Computer Science, asked by sumaiya91, 1 year ago

why are BJTs ideal for analog applications

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Answered by jag3
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Analog Devices Wiki
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Activity: BJT device as a switch
Objective:
Background:
Materials:
Directions:
Hardware Setup:
Procedure:
Questions:

Objective:

A bipolar junction transistor (BJT) can be used in many circuit configurations such as an amplifier, oscillator, filter, rectifier or just used as an on-off switch. If the transistor is biased into the linear region, it will operate as an amplifier or other linear circuit, if biased alternately in the saturation and cut-off regions, then it is being used as a switch, allowing current to flow or not to flow in other parts of the circuit. This lab activity describes the BJT when operated as a switch.
Background:

Switching circuits are significantly different than linear circuits. They are also easier to understand. Before investigating more complex circuits, we will begin by introducing discrete solid-state switching circuits: those built around BJTs.

A switch consists of a BJT transistor that is alternately driven between the saturation and cutoff regions. A simple version of the switch is shown in figure 1. When the input equals -Vin , the base-emitter junction is reverse biased or off so no current flows in the collector. This is illustrated by the load line shown in the figure. When the BJT is in cutoff, the circuit (ideally) has the following values:

VCE = VCC and IC = 0 A

This state is similar to an open switch.

When the input equals +Vin, the transistor is driven into saturation and the following conditions occur:

VCE ˜ 0V and ICsat = VCC/RC

This state is similar to a closed switch connecting the bottom of RC to ground.

Figure 1 NPN BJT switch and its load line.

The characteristics for a BJT switch assume that:

-Vin is low enough to drive the transistor into cutoff.
+Vin must produce enough base current through RB to drive the transistor into saturation.
The transistor is an ideal component.

These conditions can be assured by designing the circuit so that:

-Vin = VBE
+Vin = VBE + IBRB (VCC is a good maximum)
IB > ICsat/ß

Condition 1 guarantees that the circuit is driven into the cutoff region by the input. Conditions 2 and 3 assure that the transistor will be driven into the saturation region.

An actual BJT switch differs from the ideal switch in several aspects. In practice, even in cutoff there is some leakage current through the transistor. Also, in saturation, there is always some voltage dropped across the transistor's internal resistance. Typically, this will be between 0.2 and 0.4 V in saturation depending on the collector current and size of the device. These variations from the ideal are generally minor with a properly sized device, so we can assume near ideal conditions when analyzing or designing a BJT switch circuit.
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