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In the previous tutorial we saw that the standard Bipolar Transistor or BJT, comes in two basic forms. AnNPN (Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type, with the most commonly used transistor type being the NPN Transistor. We also learnt that the transistor junctions can be biased in one of three different ways - Common Base, Common Emitter and Common Collector. In this tutorial we will look more closely at the "Common Emitter" configuration using NPN Transistors and an example of its current flow characteristics is given below.
An NPN Transistor Configuration
Note: Conventional current flow.
We know that the transistor is a "CURRENT" operated device and that a large current (Ic) flows freely through the device between the collector and the emitter terminals. However, this only happens when a small biasing current (Ib) is flowing into the base terminal of the transistor thus allowing the base to act as a sort of current control input. The ratio of these two currents (Ic/Ib) is called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, (β). Beta has no units as it is a ratio. Also, the current gain from the emitter to the collector terminal, Ic/Ie, is called Alpha, (α), and is a function of the transistor itself. As the emitter current Ie is the product of a very small base current to a very large collector current the value of this parameter α is very close to unity, and for a typical low-power signal transistor this value ranges from about 0.950 to 0.999.
α and β Relationships
By combining the two parameters α and β we can produce two mathematical expressions that gives the relationship between the different currents flowing in the transistor.
The values of Beta vary from about 20 for high current power transistors to well over 1000 for high frequency low power type bipolar transistors. The equation for Beta can also be re-arranged to make Ic as the subject, and with zero base current (Ib = 0) the resultant collector current Ic will also be zero, (β x 0). Also when the base current is high the corresponding collector current will also be high resulting in the base current controlling the collector current. One of the most important properties of the Bipolar Junction Transistor is that a small base current can control a much larger collector current. Consider the following example.
An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate the base current Ib required to switch a resistive load of 4mA.
Therefore, β = 200, Ic = 4mA and Ib = 20µA.
One other point to remember about NPN Transistors. The collector voltage, (Vc) must be greater than the emitter voltage, (Ve) to allow current to flow through the device between the collector-emitter junction. Also, there is a voltage drop between the base and the emitter terminal of about 0.7v for silicon devices as the input characteristics of an NPN Transistor are of a forward biased diode. Then the base voltage, (Vbe) of an NPN Transistor must be greater than this 0.7 V otherwise the transistor will not conduct with the base current given as.
Where: Ib is the base current, Vb is the base bias voltage, Vbe is the base-emitter volt drop (0.7v) and Rbis the base input resistor.
An NPN Transistor has a DC base bias voltage, Vb of 10v and an input base resistor, Rb of 100kΩ. What will be the value of the base current into the transistor.
Therefore, Ib = 93µA.
The Common Emitter Configuration.
As well as being used as a switch to turn load currents "ON" or "OFF" by controlling the Base signal to the transistor, NPN Transistors can also be used to produce a circuit which will also amplify any small AC signal applied to its Base terminal. If a suitable DC "biasing" voltage is firstly applied to the transistors Base terminal thus allowing it to always operate within its linear active region, an inverting amplifier circuit called a Common Emitter Amplifier is produced.
One such Common Emitter Amplifier configuration is called a Class A Amplifier. A Class A Amplifier operation is one where the transistors Base terminal is biased in such a way that the transistor is always operating halfway between its cut-off and saturation points, thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of the AC input signal superimposed upon the DC Biasing voltage. Without this "Bias Voltage" only the positive half of the input waveform would be amplified. This type of amplifier has many applications but is commonly used in audio circuits such as pre-amplifier and power amplifier stages.
With reference to the common emitter configuration shown below, a family of curves known commonly as the Output Characteristics Curves, relates the output collector current, (Ic) to the collector voltage, (Vce) when different values of base current, (Ib) are applied to the transistor for transistors with the same βvalue. A DC "Load Line" can also be drawn onto the output characteristics curves to show all the possible operating points when different values of base current are applied. It is necessary to set the initial value ofVce correctly to allow the output voltage to vary both up and down when amplifying AC input signals and this is called setting the operating point or Quiescent Point, Q-point for short and this is shown below.
The Common Emitter Amplifier Circuit
Output Characteristics Curves for a Typical Bipolar Transistor
The most important factor to notice is the effect of Vce upon the collector current Ic when Vce is greater than about 1.0 volts. You can see that Ic is largely unaffected by changes in Vce above this value and instead it is almost entirely controlled by the base current, Ib. When this happens we can say then that the output circuit represents that of a "Constant Current Source". It can also be seen from the common emitter circuit above that the emitter current Ie is the sum of the collector current, Ic and the base current, Ib, added together so we can also say that " Ie = Ic + Ib " for the common emitter configuration.
By using the output characteristics curves in our example above and also Ohm´s Law, the current flowing through the load resistor, (RL), is equal to the collector current, Ic entering the transistor which inturn corresponds to the supply voltage, (Vcc) minus the voltage drop between the collector and the emitter terminals, (Vce) and is given as:
Also, a Load Line can be drawn directly onto the graph of curves above from the point of "Saturation" whenVce = 0 to the point of "Cut-off" when Ic = 0 giving us the "Operating" or Q-point of the transistor. These two points are calculated as:
Then, the collector or output characteristics curves for Common Emitter NPN Transistors can be used to predict the Collector current, Ic, when given Vce and the Base current, Ib. A Load Line can also be constructed onto the curves to determine a suitable Operating or Q-point which can be set by adjustment of the base current.