Current equation for bjt amplifier

Any transistor has three terminals, the emitter , the base , and the collector. Using these 3 terminals the transistor can be connected in a circuit with one terminal common to both input and output in three different possible configurations. In every configuration, the emitter junction is forward biased and the collector junction is reverse biased. The name itself implies that the Base terminal is taken as common terminal for both input and output of the transistor. When the emitter voltage is applied, as it is forward biased, the electrons from the negative terminal repel the emitter electrons and current flows through the emitter and base to the collector to contribute collector current.

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Content:

• common collector current gain
• PCB Design & Analysis
• Transistor Formulas Pdf
• How to Design Common Emitter Amplifier
• DC Biasing in Transistors – BJTs
• Differential Amplifier Circuit Tutorial using BJT and Opamp
• Common Emitter Amplifier Circuit Working & Its Characteristics
• How to use BJT Bipolar Junction Transistor – Beginner’s Tutorial

WATCH RELATED VIDEO: Bipolar Junction Transistors - Common Emitter Amplifier

common collector current gain

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You should upgrade or use an alternative browser. BJT amplifies current or voltage? Thread starter alphacat Start date Jul 2, Status Not open for further replies. Since both of its Rin and Rout are large, then when a BJT is used as an amplifier, for example a Common Emitter amplifier, the amplifier is considered to be a transconductance amplifier, meaning the input signal is voltage vBE.

So why is it said at least in the Semiconductor's Basics course that the BJT amplifies current, if its input signal is voltage? The BJT is considered a current amplifier because Collector or Emmiter currents are a function of input or base current.

Last edited: Jul 2, I dont see any beta here. Moreover: 1. A current amplifier doesnt have a large Rin as the CE amplifier.

The input signal of a current amplifier is current not voltage. BrownOut Banned. Being able to derive quantities for transconducatance and input resistance makes it possible to construce a mathematical model convenient for analysis for specific input sources and conditions. It's just as valid to contruct a small signal model wherein the output is a current controlled by an input current. Click to expand As BrownOut notes, the BJT base-emitter input looks like a forward biased diode, thus it has a nonlinear function of voltage to current.

Rin is a small-signal characteristic for a particular input bias condition. It will vary significantly with a different bias condition. Don't know where you've read these design examples but in most usual engineering design, beta is used to calculate collector current of a BJT transistor, at least for non-RF circuits.

Still, a CE amplifier for example, is not a natural current amplifier since you cant apply a current source on its input, since the diode needs an applied voltage to start conducting. BrownOut said:. Actually, it is very common, and will be found in most any engineering text.

In one sense, I agree, but in another I disagree. There are many different tools available to engineers to analyze and design circuits. For me personally, I don't do alot of analysis in the classical sense, but I keep the governing equations in the back of my mind, and use them to guide me in my designs, along with empirical design techniques.

It is very important, IMO, for the engineer to master these analitical methods, however, to get the most out of his designs, although he'll 'streamline' the way in which he uses them. Thank you very much friends You helped me out a lot on this. By the way, if using a current source as an input to the BJT, then this current source must have a very large source resistance, since the input resistance of the BJT is also quite large as was said it, it - Rin - depends on the DC collector current.

Kohms are not usually considered a particularly high input impedance. It's not hard to make an active current source with many kohms of apparent output impedance.

For example the collector impedance of a typical small silicon transistor is in the tens of kohms. Claude Abraham Member. It wouldn't be too useful otherwise.

That's what seperates active circuits from passive ones. A transformer can increase voltage while decreasing current, or vice-versa. Transformers are passive so the net power gain cannot exceed unity.

Regarding Ic, there are 3 equations. Equation 3 describes transistor action. Ie is the emitter current. When an npn bjt emitter injects electrons towards the base, they go straight through the base region since it is very thin, and get collected by the electric field in the collector base region.

Alpha is very close to 1 for a good transistor. If the base region was not thin, most of the electrons would recombine in the base and never reach the collector. In that case, alpha is low, much less than unity. Thus Ic is determined by Ie and alpha. A low alpha value makes the device just 2 diodes back to back.

Equation 2 displays the voltage gain properties of a bjt, and eqn 1 shows its current gain. By the way, either alpha or beta always appears in all equations. Eqn 1 describes "current gain". Eqn 2 describes "voltage gain" or "transconductance". Eqn 3 describes the "physical transistor action". All 3 of these functional relations are all important. Some devices are better suited to be driven from a high impedance source, or "current driven". Others are better driven from a low impedance source, hence "voltage driven".

All 3 eqns are important. Did I help? Be very careful not to get confused by thinking that the OP's equations for collector current is the same as equation 2 stated without alpha. His equation calcualtes collector current directly as an approximation of diffusion current. First of all, Is is not the same as Ics. Neither is it the same as the diode coefficient.

