İletim Kapısı / Transmission Gate

The transmission gate is a binary switch consisting of NMOS and PMOS transistors controlled by externally implemented logic levels.

The analog switch is a solid-state semiconductor switch that controls the transmission path of analog signals.Open and closed operations of key locations are controlled by some digital logic networks with standard analog switches, which are usually found in many styles and configurations.For example, single or double normally on (NO) or normally off (NC), single-pole single-shot (SPST), unipolar, double-shot (SPDT) configurations, etc. relays and contacts.

Switching and routing digital and analog signals (both voltage and current) can be easily done using mechanical relays and their contacts, but these can be slow and costly.The obvious choice is to use solid-state electronic switches that move much faster, using metal oxide semiconductor (MOS) analog doors to direct signal currents from inputs to outputs, the most common example is the well-known CMOS 4016B two-way switch.

MOS technology uses both NMOS and PMOS devices to perform logical switching functions, allowing a digital computer or logic circuit to control the operation of these analog switches.CMOS devices, where both NMOS and PMOS transistors are produced in the same door circuit, can pass (off condition) or block an analog or digital signal depending on the level of digital logic that controls it (open condition).

The type of solid state switch that allows the transmission of a signal or data in both directions is called Transmission Gate or TG.But first, let's consider the operation of the Field Effective Transistor, or FET, as a basic analog switch.

MOSFET as Analog Switch

Both Bipolar Junction Transistors (BJTs) and Field Effective Transistors (FET's) can be used as a unipolar electronic switch in a wide range of applications.The main advantages of MOSFET or metal oxide-semiconductor FET technology compared to bipolar devices are that the door terminal is insted with a thin layer of metal oxide through the main conductive channel and the main MOSFET channel used for switching is completely resistant.

Consider the following basic N-channel and P-channel development MOSFET (eMOSFET) configurations.

MOSFET as Key

transmission door

Then, we can see that the following conditions must be correct for mosfet to operate as an open (OFF) or off (ON) device for n-channel (NMOS) and p-channel (PMOS) development:

  • An N-channel MOSFET acts as a closed switch when the door-welding voltage is greater than V T than the V GS threshold voltage.So V GS > V T
  • An N-channel MOSFET acts as an open switch when the door-welding voltage is less than V T than the V GS threshold voltage.So V GS < V  T  
  • A P-channel MOSFET acts as a closed switch when the door discharge voltage is lower than V T than the V GD threshold voltage.So V GD < V  T  
  • A P-channel MOSFET acts as an open switch when the door discharge voltage is greater than V T than the V GD threshold voltage.So V GD > V T

Note that a MOSFET Threshold Voltage is the minimum voltage applied for the V T to start transmitting to the door terminal between the drain and welding terminals for the main channel.Also, since eMOSFET is mainly used as a switching device, it usually works between cutting and saturation zones, so V GS acts as an ON/OFF control voltage for MOSFET.

Ideal Key

transmission door

An ideal analog switch, similar to a mechanical switch, will create a short circuit state when turned off and an open circuit state when it is on.

However, solid state analog switches are not ideal, as there is some loss associated with the conductive channel due to the resistance value when it is ON.

If we apply a signal to the input pin, we would like to think that the signal on the output pin will be the same and lossless, and vice versa.However, while CMOS switches make excellent transmission doors, "ON" state resistors can be several ohms that create R ON , I 2 *R power loss, while "OFF" status resistors can be several thousand ohms and allow pico amperage current. still flowing through the canal.

However, complementary metal oxide semiconductor FETs remain capable of performing as analog switches and transmission doors, and MOSFET devices are the most widely used switching transistor to make MOSFET development "OFF", which requires a voltage to be applied to the door, especially to apply "ON" and zero voltage to the door.

NMOS Switch

The N-channel metal oxide semiconductor (NMOS) transistor can be used as a transmission door for the transmission of analog signals.Assuming that the evacuation and welding terminals are the same, the entrance is connected to the evacuation terminal and the control signal to the gate terminal as shown.

NMOS FET as Analog Switch

transmission door

When the control voltage at the door is V C zero (LOW), the door terminal will not be positive compared to the entrance terminal (drainage) or the output terminal (weld), so the transistor is in the cutting zone. and the entrance and exit terminals are isolated from each other.NMOS then acts as an open switch so that any voltage at the input is not transmitted to the output.

When the positive control voltage in the door terminal is +V C, the transistor becomes "ON" and acts as a closed switch in the saturation zone.If the input voltage is V IN positive and larger than the V C current, it flows from the discharge terminal to the welding terminal, thereby connecting the V OUT to V IN.

However, if V IN is zero (LOW) while the door control voltage is still positive, the transistor channel is still open, but the voltage from drainage to source, V DS is zero, so no drainage current flows through the channel and therefore the output voltage is zero.

Therefore, as long as the door control voltage V C is HIGH, the NMOS transistor passes the input voltage to the output.If LOW, the NMOS transistor is turned off and the output terminal is separated from the entrance.Thus, the control voltage on the door, V C , determines whether the transistor is "on" or "off" as a switch.

A problem with the NMOS switch here is that the voltage from the gate to the source, V GS , must be significantly larger than the channel threshold voltage to make it fully ON, otherwise there will be a voltage drop along the channel.Thus, the NMOS device can transmit only a "weak" logic "1" (HIGH) level but a strong logic "0" (LOW) lossless.

PMOS Key

The P-channel metal oxide semiconductor (PMOS) transistor is similar, but in terms of polarity it contrasts with the previous NMOS device and the current flows from the source to the drain in the opposite direction.Then the input for a PMOS device is connected to the welding terminal and the control signal door terminal as shown.

