Question about the BJT as switch circuit

In summary, a BJT (Bipolar Junction Transistor) can act as a switch in a circuit by controlling the flow of current from the collector to the emitter. It is biased by applying a voltage to the base-emitter junction, with a positive bias resulting in the "on" state and a negative bias resulting in the "off" state. BJTs have advantages such as being cost-effective, fast switching speed, and low on-resistance, but they also have limitations such as requiring a significant amount of current and having a non-linear voltage-current relationship. To protect the BJT in a switch circuit, a current limiting resistor, diode, and proper heat sinking and temperature control can be used.
  • #1
shinsj
3
0
1. Homework Statement
my teacher wants me to design a circuit to control the on or off of 4 BJTs in the circuit.


Homework Equations





The Attempt at a Solution


so i tryed to control a BJT at once .. it workshttp://user.qzone.qq.com/259234914/infocenter#!app=4

but there were somewrong with control 2 bjt in the circuit .. the BJT can not work whatever i put the value of resistance in the circuit .http://user.qzone.qq.com/259234914/infocenter#!app=4
 

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  • #2
In the second diagram, the bases appear to be supplied from a negative voltage.
 

1. How does a BJT act as a switch in a circuit?

A BJT (Bipolar Junction Transistor) can act as a switch in a circuit by controlling the flow of current from the collector to the emitter. In its "on" state, the base-emitter junction of the BJT is forward biased, allowing current to flow from the collector to the emitter. In its "off" state, the base-emitter junction is reverse biased, blocking the flow of current. By controlling the biasing of the base-emitter junction, the BJT can be used to switch a circuit on and off.

2. How is the BJT biased in a switch circuit?

In a switch circuit, the BJT is biased by applying a voltage to the base-emitter junction. This voltage can be either positive or negative, depending on the type of BJT (NPN or PNP). When the base-emitter junction is biased in the forward direction, the BJT is in its "on" state. When the junction is biased in the reverse direction, the BJT is in its "off" state.

3. What are the advantages of using a BJT as a switch in a circuit?

There are several advantages of using a BJT as a switch in a circuit. BJTs are relatively inexpensive and readily available, making them a cost-effective option for switching applications. They also have a fast switching speed, making them suitable for high-speed circuits. Additionally, BJTs have a low on-resistance, meaning that they can handle large currents without significant voltage drops.

4. What are the limitations of using a BJT as a switch in a circuit?

One limitation of using a BJT as a switch is that it requires a significant amount of current to flow through the base-emitter junction to turn it on. This can cause power dissipation and limit its use in high-power applications. Additionally, BJTs have a non-linear voltage-current relationship, which can result in distortion in the output signal.

5. How can the BJT be protected in a switch circuit?

To protect the BJT in a switch circuit, a current limiting resistor can be placed in series with the base-emitter junction. This will limit the amount of current flowing through the junction and prevent damage to the BJT. Additionally, a diode can be placed in parallel with the BJT to protect it from reverse voltage spikes. Proper heat sinking and temperature control can also help to protect the BJT in high-power applications.

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