Tutorial 6 - Transistors and MOSFETs

 

Learning Objectives:

Bipolar Junction Transistors

 

Transistors are solid state electronic switches.  Solid state means that there are no moving parts, and the switching action is regulated by the arrangement of the semiconductor materials.  A transistor has three terminals:

  • Collector takes current form the top rail (wire connected to the positive of the power supply).

  • Emitter delivers current to the load.

  • Base this is connected to the sensor part of the circuit.  The increase in voltage at the base turns the transistor on.

 

Click on the button to find out how a transistor works.

Bipolar Transistor

 

 

 

Question 1

Complete the gap-fill exercise

Question

 

Current in a transistor

If the base emitter voltage is higher than about 0.5 - 0.6 V, the transistor turns on.  We say that it is forward-biased.  Current flows through an npn transistor from two sources:

For a pnp transistor, the emitter current splits to the emitter-base current and the emitter-collector current.

 

If the voltage is reversed on a transistor, no current will flow.  We call this reverse bias.

 

 

 

 

Notice here that

It doesn't take a genius to see that:

 

5 mA + 495 mA = 500 mA

 

Ie = Ibe + Ice

 

This is consistent with Kirchhoff's First Law.

 

We can measure the current gain of a transistor, which is the ratio of the collector-emitter current to the base-emitter current. 

 

Gain = Ice Ibe

 

Gain is just a number; it has no units.  A gain of 40 tells us that the collector-emitter current is 40 times the base-emitter current.  So if the base-emitter current is 10 mA, the collector-emitter current is 400 mA = 0.4 A.  In catalogues, you may see gain as hfe.

 

Question 2

What is the current gain of the transistor shown above?

Answer

 

Here is a typical circuit involving a transistor.  It is a light operated switch that uses a relay to turn on a mains-powered bulb.

 

 

  1. When the LDR is in the dark, its resistance is high.

  2. The LDR is part of a potential divider.

  3. Therefore there is a large voltage across the LDR.

  4. There is a voltage drop across the base of the transistor so that the base is at 0.7 V.

  5. The resistor R3 limits the current to the base.

  6. The transistor is turned on.

  7. It can allow a big enough current to flow to turn the relay on.

  8. The reverse biased diode D1 is there to protect the transistor from high voltage spikes that can occur when the relay turns off.  These could wreck the transistor.

 

Question 3

A transistor circuit is shown below:

V out = 7.0 V.  Calculate:

(a) The voltage across the load (1 k) resistor.

(b) The current through the load resistor

(c) The base - emitter current, assuming that V be = 0.5 V

(d) The current gain of the transistor

Answer

 

 

Characteristic Graphs for the Bipolar Transistor

Semiconductor transistors need a base-emitter voltage of about 0.6 V before they switch on, about the same as a diode.  We can plot a voltage current graph.  The gradient gives the conductance of the transistor. 

 

 

The voltage current characteristic sometimes called the transconductance.

 

We can measure the collector-emitter current and plot it against the base-emitter current:

 

 

There is a linear region which gives the gain.  The gain is the gradient.  At very low currents, the graph is not quite linear, but we can ignore this.  At large values of base-emitter current, there is saturation.  We call this graph the transfer characteristic.  The transfer characteristic is specific for the kind of transistor.  In some texts, you may see that the transfer characteristic is a curve.  In which case, you need to take a tangent to find the gradient at a particular point.

 

The gain is referred to in some texts as hfe (h-parameter, forward bias, common emitter).  If the transistor is in a DC circuit, the gain parameter is written as hFE.  There is a subtle difference, but practical electronic engineers ignore it.  The circuits we are looking at are in the common emitter arrangement.  It is possible to have common base, and common collector arrangements. 

 

The theory of bipolar transistors is very complex, and is beyond the scope of these notes.

 

 

 

MOSFETs

The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a Voltage controlled device.    This means that a voltage at the gate means that a current flows from the drain to the source.  For more about the MOSFET, press the button.

 

MOSFETs

 

There are three terminals:

        

 

We can use MOSFETs where we have a source of voltage that can provide very little current.  This circuit is a touch sensor:

 

 

 

 

 

The general characteristics for a MOSFET are:

The transconductance characteristic of a MOSFET is shown here.

 

 

From this graph, the MOSFET turns on with a gate-source voltage of 2 V.  At 2.77 V the MOSFET is saturated and cannot carry a bigger current.

 

We need to note the following about this graph:

 

Gain = DID

            DVGS

[DID change in drain current caused by the change DVGS.  Units are milliamps per volt (mA/V), or milliseimens (mS)*]

 

Transconductance can be measured by working out the gradient of the graph.

 

 

 

Here the MOSFET is used as a switch.  There are advantages and disadvantages when compared to the bipolar transistor as a switch:

 

Advantages of a MOSFET

Disadvantages

Switching time is about 10 times faster than a bipolar transistor

Higher resistance than a bipolar transistor

Very much smaller switching current

Can be destroyed by high voltages, especially static electricity

Less affected by temperature

 

 

 

Darlington Pair

In this experiment a MOSFET is compared with a bipolar transistor to switch on a motor.

 

 

 

The motor ran when the sensor (two 4 mm plugs) was put in the coffee. 

 

But when a bipolar transistor was used, there was no effect.  However when a power transistor was added to the output of the bipolar transistor, the motor did run.

 

 

 

The arrangement on the right is called a Darlington pair.  On prototype board, it looks like this.

 

 

Summary

Transistor

  • Current controlled device.

  • Switches on when base-emitter voltage is 0.7 V

  • Requires a base current of about 1 10 mA.

MOSFET

  • Voltage controlled device

  • Input resistance is very high (1012 W).

  • Very sensitive to static electricity.

 

Links

How transistors work (St Andrews University)

Link

Measuring transistor properties

Link

History of the transistor

Link

Video Tutorial

Link

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