How does a transistor work as a switch and amplifier?

In summary: R1 and R2 keep the transistor from being damaged by the input signal. They block any DC voltage that might be on the input signal that would be harmful to the transistor. So the only thing that actually reaches the transistor is the AC signal.
  • #1
Hysteria X
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How does a transistor work as a switch and amplifier??

i know a transistor is a device that controls the flow of current but how does it act as a switch and an amplifier? i tried reading about it on so many places but i just can't seem to get it :confused: ...the diagrams just don't make sense to me :cry: ...Can anyone just explain to me in a real simple language about the mechanism would be really grateful o:)
 
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  • #2


A transistor in a circuit has three currents associated with it - base current (Ib), collector current (Ic) and emitter current (Ie). Base current enters the base and exits the emitter. Collector current enters the collector and exits the emitter. Emitter current is the sum of base and collector currents.

Collector current is related to base current by a relatively constant factor called beta or Hfe. A typical small signal transistor data sheet will indicate an Hfe of 70 - 300 meaning that the ratio of collector current to base current of an individual transistor of that type can be anywhere in that range.

Because a small current controls a much larger current, this allows the transistor to be used as an amplifier. If the amount of current supplied to the base is large then the amount of collector current causes the transistor to saturate or to conduct so hard the voltage from collector to emitter is nearly zero. This makes the transistor into a switch.
 
  • #3


Do you have a good grasp of basic electronics? Do you understand what voltage and current are? It would greatly help if we understood your knowledge level. Have you taken any electronics classes before?
 
  • #4


Drakkith said:
Do you have a good grasp of basic electronics? Do you understand what voltage and current are? It would greatly help if we understood your knowledge level. Have you taken any electronics classes before?
This is highly relevant. It is no good trying to run before you can walk. To understand how an amplifying device / component amplifies, you must first be really comfortable with what current, volts and resistance are and the mathematical relationships between them.
 
  • #5


Drakkith said:
Do you have a good grasp of basic electronics? Do you understand what voltage and current are? It would greatly help if we understood your knowledge level. Have you taken any electronics classes before?

Yes i know all the basics pretty well
 
  • #6


skeptic2 said:
A transistor in a circuit has three currents associated with it - base current (Ib), collector current (Ic) and emitter current (Ie). Base current enters the base and exits the emitter. Collector current enters the collector and exits the emitter. Emitter current is the sum of base and collector currents.

Collector current is related to base current by a relatively constant factor called beta or Hfe. A typical small signal transistor data sheet will indicate an Hfe of 70 - 300 meaning that the ratio of collector current to base current of an individual transistor of that type can be anywhere in that range.

Because a small current controls a much larger current, this allows the transistor to be used as an amplifier. If the amount of current supplied to the base is large then the amount of collector current causes the transistor to saturate or to conduct so hard the voltage from collector to emitter is nearly zero. This makes the transistor into a switch.


okay that helped but can you please explain me why there are so many other resistors in this diagram
Integrated-Transistor-Amplifier.jpg
 
  • #7


R1 and R2 form a voltage divider that sets the bias point for the transistor. If it's a class A amp then RC would be picked to match the on resistance of transistor from C to E at the bias point so the voltage a C would be about 1/2 Vcc. The circuit could operate with only R1 setting the bias point but small changes in the base input Resistance would cause large shifts in the collector current so R2 is selected to stabilize the input Resistance.

http://www.electronics-tutorials.ws/blog/biasing-transistor-tutorial.html
 
  • #8


Hysteria X said:
okay that helped but can you please explain me why there are so many other resistors in this diagram
Integrated-Transistor-Amplifier.jpg

I'll give it a try.

Rc is what develops the load voltage.

R1 and R2 serve as biasing resistors. They develop the bias voltage on the transistors base. The bias voltage is what determines the size of the depletion region in between the emitter and collector of the transistor. This depletion region directly affects the resistance of the transistor to current through the emitter and collector. When a small voltage, say 0.5 volts, is applied to the transistor base, the resistance of the transistor is very high and very little current flows. Increasing the voltage to, say 0.9 volts, greatly decreases the resistance of the transistor and a much larger amount of current flows through the transistor. So increasing the bias voltage by 0.4 volts GREATLY amplified the current flow through the resistor, far more than increasing the transistor's emitter-collector voltage by 0.4 volts.