Secondly, the equation that calculates collector directly does not come from the ebbers-moll model. Instead, it uses the difussion profile in the base as an approximation to collector current. It's a very common way to calcualte collector current as a function of base-emitter voltage.

But it's not ebbers-moll Thirdly, I'm not concerned about your confusion. I'm concerned about the student that's learning transistors, and hoping that he doesn't get confused into thinking his equation is the same as eqn2 and ignoring alpha. It's a very common way to calcualte collector current as a function of base current. I did look it up. They are not the same thing. Ies is given by ebbers-moll, which is a very accurate model but is not being used by the OP's equation.

There are more than one way to calculate IC. They are two different models using two different coefficients. Both coefficients are a functions of geometry, carrier mobility and doping levels, among other things. It's not a matter of what happens first, the equaions follow simple algebraic manipulation without losing validity.

Using a different model for analysis does not in any way attempt to invalidate ebbers-moll. It remains a relative accurate model to use, if one wishes. However, the equations that the rest of us are using gives very good results too.

PCB Design & Analysis

A common base amplifier is one of three basic single-stage bipolar junction transistor BJT amplifier configuration, typically used as a current buffer or voltage amplifier. In this configuration, the emitter terminal of the transistor serves as the input, the collector the output, and the base is common and connected to ground through C b. This circuit is usually found in high-frequency amplifiers because its input capacitance does not suffer from the Miller effect, which degrades the bandwidth of the common emitter configuration, and because of the relatively high isolation between the input and output. It is also used as current buffer since it has a current gain of approximately unity. When the circuit is preceded by a common emitter stage, it is called a cascode circuit. The cascode circuit has the benefits of both configurations, such as high input impedance and isolation. Capacitors are considered open circuit in DC and therefore are excluded.

Transistor Formulas Pdf

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How to Design Common Emitter Amplifier

Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. I am doing a design of a BJT amplifier with the voltage divider biasing.

DC Biasing in Transistors – BJTs

Sometimes the high current gain of a single-transistor, common-collector configuration isn't enough for a particular application. The common collector circuit configuration is more widely known as the emitter follower and it provides a high input impedance and a low output impedance. The various notations for transistor gain can be summarised as below. Of course, zener diodes already provide this function of voltage regulation: However, when used in this direct fashion, the amount of current that may be supplied to the load is usually quite limited. In essence, this circuit regulates voltage across the load by keeping current through the series resistor at a high enough level to drop all the excess power source voltage across it, the zener diode drawing more or less current as necessary to keep the voltage across itself steady.

Differential Amplifier Circuit Tutorial using BJT and Opamp

Transistors are the building blocks of the modern electronic era. They function as small amplifiers that amplify electrical signals as necessary to facilitate circuit functions. Transistors have three basic parts: the base, collector and emitter. The transistor parameter "Vce" signifies the voltage measured between the collector and emitter, which is extremely important because the voltage between the collector and the emitter is the output of the transistor. Moreover, the primary function of the transistor is to amplify electrical signals, and Vce represents the results of this amplification. For this reason, Vce is the most important parameter in transistor circuit design.

Common Emitter Amplifier Circuit Working & Its Characteristics

An often-used circuit applying the bipolar junction transistor is the so-called current mirror , which serves as a simple current regulator, supplying nearly constant current to a load over a wide range of load resistances. Therefore, if we have a way of holding emitter current constant through a transistor, the transistor will work to regulate collector current at a constant value. If both junction voltage and temperature are held constant, then the PN junction current will be constant.

How to use BJT Bipolar Junction Transistor – Beginner’s Tutorial

RELATED VIDEO: Current amplification factor of CE , CB and CC configurations

The term amplifier as used in this chapter means a circuit or stage using a single active device rather than a complete system such as an integrated circuit operational amplifier. An amplifier is a device for increasing the power of a signal. This is accomplished by taking energy from a power supply and controlling the output to duplicate the shape of the input signal but with a larger voltage or current amplitude. In this sense, an amplifier may be thought of as modulating the voltage or current of the power supply to produce its output. The basic amplifier, figure 9.

In this post, differential amplifier using BJT and differential amplifier using op-amps are explained in detail. Please go through both of them to get a better understanding. The circuit diagrams and detailed equations are provided along with the article. Please go through them. A differential amplifier is designed to give the difference between two input signals. The circuit is shown below.

The working of a Bipolar transistor or BJTs at DC levels is governed by several factors, that includes a range of operating points over the characteristics of the devices. Under the section 4. Once the specified DC supplies are calculated, a circuit design may be created for determining the required operating point.