# 5 - Transistors and MOSFET's

##### 2016-04-20 23:55:00 +0000, 2 years and 4 months ago

5.01 - junction transistors

Because Logic gates are only able and other such processing devices can only supply a small current (a few mA), enough to perhaps operate an LED. In order to operate more powerful output devices it is necessary to interface a ‘driver’. Transistors and MOSFET’s are able to switch currents of up to 100A and voltages of 1kW.

The symbol for an NPN transistor is:

Where, C = collector, B = base and E = emitter

+------+
+Supply+-->--+
+------+     |
+++
| |
| | R1
+++
|
*-->----
C  |        ^
+---+    +++++      |
--+R1 +----+NPN|      |  Vout
+---+  B +++++      |
| E      V
OV -------<--*--<----


In essence, when the base voltage exceeds the switching value of the transistor, it no longer has any resistance, because it closes, and becomes much like a SPST switch, which allows the Vsupply current to flow, since it is now a closed circuit loop.

When Vin exceeds the switching voltage, the transistor is said to ‘saturate’ high.

A typical transistor will begin to enter saturation at 0.5V and then fully saturate at 0.7V, this behaviour is shown in the following graph:

+Vs-+---.                          x = Vin
|    \ entering saturation    y = Vout
|      |
|      |
|      |
|       \   fully saturated
0.2V|        ---------------------
OV -+------------------------------
0.5 0.7


When Vin < 0.5v, Vout = +Vs (on)
When Vin > 0.7v, Vout = 0.2v (off)

Vce (voltage across the collector and emitter) > 12v

5.02 - selecting a transistor value

A selected transistor should be able to:

• Operate at the supply voltage (Vce, Voltage across the collector and emitter)
• Pass sufficient collector current (Ic)
• Dissipate sufficient power (Ptot)
• Give a large current gain (hFE)

The following example will be used:

+------+
+Supply+-->--+
+------+     |
+++++++++ }
|12V, 6W| } Vout
| LAMP  | }
+++++++++ }
|
V
C  |
+---+    +++++
--+R1 +->--+NPN|
+---+  B +++++
| E
OV -------<--+


Since the lamp is operating at 12V, the Vce must be greater than 12V.
For the 6W bulb to operate on 12V, the current flowing through it, Ic, must be 0.5A

When a transistor is on, there is approximately 0.2V across the collector and emitter. The current flowing, Ic, is 0.5A, the total power that must be dissipated by the transistor, Ptot, is therefore:

A transistors’ hFE is a measure of how many times larger the collector current is than the base current, a transistor should be chosen so that the hFE is 100 times greater than the collector current.

In selecting a suitable series resistor, R1, with the base of the transistor sufficient current needs to flow in order to fully saturate the transistor. However, a suitable resistor must be chosen so that the base-emitter is not damaged by high currents. Around 5mA~ is the maximum amount of current that can flow through a base of a small transistor.

To calculate the voltage across the resistor, you must take into account the 0.7V base-emitter voltage, and take that away from Vin (shown on the diagram). So that:

VR1 = Vin - 0.7_{base-emitter}

Allowing a maximum of 5mA into the base, the resistance of R1 can be found by:

5.03 - enhancement mode MOSFETS

MOSFET stands for Metal-oxide semiconductor field effect transistor.
MOSFET’s have very large input resistances (50MΩ), and a correspondingly large current gain.

The symbol for an n-channel MOSFET is shown below:

Where D = drain, G = gate and S = source
In order to switch the MOSFET, and thus allow drain current, Id, to flow, a voltage Vgs (gate-source), is applied, this is the same as the Vin of a transistor.
The behaviour of the MOSFET is also similar to a transistor, and is expressed in the following graph:

+Vs |---.                          x = Vgs
|    \ entering saturation    y = Vout
|      |
|      |
|      |
|       \   fully saturated
0.1V|        ---------------------
OV  +------------------------------
1     2


As shown by the graph, Vout does not change until Vgs is greater than 1V, drain current begins to flow through the MOSFET and it saturates at 0.1V, when Vgs is about 2V. Thus the MOSFET is acting in the same way as a transistor, as a switch.

The traditional layout of a MOSFET is shown below:

+------+
+Supply+-->--+
+------+     |
+++
| |
| | R1
+++
|
*-->----
|        ^
++++++++    |
----*-->--+MOSFET|    |
|     ++++++++    |
+++       |        | Vout
| |       |        |
| | R2    v        |
+++       |        |
|        |        V
OV -+-----<--*--<----


R2 is included to prevent damage from static electricity caused by its high input resistance, a typical value is 1MΩ.

A selected MOSFET should be able to:

• Operate at the supply voltage (Vds)
• Pass sufficient drain current (Id)
• Dissipate sufficient power (Pd)
• Give a very low drain resistance (rds)

The following example will be used to calculate rds

+------+
+Supply+-->--+
+------+     |
+++++++++ }
|12V, 6W| } Vout
| LAMP  | }
+++++++++ }
|
V
|
++++++++
----*-->--+MOSFET|
|     ++++++++
+++       |
| |       |
| | R2    V
+++       |
|        |
OV -+-----<--+


Since the lamp operates at 12V, Vds must be greater than 12V.
Id must be greater than 0.5A, since a 6W lamp on 12V will use 0.5A.
Rds must be low, less than 0.1Ω
With a Rds value ov 0.1Ω, the power dissipation required, Pd is:

The main advantage of a MOSFET over a transistor are:

• Very large input resistance
• Very large current gain
• Positive thermal coeffcient (if temperature increases, Rds increases to reduce current flowing into the drain, thus cooling the MOSFET)
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