Switches in Electronics Circuit

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BJT or MOSFET each of these act as a switch in vivid applications. By the not just a switch!! These are high performance switches which work at very high switching speeds...may be in fraction of secs or milli secs!

Now, What do you understand by the term Switch??

Definition: A switch is a device for making and breaking the connection in an electric circuit.


Since the basic definition is lucid now, lets go into further details. Switches can be of two types :

1. Electromechanical switch
2. Electronic switch

Electromechanical Switch:

The switches that you use in your homes for day-to-day purposes like turning on your fans and lights or the motorized toy cars we used to play with. The fig below shows the inside out diagram of a basic level electromechanical switch.








As we are more concerned with the electronics side, we shall not go in depths with this electromechanical switch.

Electronic Switch:

BJT and MOSFET are the prominent transistors which serve as high speed switching devices in the electronic circuitry.

BJT as Switch:

Both types of BJT function by letting a small current input to the base control an amplified output from the collector. The result is that the transistor makes a good switch that is controlled by its base input.



The areas of operation for a transistor switch are known as the Saturation Regionand the Cut-off Region. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its "fully-OFF" (cut-off) and "fully-ON" (saturation) regions as shown below.

I - V Curve

MOSFET as Switch :

Just like the BJT the MOSFET can also be used as a fast transition switching device. As the MOSFET is the most popularly used transistor for switching circuits lets see its types and working.

MOSFET analog switchMOSFET analog switches use the MOSFET to pass analog signals when on, and as a high impedance when off. Signals flow in both directions across a MOSFET switch. In this application, the drain and source of a MOSFET exchange places depending on the relative voltages of the source/drain electrodes. The source is the more negative side for an N-MOS or the more positive side for a P-MOS. All of these switches are limited on what signals they can pass or stop by their gate–source, gate–drain and source–drain voltages; exceeding the voltage, current, or power limits will potentially damage the switch.

Single-type MOSFET switchThis analog switch uses a four-terminal simple MOSFET of either P or N type.
In the case of an n-type switch, the body is connected to the most negative supply (usually GND) and the gate is used as the switch control. Whenever the gate voltage exceeds the source voltage by at least a threshold voltage, the MOSFET conducts. The higher the voltage, the more the MOSFET can conduct. An N-MOS switch passes all voltages less than Vgate–Vtn. When the switch is conducting, it typically operates in the linear (or ohmic) mode of operation, since the source and drain voltages will typically be nearly equal.
In the case of a P-MOS, the body is connected to the most positive voltage, and the gate is brought to a lower potential to turn the switch on. The P-MOS switch passes all voltages higher than Vgate–Vtp (threshold voltage Vtp is negative in the case of enhancement-mode P-MOS).
A P-MOS switch will have about three times the resistance of an N-MOS device of equal dimensions because electrons have about three times the mobility of holes in silicon.

Dual-type (CMOS) MOSFET switchThis "complementary" or CMOS type of switch uses one P-MOS and one N-MOS FET to counteract the limitations of the single-type switch. The FETs have their drains and sources connected in parallel, the body of the P-MOS is connected to the high potential (VDD) and the body of the N-MOS is connected to the low potential (Gnd). To turn the switch on, the gate of the P-MOS is driven to the low potential and the gate of the N-MOS is driven to the high potential. For voltages between VDD–Vtn and Gnd–Vtp, both FETs conduct the signal; for voltages less than Gnd–Vtp, the N-MOS conducts alone; and for voltages greater than VDD–Vtn, the P-MOS conducts alone.
The voltage limits for this switch are the gate–source, gate–drain and source–drain voltage limits for both FETs. Also, the P-MOS is typically two to three times wider than the N-MOS, so the switch will be balanced for speed in the two directions.
Tri-state circuitry sometimes incorporates a CMOS MOSFET switch on its output to provide for a low-ohmic, full-range output when on, and a high-ohmic, mid-level signal when off.