Please bear with me. I'm a total newb to electronics. I apologize if this question is stupid.
I'm having trouble working out how the attached circuit is able to maintain a constant current.
Transistor Q2 has no current flowing through it, but if I remove it from the circuit, then the current is no longer regulated.
I'm trying to apply KCL and Ohm's law but things just keep looping back and I end up just restating what I already know.
I(R1) = Ice(Q2) + Ibe(Q1)
I(Load) = Ice(Q1)
I(R2) + Ibe(Q2) = Ibe(Q1) + Ice(Q1)
I(R1) + I(Load) = Ibe(Q2) + Ice(Q2) + I(R2)
V(R1) = I(R1) * 10k
V(R1) = 10k * (Ice(Q2) + Ibe(Q1))
I(R2) = Ibe(Q1) + Ice(Q1) - Ibe(Q2)
V(R2) = (Ibe(Q1) + Ice(Q1) - Ibe(Q2) ) / 33
Vce(Q2) = V(R1)
Vce(Q2) = 10k * (Ice(Q2) + Ibe(Q1))
Vce(Q2) = Ice(Q2) * Rce(Q2)
Vce(Q2) = 10k * Ice(Q2) + 10k * Ice(Q1)
10k * Ice(Q2) + 10k * Ice(Q1) = Ice(Q2) * Rce(Q2)
9.999k * Ice(Q2) + 10k * Ice(Q1) = Rce(Q2)
9.999k * Ice(Q2) = Rce(Q2) - 10k * Ice(Q1)
Ice(Q2) = (Rce(Q2) - 10k * Ice(Q1) ) / 9.999k ... at this point my mental stack overflows and I'm lost.
Intuitively it seems like the base emitter junction of Q2 prevents the voltage across R2 from exceeding its forward voltage drop which in turn prevents the current from exceeding the (forward voltage drop of base emitter junction for Q2 / 33 ) amps. But what prevents Ice(Q1) from becoming so high that it swamps Q2 and causes the voltage across R2 to rise? I can see that Ice(Q2) can pull some of the I(R1) current away from Ibe(Q1) but I can't figure out how to calculate the magnitude of the effect.
What's the proper way to analyze this circuit so that I can calculate the effects modifying R1 and R2 will have on the circuit.
Thanks,
Dave
I'm having trouble working out how the attached circuit is able to maintain a constant current.
Transistor Q2 has no current flowing through it, but if I remove it from the circuit, then the current is no longer regulated.
I'm trying to apply KCL and Ohm's law but things just keep looping back and I end up just restating what I already know.
I(R1) = Ice(Q2) + Ibe(Q1)
I(Load) = Ice(Q1)
I(R2) + Ibe(Q2) = Ibe(Q1) + Ice(Q1)
I(R1) + I(Load) = Ibe(Q2) + Ice(Q2) + I(R2)
V(R1) = I(R1) * 10k
V(R1) = 10k * (Ice(Q2) + Ibe(Q1))
I(R2) = Ibe(Q1) + Ice(Q1) - Ibe(Q2)
V(R2) = (Ibe(Q1) + Ice(Q1) - Ibe(Q2) ) / 33
Vce(Q2) = V(R1)
Vce(Q2) = 10k * (Ice(Q2) + Ibe(Q1))
Vce(Q2) = Ice(Q2) * Rce(Q2)
Vce(Q2) = 10k * Ice(Q2) + 10k * Ice(Q1)
10k * Ice(Q2) + 10k * Ice(Q1) = Ice(Q2) * Rce(Q2)
9.999k * Ice(Q2) + 10k * Ice(Q1) = Rce(Q2)
9.999k * Ice(Q2) = Rce(Q2) - 10k * Ice(Q1)
Ice(Q2) = (Rce(Q2) - 10k * Ice(Q1) ) / 9.999k ... at this point my mental stack overflows and I'm lost.
Intuitively it seems like the base emitter junction of Q2 prevents the voltage across R2 from exceeding its forward voltage drop which in turn prevents the current from exceeding the (forward voltage drop of base emitter junction for Q2 / 33 ) amps. But what prevents Ice(Q1) from becoming so high that it swamps Q2 and causes the voltage across R2 to rise? I can see that Ice(Q2) can pull some of the I(R1) current away from Ibe(Q1) but I can't figure out how to calculate the magnitude of the effect.
What's the proper way to analyze this circuit so that I can calculate the effects modifying R1 and R2 will have on the circuit.
Thanks,
Dave