If it is LOW, the NMOS transistor is turned “OFF”, and the output terminal is disconnected from the input. Therefore, as long as the gate control voltage, V C is HIGH, the NMOS transistor passes the input voltage to the output. If however V IN becomes zero (LOW) while the gates control voltage is still positive, the transistor channel is still open but the drain-to-source voltage, V DS is zero, so no drain current flows through the channel and thus the output voltage is zero. If the input voltage, V IN is positive and greater than V C current will flow from the drain terminal to the source terminal, thus connecting V OUT to V IN. When there is a positive control voltage +V C at the gate terminal, the transistor is turned “ON” and in its saturation region acting as a closed switch. Then the NMOS is acting an open switch so any voltage at the input will not be passed to the output. When the control voltage, V C on the gate is zero (LOW), the gate terminal will not be positive with respect to either input terminal (drain) or the output terminal (source), thus the transistor is in its cut-off region and the input and output terminals are isolated from each. Assuming that the drain and source terminals are identical, the input is connected to the Drain terminal and the control signal to the gate terminal as shown. The N-channel metal-oxide semiconductor (NMOS) transistor can be used as a transmission gate for the passing of analogue signals. Nevertheless, the ability of complementary metal-oxide semiconductor FETs to perform as analogue switches and transmission gates remains high, and MOSFET devices, in particular the enhancement MOSFET which requires a voltage to be applied to the gate to turn it “ON” and zero voltage to turn it “OFF” are the most commonly used switching transistor. However, while CMOS switches do make excellent transmission gates, their “ON” state resistance, R ON can be several ohms creating an I 2*R power loss, while their “OFF” state resistance can be several thousand ohms allowing pico amperes of current to still flow through the channel. We would like to think that if we applied a signal to its input pin this would result in the signal being identical and without loss at the output pin, and vice versa. However, solid-state analogue switches are not ideal as there is always some loss associated with the conducting channel due to its resistive value when ON.

The Ideal SwitchĪn ideal analogue switch would create a short-circuit condition when closed and an open-circuit condition when open, in a similar fashion to a mechanical switch. Also, since the eMOSFET is used mainly as a switching device, it generally operates between its cut-off and saturation regions thus V GS acts as an ON/OFF control voltage for the MOSFET.

Note that a MOSFETs Threshold Voltage, V T is the minimum voltage applied to the gate terminal for the main channel between the drain and source terminals to start conducting. An N-channel MOSFET behaves like an open switch when the gate-source voltage, V GS is less than the threshold voltage, V T.An N-channel MOSFET behaves like an closed switch when the gate-source voltage, V GS is greater than the threshold voltage, V T.Then we can see that for the n-channel (NMOS) and p-channel (PMOS) enhancement MOSFET to operate as an open (OFF) or closed (ON) device the following conditions must be true: CMOS devices where both NMOS and PMOS transistors are fabricated into the same gate circuit, can pass (closed-condition) or block (open-condition) an analogue or digital signal, depending on the digital logic level that controls it. MOS technology uses both NMOS and PMOS devices to perform the logical switching functions, thus allowing a digital computer or logic circuit to control the operation of these analogue switches. The obvious choice is to use much faster acting solid state electronic switches which use metal oxide semiconductor (MOS) analogue gates to route the signal currents from their input to their output, with the well-known CMOS 4016B bilateral switch being the most common example. The switching and routing of digital and analogue signals (both voltage and current) can easily be done using mechanical relays and their contacts, but these can be slow and costly. The open and closed operations of the switch positions are usually controlled by some digital logic network, with standard analogue switches available in many styles and configurations that we can use as a transmission gate.įor example, single or dual normally open (NO) or normally closed (NC), single-pole single-throw (SPST), single-pole, double-throw (SPDT) configurations etc, in much the same way as for conventional electromechanical relays and contacts. The analogue switch is a solid-state semiconductor switch that controls the transmission path of analogue signals.

Cmos transistor diagram