# How does a transistor work as a switch and amplifier?

1. Jan 30, 2013

### Hysteria X

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 cant seem to get it ...the diagrams just dont make sense to me ...Can anyone just explain to me in a real simple language about the mechanism would be really grateful

2. Jan 30, 2013

### skeptic2

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

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. Jan 30, 2013

### Drakkith

Staff Emeritus
Re: How does a transistor work as a switch and amplifier??

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. Jan 30, 2013

### sophiecentaur

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

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. Jan 30, 2013

### Hysteria X

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

Yes i know all the basics pretty well

6. Jan 30, 2013

### Hysteria X

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

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

7. Jan 30, 2013

### nsaspook

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

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. Jan 30, 2013

### Drakkith

Staff Emeritus
Re: How does a transistor work as a switch and amplifier??

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.

9. Jan 31, 2013

### sophiecentaur

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

Oh boy! That's just about the simplest amplifier diagram you will ever see.
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. Jan 31, 2013

### cosmik debris

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

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.