This leads to considerable power consumption. Since the drain terminal is connected to the supply voltage, a current constantly flows through the transistor via the resistor when the transistor is on.
We have already established that the LED does not light up when the transistor is on, because the switched-on transistor connects the LED to ground via the source/drain path. Let’s look again at the basic circuit of the NMOS transistor, which implements a NOT gate, in a little more detail. But we still haven’t answered the question why there are two types of transistors, NMOS and PMOS. We have now built a NAND gate and a NOR gate with NMOS transistors. Notice how the LED is connected to ground when at least one of the two transistors is on, and the LED goes off in all of these cases. If we are using NMOS transistors, we can accomplish this by connecting two NMOS transistors in parallel. So either of the two inputs can be set to a 1 to cause the LED to go out. We need to find a circuit where the LED is “normally” not lit, and only lit when both inputs are 0. Let’s now try to build the NOR gate with NMOS transistors. This means that the output of a NOR gate is only 1 if both inputs are 0. Can we also build a NOR gate with NMOS transistors? NMOS NOR GateĪ NOR gate is the negation of an OR gate. Notice how the LED is only connected to ground if both transistors are on, and the LED only goes out in that one case. If we use NMOS transistors, we can achieve this by connecting two NMOS transistors in series. So both inputs together must use a 1 to make the LED go out. We need to find a circuit where the LED is “normally” on, and only off if both inputs are 1. Now let’s try to build the NAND gate with NMOS transistors. This means that the output of a NAND gate is 0 only if both inputs are 1. NMOS NAND GateĪ NAND gate is the negation of an AND gate. Is it possible to build other types of logic gates? So why do we need two different types at all? Patience! We will let the PMOS transistor do its magic when we will combine it with NMOS transistors. Like the NMOS transistor, the PMOS transistor in this circuit works like an inverter: When the switch at the gate is off, the LED is on. To distinguish between the two types, the PMOS transistor has a small circle drawn on the gate terminal. The PMOS transistor functions analogously to the NMOS transistor, but with reversed logic: The transistor is on when a 0 is present at the gate. Now let’s turn to the second type of MOSFET transistor, the PMOS transistor. Thus we have built our first logic gate: The NOT gate. You can see that the NMOS transistor in this circuit works like an inverter: When the switch at the gate is off, the LED is on. Without it, there would be a short circuit when the transistor is switched on. It’s purpose is to avoid that the LED and ground are directly connected to the power supply. An NMOS transistor is on when a 1 is present at the gate terminal.ĭon’t be confused by zigzag-shaped component at the power supply. Start the simulation in the circuit below and observe how the switch on the gate input turns the transistor on and off. If the transistor is off, the source/drain path is interrupted. When the transistor is on, signals can flow along the source/drain path. The signal at the gate terminal determines whether the transistor is on or off. MOSFETs have three terminals, labeled “Gate”, “Source” and “Drain”. However, as we will see below, they show their true power when transistors of both types are combined. Both types can be used individually to build logic gates.
There are two different types of MOSFET transistors: NMOS (n-type MOS) and PMOS (p-type MOS). It is only important to know that a transistor is an electronic switch. What this exactly means is not relevant for this tutorial. Nowadays, computers mainly use so-called “MOSFET” (Metal-Oxide-Semiconductor Field-Effect Transistor). There are many different types of transistors.
#SIGNAL PATH OF A TRANSISTOR MANUAL#
Unlike manual switches, which are operated by hand, electronic switches can be controlled by an electrical input signal. Transistors are electronic components that are essentially switches. Logic gates are constructed from so-called “transistors”. Move the mouse over the NAND gate to display the truth table of the NAND gate.īut how does a NAND gate work inside? Transistors Start the simulation and press different combinations of the two switches to observe which combinations make the LED light up. A modern microprocessor like the one in your smartphone contains billions of logic gates.Īn example of such a gate is the NAND gate shown in the circuit below. The heart of every computer consists of logic gates that perform the basic logical operations necessary to run programs.