# How do I choose currents through my circuit?

Beginner here. I've recently "almost" finished a project in which I am using a relaxation oscillator to get a square wave output with a certain frequency. The circuit works as I wanted but that's only in the simulator (I believe in real life things change up a bit).

My problem is that I didn't check if the values for my currents are good for real life practice. For example, using .op in LTspice I get $$\I_C\$$ currents in the hundreds of uA, which look quite small to me. I can pump up the values if I decrease R2's value, but even when I get currents in range of mA the circuit works just as fine. I know the main factor to look into is the power dissipated by each component, but there are more factors? Why choose the mA range when I can work with hundreds of uA or maybe less if I increase R2?

• The current through R2 must me be high enough to Q4,Q8 base currents doesn't distort the voltage at Q2 collector. The R2 resistance also depends on Vcc (ohms law). Nov 9 at 21:48
• Looks like you copied and simplified the diagram of a monolithic opamp. Which is OK to some extent. With discrete construction you don't get the component matching that you do with monolithic. I would emitter ballast the Q1/2/5 current mirrors just like Q11/13 are ballasted if you want good control of bias currents via R2. Nov 10 at 6:13
• Okay noted, but that didn't answer my question.I still don't know what current I can choose through the whole circuit, there are hundreds of good uA or I should look in the order of mA. Thank you for your answer though, it was good to know Nov 10 at 7:30