hello All,

I have designed the circuit below to buffer an op-amp output to deliver 4.2V to a 100 ohm load. The load is just a representative of a circuit which sink 44 mA current. I appreciate if you help me find answers for questions listed below,
1) As the output of the OP-Amp going to be 0.7V more than 4.2V, I chose Rc= 50 to provide Vs =2*(4.2+0.7)= (12-50*0.044)= 9.8 for the op-amp power supply. Is my approach correct?
2) I would like to power down or at least decrease the power consumption of my circuit.Therefore, I thought of using Q1 as a switch as shown in the figure. However I do not know if I should place R1 resistor or not and if R1 is required how should I find the proper value? I appreciate i you could give me some hints.

Regards
Samaneh

 

Depends upon whether you want to shut it completely off, or just reduce the power.

To turn it off, you don't want or need R1.

If you want to reduce the power, then the 5K resistor with R1 will act as a voltage divider to reduce the current through Q2.
Thus if R1 is made equal to 5k, then the voltage at the op amp input, and therefore the current through Q2, will be cut in half.

 

Why is the op-amp being powered from the collector of Q2? Are you trying to reduce power? Why not just running the amp from 12V?
The current in the base of Q2 will be about 400uA. (depends on the gain of Q2 at 44mA collector current) The 400uA must come from 12V. I don't see it makes any difference with or without R43.