PMOS FET as Key

transmission door

For PMOS FET, when the control voltage at the door is V C zero and therefore more negative than the input terminal (welding) or output terminal (drainage), the transistor is in the "ON" and saturation zone. acts as a closed key.If the input voltage is V IN positive and larger than the V C current, the current will flow from the source terminal to the drainage terminal, that is, I D flows out of the drainage, thereby connecting V IN and V OUT.

If the input voltage is V IN zero (LOW) while the door control voltage is still zero or negative, the PMOS channel is still open, but from the source to the drain voltage, the V SD is zero, so the current does not flow through the channel, and thus the voltage in the output (discharge) is zero.

When the positive control voltage in the door terminal is +V C, the channel of the PMOS transistor becomes "OFF" and acts as an open switch in the cutting zone.Thus, there is no drainage current, I D flows through the conductive channel.

Therefore, as long as the door control voltage V C is LOW (or negative), the PMOS transistor will transmit the input voltage to the output.If it is HIGH, the PMOS transistor is turned off and the output terminal is separated from the entrance.Thus, as with the previous NMOS device, the control voltage, V C on the door, determines whether the transistor is "on" or "off" as a switch.

The problem with the PMOS switch is that the door-weld voltage, V is that the GS should be significantly less channel threshold voltage should be larger than the full off turn or the current will still flow through the channel.Thus, the PMOS device can transmit a "strong" logic "1" (HIGH) level without loss, but a weak logic "0" (LOW).

Thus, we can see that the voltage from the positive door to the source for an NMOS device causes the current to flow in one direction from the Drain to the Source, while for the PMOS device, a negative voltage flows from the door to the source. It's in the opposite direction from the source evacuation.

However, the NMOS device passes only a strong "0" but a weak "1", while the PMOS device passes a strong "1" but a weak "0".Thus, by combining the features of NMOS and PMOS devices, it is possible to transmit both a strong logic "0" and a strong logic "1" in both directions without any disruption.This then forms the basis of a Transmission Door.

Transmission Door

By connecting PMOS and NMOS devices in parallel, we can create a basic dual CMOS switch, commonly known as the "Transmission Gate".Note that transmission doors are quite different from traditional CMOS logic gates, since the transmission door is symmetrical or two-sided, that is, the input and output are interchangeable.This two-way operation is shown in the following transmission gate symbol, which shows two overlapping triangles facing in opposite directions to indicate two signal directions.

CMOS Transmission Door

transmission door

The two MOS transistors are connected repeatedly in parallel with an inverter used between the NMOS and PMOS gate to provide two complementary control voltages.When the input control signal V C is LOW, both NMOS and PMOS transistors are disconnected and the switch is turned on.V Any Is High, both devices are pressed to the transmission and the switch is turned off.

Thus, the transmission door acts as a "closed" switch when V C = 1, while when the door is V C = 0, it acts as an "open" switch when it is operating as a voltage-controlled switch.Balloon of the symbol showing the door of PMOS FET.

Transmission Gate Boolean Expression

As with traditional logic gates, we can define the operation of a transmission door using both a truth table and a boolean expression as follows.

Transmission Gate Accuracy Table

transmission door

From the accuracy table above, we can see that the output in B depends not only on the logic level of input A, but also on the level of logic found in the control input.Therefore, the logic level value of B is defined as both A AND Control, which gives us the boolean expression for a transmission gate:

B = A.Control

Because the boolean expression of a transmission pass contains the logical AND function, it is possible to implement this operation using a standard 2-input AND gateway; this is one input data entry and the other is the control entry as shown.

AND Door Application

transmission door

Another point to consider about transmission gates is that a single NMOS or a single PMOS can be used as a CMOS switch, but the combination of two transistors in parallel has some advantages.A FET channel is resistant, so the ON resistors of both transistors are effectively connected in parallel.

On-resistance door-to-source voltage, such as a FET, V is a function of GS as a transistor becomes more conductive due to the less gate drive, the other transistor takes over and becomes more conductive.Therefore, the combined value of the two ON resistances (as low as 2 or 3Ω) remains constant more or less than if a single switching transistor was on its own.

I can show you when in the diagram below.

Transmission Gate ON-resistance

transmission door

Summarize

Here we found that by connecting a P-channel FET (PMOS) to an N-channel FET (NMOS), we can create a solid state switch that is digitally controlled using logic level voltages and is often called the "transmission gate".

Transmission Door (TG) is also a binary key that can be terminal input or output.The entrance and exit terminals, as well as the transmission door, have a third connection called control; where the control input determines the switching status of the door as an open or closed (NO/NC) switch.

This input is typically driven by a digital logic signal that switches between soil (0V) and a DC voltage, usually VDD.When the control input is low (Control = 0), the switch is turned on and the switch is turned off when the control input is HIGH (Control = 1).

Transmission doors act like voltage-controlled switches, and as switches, CMOS transmission doors can be used to switch both analog and digital signals that, as discussed, exceed the entire voltage range (from 0V to V DD) in both directions. a single MOS device.

The combination of an NMOS and a PMOS transistor together within a single door means that the NMOS transistor will convey a good logic "0" but a weak logic "1", while the PMOS transistor will transfer a good logic "1" but a weak logic. "0".Therefore, connecting an NMOS transistor in parallel with a PMOS transistor provides a single binary switch that offers efficient output drive capability for CMOS logic gates controlled by a single level of input logic.