# Air's Bipolar Junction Transistor Q&A Thread

• Air
In summary, when using hybrid pi and t model for bipolar junction transistors, it is necessary to determine the DC operating point first. This involves making the AC inputs zero and solving for the DC currents and voltages, including Ib, Ic, and Ie. The specific variables used in the KVL loops will depend on the circuit, but the available device parameters like Iceo, Vceo, and Hfe are typically enough. Remember to include diagrams if necessary and to show your own efforts when posting homework questions.
Air
Hi, I'm learning Bipolar Junction Transistor and need help to confusion which arise so I will keep posting questions which I don't understand to this thread. Thank you in advance. So my questions start:

When using hybrid pi model and t model, we determine the dc operating point first. In one example, I saw that they make vi=0. My question is that should all sources be made zero to find dc currents? Also, determining dc operating point means only finding dc collector current or do we find other currents (such as dc emitter current) or voltages?

If you want to refer to voltages on a diagram, you need to include the diagram.
See the "paper clip" thing at the top of the edit screen. That is for attaching stuff.

Also, since this is the homework section, you need to show your own efforts at finding this information.

1st question: yes, for bias you can/should assume the AC inputs are zero - they are presumed to be small signal in most designs anyway - negligible excursion away from the DC Q-point.

2nd question: usually you are doing some KVL loops which include both Ib and Ic (and by KCL, Ie) along with Vbe. The exact variables depend on the circuit (base bias, degeneration, etc.) but remember the device parameters typically available for BJTs: Iceo, Vceo & Hfe. These are enough to do pretty all the KVL loops.

## 1. What is a bipolar junction transistor (BJT)?

A BJT is a type of semiconductor device that is commonly used as an electronic switch or amplifier. It is composed of three layers of doped semiconductor material, usually silicon, and has three terminals: the base, emitter, and collector. The behavior of a BJT is controlled by the amount of current flowing through the base terminal, which allows it to amplify or switch electronic signals.

## 2. How does a BJT work?

A BJT works by using the flow of electrons between the base and emitter terminals to control the flow of larger currents between the collector and emitter terminals. When a small current is applied to the base terminal, it creates a larger current flow between the collector and emitter terminals. This behavior can be used to amplify small signals or to switch on and off larger currents.

## 3. What are the different types of BJTs?

There are two main types of BJTs: NPN and PNP. These refer to the type of doping used in the semiconductor material. In an NPN transistor, the base is made of P-type material and the emitter and collector are made of N-type material, while in a PNP transistor, the base is N-type and the emitter and collector are P-type. Additionally, BJTs can be categorized as either general purpose, high frequency, or power transistors, depending on their intended use.

## 4. What are some common applications of BJTs?

BJTs have a wide range of applications in electronic circuits. They are commonly used as switches in digital logic circuits, as amplifiers in audio and radio frequency circuits, and as drivers for larger power transistors in power supplies. BJTs are also used in various electronic devices such as computers, televisions, and mobile phones.

## 5. How do BJTs compare to other types of transistors?

BJTs are one of the oldest types of transistors and are still widely used in electronic circuits. They have a higher gain (amplification) and lower input impedance compared to other types of transistors, such as MOSFETs. However, BJTs also have a higher power consumption and are more prone to thermal runaway, which can limit their use in high-power applications. Other types of transistors, such as JFETs and MOSFETs, have different characteristics that make them more suitable for certain applications.

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