BJT SR FF

An SR FF made with 2 BJTs, 4 resistors, 2 switches, and a 5V DC supply. Setting Q • Switch nS (not Set) to low (0V) (setting) • Q goes high (4.2V) • T2 is saturated Vbe = 0.8V • Current in T2 b (4.2 – 0.8)/10 = 0.3mA • T2 β = 100 ⇒ T2 Ic up to 30mA • nQ is low (0.1V) • T1 is cut-off even if nS is now open (memory) Resetting Q • Switch nR (not Reset) to low (0V) (resetting / clearing) • nQ goes high (4.2V) • T1 is saturated Vbe = 0.8V • Current in T1 b (4.2 – 0.8)/10 = 0.3mA • T1 β = 100 ⇒ T1 Ic up to 30mA • Q is low (0.1V) • T2 is cut-off even if nR is now open (memory) We can observe how the state Q switches to high (4.2V) when nS (green) is switched to low (0V) and stays high even when nS switch is open. Similarly, Q switches to low (.1V) when nR (red) is switched to low (0V) and stays low even when nR switch is open.