- MOSFET as a Switch
- BJT Transistor as a Switch, Saturation Calculator
- The Bipolar Junction Transistor (BJT) as a Switch
MOSFET as a SwitchA bipolar junction transistor BJT can be used in many circuit configurations such as an amplifier, oscillator, filter, rectifier or just used as an on-off switch. If the transistor is biased into the linear region, it will operate as an amplifier or other linear circuit, if biased alternately in the saturation and cut-off regions, then it is being used as a switch, allowing current to flow or not to flow in other parts of the circuit. This lab activity describes the BJT when operated as a switch. Switching circuits are significantly different than linear circuits. They are also easier to understand. Before investigating more complex circuits, we will begin by introducing discrete solid-state switching circuits: those built around BJTs. A switch consists of a BJT transistor that is alternately driven between the saturation and cutoff regions. A simple version of the switch is shown in figure 1. When the input equals - V inthe base-emitter junction is reverse biased or off so no current flows in the collector. This is illustrated by the load line shown in the figure. When the BJT is in cutoff, the circuit ideally has the following values:. Condition 1 guarantees that the circuit is driven into the cutoff region by the input. Conditions 2 and 3 assure that the transistor will be driven into the saturation region. An actual BJT switch differs from the ideal switch in several aspects. In practice, even in cutoff there is some leakage current through the transistor. Also, in saturation, there is always some voltage dropped across the transistor's internal resistance. Typically, this will be between 0. These variations from the ideal are generally minor with a properly sized device, so we can assume near ideal conditions when analyzing or designing a BJT switch circuit. An LED driver is shown in figure 2. The driver shown in this figure is used to couple a low current part of the circuit to a relatively high current device the LED. When the output from the low current circuit is low 0 Vthe transistor is in cutoff and the LED is off. The driver is used because the low-current part of the circuit may not have the current capability to supply the 20 mA typical required to light the LED to full brightness. Build the LED switch circuit shown in figure 2 on your solder-less breadboard. The switch is controlled by digital output DO from the Discovery connector. Scope channel 1 will display the voltage across the switch transistor Q 1 V CE and scope channel 2 will display the voltage across the LED.
BJT Transistor as a Switch, Saturation Calculator
Bipolar junction transistors Also known as BJTs can be used as an amplifier, filter, rectifier, oscillator, or even a switch, which we cover an example in the first section. The transistor will operate as an amplifier or other linear circuit if the transistor is biased into the linear region. The transistor can be used as a switch if biased in the saturation and cut-off regions. This allows current to flow or not in other parts of a circuit. A relatively small flow of electrons sent through the base of the transistor has the ability to exert control over a much larger flow of electrons through the collector. Suppose we had a lamp that we wanted to turn on and off with a switch. Such a circuit would be extremely simple, as in the figure below a. Remember that the controlled current through a transistor must go between collector and emitter. Since it is the current through the lamp that we want to control, we must position the collector and emitter of our transistor where the two contacts of the switch were. A PNP transistor could also have been chosen for the job. Its application is shown in the figure above c. Consequently, the transistor cannot turn on. Perhaps, the simplest thing to do would be to connect a switch between the base and collector wires of the transistor as in figure a below. In this state, the transistor is said to be cutoff. If the switch is closed as in figure bcurrent will be able to flow from the base to the emitter of the transistor through the switch. This base current will enable a much larger current flow from the collector to the emitter, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated. Of course, it may seem pointless to use a transistor in this capacity to control the lamp. A regular switch will suffice the function instead of a transistor. Two points can be made here. This may be an important advantage if the switch has a low current rating: a small switch may be used to control a relatively high-current load. More importantly, the current-controlling behavior of the transistor enables us to use something completely different to turn the lamp on or off. Consider the figure below, where a pair of solar cells provides 1 V to overcome the 0. Or, we could use a thermocouple many connected in series to provide the necessary base current to turn the transistor on in the figure below. Even a microphone see the figure below with enough voltage and current from an amplifier output could turn the transistor on, provided its output is rectified from AC to DC so that the emitter-base PN junction within the transistor will always be forward-biased:. The point should be quite apparent by now. Any sufficient source of DC current may be used to turn the transistor on, and that source of current need only be a fraction of the current needed to energize the lamp. Here we see the transistor functioning not only as a switch, but as a t rue amplifier: using a relatively low-power signal to control a relatively large amount of power. Please note that the actual power for lighting up the lamp comes from the battery to the right of the schematic. It is not as though the small signal current from the solar cell, thermocouple, or microphone is being magically transformed into a greater amount of power. Don't have an AAC account? Create one now. Forgot your password? Click here.
The Bipolar Junction Transistor (BJT) as a Switch
With the help of such analogue switches, digital systems can control the flow of signals in analogue circuits. It is a type of field effect transistor with an insulated gate from the channel hence, sometimes called as Insulated Gate FET or IGFET and the voltage at the gate terminal determines the conductivity. A channel is created when the voltage between gate and source terminals V GS is greater than the threshold voltage V TH. The gate current in either regions of operation is very less almost equal to zero. They are:. In both these regions, the MOSFET is in ON state but the difference is in linear region, the channel is continuous and the drain current is proportional to the resistance of the channel. Semiconductor switching in electronic circuit is one of the important aspects. Assume the device is initially OFF. The voltage across Gate and Source i. This makes the Light to turn ON. This in turn will make the light to turn OFF. It is obvious that you cannot directly control the LED with the help of the microcontroller. You need a device that bridges the gap between the microcontroller and the LED. This device should take in a control signal from the microcontroller usually the voltage of this signal is in the working voltage range of the microcontroller, 5V for example and supply power to the LED, which in this case is from a 12V supply. The setup of the above mentioned scenario is shown in the following circuit. In the above case i. The starting current also called as in-rush current of a motor will be very high. So, even with R DS of 0. Also, when driving a motor, the back emf is an important factor that has to be considered while designing the circuit. Your email address will not be published. Table of Contents. Leave a Reply Cancel reply Your email address will not be published.
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