  24 February 2009
Author: Giorgos Lazaridis
BJT Transistor theory

Transistor Circuit Essentials

Before we start talking about the different types of transistor connections, we first need to declare some characteristic sizes and symbols that will be used from now on.

IE is the Emitter current, IC is the Collector current and IB is the Base current. The direction of each current has to do with the type of the transistor (PNP or NPN). The voltage across two leads will be symbolizes by the letter V, with the two letters of the corresponding leads of the transistor as pointers. The second letter will always be the one that also characterizes the connection type of the transistor. So, for example, in Common Base connection, the voltage across the emitter and the base is VEB, and the voltage across the collector and the base is VCB. Similarly, in Common Emitter connection, VBE is the voltage across the base and the emitter and VCE is the voltage across the collector and the emitter. We will symbolize the power supplies of the leads with the letter V followed by the letter of the corresponding lead, twice. The symbol VEE is for the emitter supply, VCC for the collector supply and VBB for the base supply.

Choosing the right connection

There are 3 methods that a transistor can be connected, each one having advantages and disadvantages and specific application uses. So it is very important before you start designing your circuit, to be able to choose the proper connection according to your application requirements. First we will see these 3 connections in a glance, and then we will discuss each one thoroughly.

The Common Base (CB) Connection in a glance A transistor is connected with Common Base, when the emitter-base diode is forward biased and the collector-base diode is reverse-biased, the input signal is applied in the emitter, and the output is taken from the collector. It is called "Common Base" because the input and output circuits share the base in common.

The Common Base connection is probably the most rarely used type, due to the strange behavior that it has. As we saw in the operation of a transistor in the previous page, the emitter current is the strongest current of all within a transistor (IE=IB+IC). What this means is that the input of this circuit must be able to provide enough current to source the output. Moreover, the output current (IC) will be slightly less than the input current (IE). So this connection type is absolutely improper for a current amplifier, since the current gain is slightly less than unity (0.9 < hfb<1), in other words it acts as a current attenuator rather than a current amplifier.

On the other hand, it does provide a small voltage amplification. The output signal is in phase with the input signal, so we can say that this is a non-inverting amplifier. But here comes another strange behavior: The voltage amplification ratio of this circuit is very difficult to be calculated, because it depends on some operational characteristics of the transistor that are difficult to be measured directly. The emitter-base internal resistance of the transistor for example and the amount of DC bias of the input signal play a major role in the final amplification ratio, but these are not the only ones. The current that flows within the emitter changes the internal emitter-base resistance which eventually changes the amplification ratio. This connection type has a unique advantage. Due to the fact that the base of the transistor is connected to the ground of the circuit, it performs a very effective grounded screen between the input and the output. Therefore, it is most unlikely that the output signal will be fed back into the input circuit, especially in high frequency applications. So, this circuit is widely used in VHF and UHF amplifiers.

The Common Collector Connection (CC) in a glance (Emitter Follower) A transistor is connected with Common Collector, when the base-collector and emitter-collector diodes are forward biased, the input signal is applied in the base, and the output is taken from the emitter. It is called "Common Collector" because the input and output circuits share the collector in common.

The common collector connection is used in applications where large current amplification is required, without voltage amplification. As a matter of fact, this circuit has the highest current gain factor. Remember that hfb=IC/IE (current gain in CB), hfe=IC/IB (current gain in CE) and hfc=IE/IB (current gain in CC). Taking into account that IE>IC (IE=IB+IC) and that IB is the smallest current, from the previous 3 formulas we can easily conclude that hfc>hfe>hfb. The current that this circuit can provide at its output is indeed the highest, and it it is the sum of the IC current plus the IB current from the input.

The most distinctive characteristic though that this connection type has, is that the output voltage is almost equal to the input voltage. As a matter of fact, the output voltage will be equal to the input voltage, slightly shifted towards ground (VE=VB-VBE). This voltage drop depends on the material that the transistor is made of. A Germanium transistor has VBE=0.3V and a Silicon transistor has VBE=0.7 volts. So, the output signal on a silicon transistor will be exactly the same as the input signal, only that it will be shifted by 0.7 volts. This is why this connection is also called "Emitter Follower". The output signal is in phase with the input signal, thus we say that this is a non-inverting amplifier setup.

This is a very efficient circuit to match impedance between two circuits, because this mode has high input impedance and low output impedance. It is widely used for example to drive the speakers in an audio amplifier, since the speakers have usually very low impedance. It is also used as a current amplifier in applications where the maximum current is required, such as driving solenoids, motors etc. This feature makes this type also perfect for designing Darlington pair transistors, since the maximum current amplification is the requirement.

It is also a very effective circuit to make voltage regulators with high current supply. A Zener diode for example at the base of the transistor will provide a fixed voltage, and the output will be 0.7 (or 0.3) volts less than the Zener diode's regulation voltage, since it will follow the input, no matter how much current it provides.

The Common Emitter (CE) Connection in a glance A transistor is connected with Common Emitter connection, when the base-emitter and emitter-collector diodes are forward biased, the input signal is applied in the base, and the output is taken from the collector. It is called "Common Emitter" because the input and output circuits share the emitter in common.

This is the most common transistor connection used. The reason is because it can achieve high current amplification as well as voltage amplification. This results in very high power amplifications gain (P = V x I). Although -in maths- the current amplification of a Common Collector circuit is larger than a Common Emitter circuit, typically we can safely say that they both have almost the same gain:

hfc = IE / IB
hfe = IC / IB (1)
IE = IB + IC (2)
(1)(2)=> hfe = (IE - IB) / IB = IE / IB - IB / IB = hfc - 1 => hfe = hfc - 1

Suppose that a transistor has hfc=100 (in CC). From the above analysis we see that if we connect this transistor with common emitter, it will have hfe=99 (hfc-1), which is not a significant decrement. Additionally, the output voltage can also be predictably amplified. This is what makes this circuit so widely used. We will discuss this specific connection extensively with different biasing techniques.

This mode is used in several applications, such as audio amplifiers, small signal amplification, load switching and more. A distinctive characteristic for this connection is that the output signal has 180 degrees phase difference from the input signal, thus we call it inverting amplifier.

General Connection Characteristics

Here is a table with the characteristic sizes of the 3 different transistor connections, so that you can directly compare them

 Common Base Common Emitter Common Collector Input Impedance Low (about 50 Ω) Medium (1-5 KΩ) High (300-500 KΩ) Output Impedance High (500KΩ-1MΩ) Medium (about 50KΩ) Low (up to 300 Ω) Current Gain Low (<1) High (50 - 800) High (50-800) Voltage Gain Low (about 20) High (about 200) Low (<1) Power Gain Low (about 20) High (up to 10000) Medium (about 50)

Relative pages
• Basic transistor circuits
• 555 theory of operation
• Learn how to interface ICs
• Learn how dimmers work
• The TRIAC theory
• Learn about the most popular PC Cooling methods
• Dr.Calculus: Checking transistor functionality
• Op-Amp IC Pinouts

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