If the base voltage (bias voltage) is too low, practically no current can flow through the transistor and it effectively operates as a switch that is opened. Too high and the transistor hits saturation, which is where further increase of the bias voltage has little or no effect on the current flow through the transistor.

If we put some numbers in and did the math, we would find that by varying the voltage on the transistor base the output voltage (V out) is greatly affected. For example, hooking an antenna up to this transistor allows us to detect a very very small change in voltage, say a few microvolts from a signal, which in turn is applied to the base of the transistor. The output voltage changes with the input signal but is greatly amplified. Couple of whole bunch of these together and you have yourself a radio amplifier circuit.
 
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  • #9


Hysteria X said:
okay that helped but can you please explain me why there are so many other resistors in this diagram

Oh boy! That's just about the simplest amplifier diagram you will ever see. :wink:
Actually, you don't need that load resistor Rc. That resistor could actually be replaced by a device like a loudspeaker, headphones or a motor. It's just that it is easier to analyse a 'simpler' circuit as shown. The Load Resistor is where the 'output power' is dissipated in that basic diagram and the Vout terminal is just for measuring the volts. In order to 'amplify' a signal, you need to be able to vary / control the flow of power into a load (output), using a lower amount of power at the input. The easiest way to look at an amplifying device like a transistor is to liken it to a variable resistor on which you can move the knob very very quickly. The current into the base of the transistor is acting, essentially, as a current control knob. In some other devices, the current may be controlled by varying a voltage (thermionic valve and FET).
 
  • #10


Simply and not quite correct but close enough to understand the basics; an amplifier follows an input voltage and "amplifies" it by translating that to a swing between a higher supply voltage. A switch just has a lot of gain and as the input goes one way the output is slammed against one supply rail and as the input goes the other way the out is forced to the other supply rail. So, any small input swing results in a square wave output or switch.
 

FAQ: How does a transistor work as a switch and amplifier?

How Does a Transistor Work as a Switch and Amplifier?

Q1: What is a Transistor?

A transistor is a semiconductor device that can be used as both a switch and an amplifier in electronic circuits. It is a fundamental building block of modern electronics.

Q2: How Does a Transistor Work as a Switch?

A transistor can work as a switch in two states: ON (conducting) and OFF (non-conducting).

ON State: When a small voltage is applied to the base of a transistor (in a bipolar junction transistor, or BJT), or when the gate-source voltage exceeds a threshold (in a field-effect transistor, or FET), the transistor becomes conductive, allowing current to flow between the collector and emitter (BJT) or drain and source (FET). This is the ON state, and the transistor acts as a closed switch, allowing current to pass through.

OFF State: When there is no voltage applied to the base (BJT) or when the gate-source voltage is below the threshold (FET), the transistor becomes non-conductive. In this OFF state, the transistor acts as an open switch, preventing current flow between the collector and emitter (BJT) or drain and source (FET).

Q3: How Does a Transistor Work as an Amplifier?

A transistor can work as an amplifier by controlling a larger output current or voltage in proportion to a smaller input signal. There are two common types of transistor amplifiers: common-emitter (CE) for BJTs and common-source (CS) for FETs.

Common-Emitter Amplifier (BJT): In this configuration, a small input current applied to the base controls a larger output current flowing between the collector and emitter. The transistor amplifies the input signal by a factor known as the current gain (\( \beta \)).

Common-Source Amplifier (FET): In this configuration, a small input voltage applied to the gate controls a larger output current flowing between the drain and source. The transistor amplifies the input signal by a factor known as the transconductance (\( g_m \)).

Q4: What are the Applications of Transistor Switching?

Transistor switching is widely used in digital electronics. It forms the basis of digital logic gates, memory cells, and microcontrollers. Transistors as switches are used in computers, smartphones, and various electronic devices to process and control binary information (0s and 1s).

Q5: What are the Applications of Transistor Amplification?

Transistor amplifiers are used in a wide range of applications, including audio amplification, radio frequency (RF) amplification, signal processing, and communication systems. They are essential components in radios, televisions, audio amplifiers, and wireless communication devices.

Q6: How Can I Learn More About Transistors?

You can learn more about transistors and their applications by studying electronics textbooks, taking electronics courses, and exploring online resources dedicated to electronics and semiconductor devices. Practical experimentation and projects can also enhance your understanding of transistor operation.

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