Basic Idea of Transistor (BJT)

Transistor:

Transistor is a three terminal active electronic device that is made of two p-n junctions practically p-n-p or n-p-n junction in itself. It can amplify the amplitude of AC signal and acts as a switch also. In ancient days a special type of vacuum tube is used as triode or amplifier.

Symbols:

Schematic symbols of transistors are different according to its junction types which are shown below:

In above symbols we can see an arrow sign with one terminal which name is emitter. The arrow is drawn in outer direction for n-p-n transistor and it is drawn in inner direction for p-n-p transistor. The arrow indicates the direction of emitted current from/into transistor.

Terminals:

Transistor has three terminals, namely (i) Collector (ii) Base (iii) Emitter.

Above figure shows back to back diode model, internal structure and symbolic presentation with respective terminals for both n-p-n and p-n-p transistors.

Identifying Terminals of an Unknown Transistor: 

It is necessary to identify terminals of a transistor correctly before connecting with any electronic circuit. We can find out terminals’ name and type of transistor from data sheet of related part number downloaded from internet. But practically we can use either digital multimeter or analog ohm-meter to identify the terminals.

With a Digital Multimeter: Modern digital multimeters have transistor checking ports with some connecting holes named E, C, B on its front face. One port used to check p-n-p and another for n-p-n transistor. Place the selector knob of multimeter on hfe then connect a transistor with one of these ports and check the reading, if no reading displays on meter reverse the connection and/or change the port. A good workable transistor shows the value of forward current gain, hfe on meter’s display and faulty transistor can’t show the value of hfe. Now the type and terminals’ name can be identified from label written with related port and holes name where the transistor is connected.

Another type of manual test shown in figure (a), (b), (c) and (d). The Base-emitter and base-collector junction of transistor is just like a p-n junction diode, so the diode test is good enough to identify terminals of transistor. Follow the steps:

(i) Place the selector knob of digital multimeter to diode symbol on its range.

(ii) Connect two probes of multimeter with any two terminals of transistor and observe the reading. If no change appears on display reverse the connection or change the terminal, now you can found a reading 0.3 to 0.7 on display. In this connectivity any one of these two terminals must be Base.

(iii) Keep one of probes remain connected with terminal and release another probe to reconnect with rest of terminal, and observe the reading also. If change of reading appears on display it is sure that the fixed/remained terminal is connected with Base, Or if no change of reading appears on display keep another probe remain fixed and release another probe to reconnect with rest of terminal, and observe the reading. Now change of reading must appears on display it is sure that the fixed/remained terminal is connected with Base.

(iv) In the mean time of identifying terminals we should concentrate that which colored probe was being connected with Base. If it was red color the Base is p-type and transistor’s type is n-p-n otherwise it is p-n-p transistor which will be cleared from figure (a) and (b).

(v) After identifying Base place the selector knob of multimeter on Mega Ω range and reconnect probes between collector and emitter then touch the base with hand and observe the reading also. If no change appears on display reverse the connection and touch the base with hand and observe the reading again. Now you can found change on display. In this condition the collector has connected with red probe and emitter has connected with black probe for an n-p-n transistor or collector has connected with black probe and emitter has connected with red probe for a p-n-p transistor which will be cleared from figure (c) and (d).

Identifying a Faulty Transistor:

Short Circuit Fault: During above test if any transistor shows near to zero resistance in either Base-Emitter or Base-Collector junction for reverse bias then we can say the transistor is short circuited.

Open Circuit Fault: If any transistor shows near to infinite resistance in either Base-Emitter or Base-Collector junction for both forward and reverse bias then we can say the transistor is open circuited.

 

Leakage Fault: If any transistor shows some amount of resistance in either Base-Emitter or Base-Collector junction for reverse bias then we can say the transistor has leakage fault.

Connecting with Circuits: 

After identifying terminals we can connect a transistor with circuit easily. We can find some schematic symbols of transistor on a PCB where we have to connect transistors with relevant holes. Three different symbols often founded on PCBs shown in figure right.

 

Characteristics:

An n-p-n transistor becomes “on” or short circuit between collector and emitter when a positive voltage arises to base and it becomes “off” or open circuit between collector and emitter when the base triggering voltage decreases to zero. This idea will be cleared from a practical circuit drawn below:

In above circuit there are two transistors. In the first transistor the base is grounded and no current can flow into it. As a result the transistor is “off” and no current can flow through the bulb. In the other case there is a current flowing into the base and so the transistor is “on” and the current can flow through it resulting in the light bulb being on.

A p-n-p transistor becomes short circuit between collector and emitter when a positive voltage arises to base and it becomes open circuit between collector and emitter when a positive voltage decreases to base. This idea will be cleared from figure below:

In above circuit there are two transistors. In the first transistor the base is grounded and current can flow into it. As a result the transistor is “on” and current can flow through the bulb. In the other case there is a no current flowing into the base and so the transistor is “off” and the current cannot flow through it resulting in the light bulb being off.

Uses:

Most common uses of transistor are (i) Amplifier and (ii) Switch

(i) Amplifier: A single stage amplifier is shown in figure below:

Figure: Single stage amplifier.

(ii) Switch: Following two figures present transistor switching circuits:

Figure -1: Transistor ‘Q1’ gets positive base triggering voltage via potentiometer ‘RV1’ and goes to saturation and it turns on the relay ‘RL1’.

Figure -2: Transistor ‘Q1’ gets zero base triggering voltage by changing potentiometer shaft of ‘RV1’ and goes to cut-off and it turns off the relay ‘RL1’. In above ways transistor acts as switch.

Most popular transistors’ physical images which have been using in electronic circuitry:

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