i am designing a circuit able to draw IV curves of LEDS and diodes through Arduino.
Basically i use the PWM function and an RC integrator to have an approximately constant voltage. Then i would simply connect the LED via a shunt resistor to measure current and voltage; however to not affect the integrator R_shunt must be greater than integrator resistor, but i can't use too big resistors otherwise current would be too small (max current = (5 Volt-V_breakdown,diode)/(R_shunt+R_integrator) right?). Then i put a transitor in emitter follower configuration to increase load impedance.
Problem is; i don't understand very well what i am doing (altough i thought i did). In emitter follower the voltage taken on a load after the emitter is V_base- 0.7 . I got this, but then i don't understand HOW the voltage at the collector influence the circuit, and why using now very big resistors in RC integrator (like 1 MOhm) current is low on the shunt resistor, say 0.7 mA instead of usual 14 mA. I would say: current in the shunt resistor is (excluding led for one moment) (V_base-0.7)/R_s but now seems that resistor at the base influences this current. How?
EDIT:The shunt resistor in the schematic is Rs, sorry for the confusing names. About collector voltage, i wasn't sure it would influence emitter current, that was the question, maybe that's the reason of misunderstanding. Actually i think i found the solution revising Horowitz's book. In emitter follower the output impedance (ie seeing emitter as the output with a Z_load) is Z_(load)*(beta+1), whereas the resistance on the base R becomes an input resistance of R/(beta+1). So for R in MOhm, considering the Thevenin equivalent i have a total resistance of R/(beta+1) + R_shunt in which a voltage V_base - 0.6 is flowing, that's how R of integrator influences the circuit. Am i right?