Bipolar Transistor

If we combine two separate signal diodes in a row, this will give us two serially connected pin junctions that will share a common positive (P) or negative (N) terminal. The fusion of these two diodes produces three layered, two-link, three terminal devices that form the basis of a Bipolar connection transistor, or BJT for short.

Transistors are three terminal active devices made of different semiconductor materials that can act as insulators or conductors by applying a small signal voltage. The transistor's ability to switch between these two states ensures that it has two basic functions: switching " (digital electronics) or amplification (analog electronics). Bipolar transistors are capable of working in three different regions:

  • Active zone – transistor works as an amplifier and Ic = β*Ib
  • Saturation – the transistor is accustomed to being a key. "completely on" and Ic = I (saturation)
  • Cut-off – transistor works as a switch. "completely closed" and Ic = 0

The word transistor is a combination of two Transfer Varistors, which describe the modes of operation in the early days of electronic development. There are two basic types of bipolar transistor structures, PnP and NPN, which basically define the physical arrangement of the type p and N type semiconductor materials in which they are made.

The basic structure of the bipolar transistor consists of two PN connections that produce three connection terminals, and each terminal is given a name to identify the other two. These three Terminals are known as Emitter ( E ), Base ( B ) and Collector( C ), respectively.

Bipolar Transistors control the amount of current flowing from Emitter to collector terminals in proportion to the amount of bias voltage applied to the base terminals. Thus, they are current editing devices that act as a current-controlled switch. A small current flowing into the basic terminal controls a much larger collector stream, which forms the basis of transistor movement.

The principle of operation of the two types of PNP and NPN transistors is the same, the only difference is the fainting and polarity of the power supply for each type.

Bipolar Transistor Design

Bipolar Transistor Design

The structure and circuit symbols for both the PNP and NPN bipolar transistor are given above, indicating the direction of the "traditional current flow" between the base terminal and the emitter terminal at all times with the arrow in the circuit symbol. The direction of the arrow always points from the positive P-type region to the negative N-type zone for both transistor types in exactly the same way as the standard diode symbol.

Bipolar Transistor Configurations

Since the bipolar transistor is a three-terminal device, there are basically three possible ways to connect it in an electronic circuit, and one terminal is common to both input and output signals. Because the static properties of the transistor vary according to each circuit pattern, each connection method reacts differently to the input signal within a circuit.

  • Common Base configuration – there is voltage gain but no current gain.
  • Common Emitter Configuration – there is both Current and Voltage Gain.
  • Common Collector Configuration – There is current gain but no voltage gain.

Common Base (CB) Configuration

As the name suggests, in the common base or grounded base configuration, the base connection is common for both the input signal and the output signal. The input signal is applied between the base of the transistors and the emitter terminals, while the corresponding output signal is received between the base and collector terminals as shown. The base terminal is grounded or can be connected to some fixed reference voltage points.

The input current flowing into the emitter is quite large, since it is the sum of both the base current and the collector current, respectively, so the collector current output is less than the emitter current input. For this type of circuit, it results in a current gain. "1" (unity) or less, that is, the common basic configuration "weakens" the input signal.

Common Base Transistor Circuit

Common Base Transistor Circuit

This type of amplifier configuration is an unverted voltage amplifier circuit in which VIN and Vout signal voltages are "in-phase". This type of transistor regulation is not very common due to its unusually high voltage gain properties. Entry characteristics represent the characteristics of an advanced biased diode, while the output characteristics represent the characteristics of an illuminated photo diode.

In addition, this type of bipolar transistor configuration has a high output rate that gives the value of "resistance gain" to output resistance or, more importantly, "load" resistance (rl) "input" resistance (rn). Therefore, voltage gain (Av) for a common basic configuration is given as follows:

Common Base Voltage Gain

Common Base Voltage Gain

Where: Ic/Ie is current gain, alpha (α) and rl/rin resistance gain.

The common base circuit is usually used in single-stage amplifier circuits such as microphone pre-amplifier or radio frequency (Rε) amplifiers due to very good high frequency response.

Common Emitter (CE) Configuration

In the common emitter or earthed emitter configuration, the input signal is applied between the base and emitter, while the output is taken between the collector and emitter as shown. This type of configuration is the most widely used circuit for transistor-based amplifiers. It represents the "normal" method of bipolar transistor connection.

The common emitter amplifier configuration produces the highest current and power gain of the three bipolar transistor configurations. This is because the entry impedance is low because it is connected to a forward biased PN-junction, and the output impedance is high because it is taken from an inverted biased PN-junction.

Common Emitter Amplifier Circuit

Common Emitter Amplifier Circuit

In such a configuration, the current flowing from the transistor should be equal to the currents flowing into the transistor, since the emitter current is given as Ie = Ic + Ib. Since the load resistance (RL) is connected serially with the collector, the current gain of the common emitter transistor configuration is quite large as it is the Ic/Ib ratio. A transistor current gain is given the beta Greek symbol, (β). Since the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the Ic/Ie ratio is called Alpha, given the Greek symbol of α. Note: The alpha value will always be less unity.

Since the electrical relationship between these three currents, Ib, Ic and Ie, is determined by the physical structure of the transistor itself, any small change in the base current (Ib) will cause a much larger change in the collector current (Ic).

Then small changes in the current flowing in the base will control the current in the emitter-gatherer circuit. Typically, Beta has a value of 20 to 200 for most general purpose transistors. So if a transistor has a beta value of 100, then one electron will flow through the base terminal for every 100 electrons flowing between emitter – collector terminal.

By combining expressions for both Alpha, α, and Beta, β, the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as follows:

Where:" Ic" is the current flowing into the collector terminal," Ib", is the current flowing into the base terminal, and "Ie" is the current flowing through the emitter terminal.

To recap. this type of bipolar transistor configuration has a larger input impedance, current and power gain than the common base configuration, but the voltage gain is much lower. The common emitter configuration is an inverted amplifier circuit. This means that the resulting output signal has a phase shift of 180o according to the input voltage signal.

Common Collector (CC) Configuration

In the common collector or grounded collector configuration, the collector is connected to the soil through feeding, so that the collector terminal is common for both input and output. When connecting the input signal directly to the base terminal, the output signal is taken from the emitter load resistance as shown. Configuration is commonly known as this type of Voltage tracker or Emitter Tracker circuit.

The common collector or emitter tracker configuration has a relatively low output impedance. It is also very useful for impedance matching applications due to its very high input impedance in hundreds of thousands of Ohm regions.

Common Collector Transistor Circuit

The common emitter configuration has a current gain approximately equal to the β value of the transistor. However, in the common collector configuration, the load resistance is serially connected to the emitter terminal, so that its current is equal to that of the emitter current.

Since emitter current is a combination of collector and base current, the load resistance in this type of transistor configuration has both collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as follows:

Common Collector Current Gain

This type of bipolar transistor configuration is an inverted circuit due to the fact that the Vin and Vout signal voltages are "in-phase". The common collector configuration has a voltage gain of approximately "1" (union gain). Therefore, voltage gain can be considered as a voltage buffer because it is a union.

The load resistance of the common collector transistor receives both the base and collector currents. This provides a large current gain (as in the common emitter configuration). Therefore, it provides a good current amplification with very little voltage gain.

Bipolar Transistor Configurations