Basic Transistor Circuits !

Basic Transistor Circuits !
Basic Transistor Circuits !
Basic Transistor Circuits !
Basic Transistor Circuits !
Basic Transistor Circuits !
Basic Transistor Circuits !
Basic Transistor Circuits !
Basic Transistor Circuits !
Basic Transistor Circuits !
Basic Transistor Circuits !
Basic Transistor Circuits !

common emitter circuit and the common collector circuit. The two circuits are constructed in a very similar fashion. The difference is that in the common emitter circuit the input is located between the base and the emitter and the output is between the collector and the emitter (i.e. the emitter is the common reference point for the input and output signals). In contrast, the common collector configuration employs the collector as the common reference point for the input and output signals, with the output being connected to the emitter.
The voltage properties of both circuits can be improved by incorporating a feedback loop, which takes the output signal and adds it to the input signal at the base. A feedback loop can be realised with the help of a resistor (resistive feedback) or with capacitors (capacitive feedback). The effects of the two types of feedback are examined in the following experiments.

Basic Transistor Circuits !Basic Transistor Circuits !

Common base circuit, In addition, high frequency circuits may use the common base circuit. Thanks to its high limiting frequency, it is often employed in input stages for signal frequencies greater than 100 MHz. Furthermore, its low input resistance allows easy matching to antenna impedances.

Basic Transistor Circuits !

 

Transistor as a Switch:

 

One of the main applications for transistors is transistor switching stages. They are used for contactless, fast switching of small and medium loads. When used in this manner, the transistor is simply shifted between two states. Switching between these two states happens within a few microseconds.

Basic Transistor Circuits !

Transistor blocked

In this state, the input base current IB = 0 A. The transistor is then blocked and no collector current is able to flow. Because there is no voltage drop across the resistor RC, the output voltage Uo corresponds to the supply voltage UB.

Transistor driven

In this state, a base current is fed to the transistor via RB by applying an input voltage. This causes a collector current to flow, which must be sufficiently large that most of the supply voltage UB is dropped across the collector resistor RC. The output voltage Uo therefore sinks almost to 0 V. The voltage does not fall right to 0 V due to the fact that the resistance between the collector and the emitter of the transistor is not 0 Ω. This residual voltage is also called the saturation voltage UCEsat.

Transistor as an Amplifier:

 

The response of a transistor to changes in the input voltage at the base are often described in terms of particular characteristic curves. The output characteristic and the transfer characteristic are among the most important of the characteristic curves.

The following output characteristic illustrates the relationship between the collector current IC and the collector-emitter voltage UCE for different values of the base current IB.

Basic Transistor Circuits ! 

The curves all have a similar shape. From zero current and zero voltage, they climb until they make a clearly recognisable sharp bend, which occurs at a rather low voltage, usually at about 0.2 V. After this point, the curve flattens out and the dependency of IC on VCE continues in a straight line (is linear).

The transfer characteristic represents the relationship between IC and the base current IB, and is approximately linear. This means that a changed signal at the base is reproduced exactly by the corresponding change in the current flow at the collector. The ratio of the current strengths is given by what is termed the gain. The current gain β is defined as the change in the collector current divided by the change in the base current: β = ΔIC/ΔIB.

Basic Transistor Circuits !

If the ratio between the base current and the collector current is linear, the value of the gain is constant.

In the following experiments, a common emitter circuit will be used as a voltage amplifier. The collector is connected via a resistor to the supply voltage, while the emitter is connected to 0 volts. Hence there are limiting values for the output voltage. It cannot rise above +15 volts or fall below 0 volts. If the transistor is already fully conducting, any further increase in the base voltage does not have any additional effect.

The circuit is dimensioned such that the output voltage UCE between the collector and the emitter is set to the midway point between the upper and lower limiting values which, in our circuit, is 7.5 V. If a symmetrical signal is supplied to the base, this design allows the output signal to be increased by 7.5 V in either direction without distortion occurring. The pre-set output voltage is called the operating point of the transistor circuit.

Biasing (Setting the Operating Point of a Transistor)

Since the output voltage is dependent on the base voltage, the latter can be set using a voltage divider to the value that results in the desired output voltage, as shown in the following circuit diagram. This method of setting the operating point is referred to as biasing, and the resistors constituting the voltage divider are called biasing resistors. The optimum balance can be achieved by using a potentiometer for fine tuning. 

Basic Transistor Circuits !

Once the base voltage has been preset to the desired value by biasing, small signals can be fed in, which cause the base voltage to rise or fall accordingly. These signals are then reproduced in amplified form at the output. The DC voltage component of the signal can be suppressed by the use of capacitors at the input and output.

In order to ensure that this saturation level is achieved, a base current is supplied that is not just the amount required for a collector current, but several times that current. As a consequence, the base is overdriven. This multiple is also referred to as the overdrive factor and, in practice, has a value of between 2 and 10.

Other Transistor Types:

 

Alongside the bipolar transistors that we have already described, there is the group consisting of the unipolar transistors. These include the field effect transistors (FETs) and the unijunction transistors (UJTs). In contrast to the bipolar transistors, which are controlled by currents, unipolar transistors are controlled by a voltage. The voltage establishes an electric field in these transistors. This field envelops a channel whose conductivity increases or decreases at it expands or shrinks respectively. This allows control with almost no power dissipation.

Field Effect Transistors:

Field effect transistors are divided into two groups: junction (or depletion mode) FETs and MOS-FETs. The following diagram gives an illustrative overview.

Basic Transistor Circuits !

With the exception of a few special designs, these transistors have three terminals. They are referred to as the source, gate and drain. The source terminal corresponds to the emitter, the gate terminal to the base, and the drain terminal to the collector of bipolar transistors. 

The corresponding circuit symbols are portrayed in the following overview:

Basic Transistor Circuits !


The unijunction transistor also possesses three terminals. These terminals are referred to as the emitter, base 1 and base 2. Because of its terminal configuration, this transistor is also called a double-base diode. It is hardly used any more nowadays and is only to be found on occasion in some oscillator circuits.Unijunction Transistor

Basic Transistor Circuits !

IGBT:

The IGBT (Insulated Gate Bipolar Transistor) is increasingly employed in power electronics. It combines the advantages of the bipolar transistor (good forward response, high reverse voltage, robustness) with the advantages of the field effect transistor (switching with virtually no power dissipation). A certain degree of resistance to short circuiting is also provided. The terminals are designated accordingly as the gate, collector and emitter.

Circuit symbol

Basic Transistor Circuits !

More From Iamtechnical.com

Advertisement: