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This circuit illustrates how an applied gate voltage has the opposite effect on NMOS and PMOS transistors. The NMOS and PMOS transistors are configured to give behaviours close to that of the BS170 and BS250 discrete MOSFETs, respectively. The left NMOS transistor controls a green LED circuit, and the right PMOS transistor controls an orange LED circuit. The top voltage supply represents V_DD, and the bottom voltage source applies V_G to both transistors. Since the NMOS source is grounded (V_S = 0), the gate voltage alone determines V_GS. Since the PMOS source is tied to V_DD (V_S = 5 V), the gate voltage has an opposite effect: V_SG = V_S - V_G = 5 - V_G. So, when V_G = 5 V, V_SG = 0 V and the PMOS is off. When V_G = 0 V, then V_SG = 5 V and the PMOS is on. This 'inverted' action is why the PMOS digital symbol has dot on the gate.
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