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Chapter 6: Problem 6

What are the major differences between the collector characteristics of a BJT transistor and the drain characteristics of a JFET transistor? Compare the units of each axis and the controlling variable. How does \(I_{C}\) react to increasing levels of \(I_{B}\) versus changes in \(I_{D}\) to increasingly negative values of \(V_{G S}\) ? How does the spacing between steps of \(I_{B}\) compare to the spacing between steps of \(V_{G S}\) ? Compare \(V_{C_{\text {st }}}\) to \(V_{P}\) in defining the nonlinear region at low levels of output voltage.

Short Answer

Expert verified
The major differences between BJT and JFET transistors lie in their control variable, units of each axis and reactions to different parameters. BJT is a current controlled device where \(I_{C}\) is dependent on \(I_{B}\). JFET is a voltage controlled device where \(I_{D}\) is dependent on \(V_{G S}\). \(I_{C}\) increases with \(I_{B}\) in BJT while \(I_{D}\) decreases with increasingly negative \(V_{G S}\) in JFET. The spacing between steps of \(I_{B}\) or \(V_{GS}\) depends on specific device values and operating conditions. \(V_{C_{\text{st}}}\) and \(V_{P}\) both define the non-linear region at low output voltage levels in BJT and JFET respectively.

Step by step solution

01

BJT vs JFET Characteristics

The collector current \(I_{C}\) in a BJT depends on the base current \(I_{B}\), while in a JFET, the drain current \(I_{D}\) depends on the gate-source voltage \(V_{G S}\) . BJTs are current controlled devices, JFETs are voltage controlled.
02

Units Comparison

The BJT's collector current \(I_{C}\) is measured in Amperes (A) and the base current \(I_{B}\) in Amperes (A). For JFET, the drain current \(I_{D}\) is also measured in Amperes (A) and the gate-source voltage \(V_{G S}\) in Volts (V).
03

Reaction to Increasing Levels

In a BJT, as \(I_{B}\) increases, \(I_{C}\) also increases because they are directly proportional. In a JFET, when \(V_{G S}\) becomes increasingly negative, \(I_{D}\) decreases. This is because JFET is a voltage controlled device.
04

Spacing Comparison

Spacing between steps of \(I_{B}\) and \(V_{G S}\) depend on the specific values for each device as well as their operating conditions but it's generally true that because JFET is a voltage-controlled device, the spacing is likely to be uniform for \(V_{G S}\)
05

Comparison between \(V_{C_{\text{st}}}\) and \(V_{P}\)

Both \(V_{C_{\text st}}\) and \(V_{P}\) define the non-linear region at low levels of output voltage. In a BJT, \(V_{C_{\text{st}}}\) is the saturation voltage - when transistor is fully on. In a JFET, \(V_{P}\) is pinch-off voltage - when transistor is fully off.

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Most popular questions from this chapter

A \(p\) -channel JFET has device parameters of \(I_{D S S}=7.5 \mathrm{~mA}\) and \(V_{P}=4 \mathrm{~V}\). Sketch the transfer characteristics.

Sketch the transfer and drain characteristics of an \(n\) -channel depletion- type MOSFET with \(I_{D S S}=12 \mathrm{~mA}\) and \(V_{P}=-8 \mathrm{~V}\) for a range of \(V_{G S}=-V_{P}\) to \(V_{G S}=1 \mathrm{~V}\).

Sketch the transfer characteristics of a \(p\) -channel enhancement-type MOSFET if \(V_{T}=-5 \mathrm{~V}\) and \(k=0.45 \times 10^{-3} \mathrm{~A} / \mathrm{V}^{2}\).

a. Sketch the transfer and drain characteristics of an \(n\) -channel enhancement-type MOSFET if \(V_{T}=3.5 \mathrm{~V}\) and \(k=0.4 \times 10^{-3} \mathrm{~A} / \mathrm{V}^{2}\) b. Repeat part (a) for the transfer characteristics if \(V_{T}\) is maintained at \(3.5 \mathrm{~V}\) but \(k\) is increased by \(100 \%\) to \(0.8 \times 10^{-3} \mathrm{~A} / \mathrm{V}^{2}\).

Given a depletion-type MOSFET with \(I_{D S S}=6 \mathrm{~mA}\) and \(V_{P}=-3 \mathrm{~V}\), determine the drain current at \(V_{G S}=-1,0,1\), and \(2 \mathrm{~V}\). Compare the difference in current levels between \(-1 \mathrm{~V}\) and \(0 \mathrm{~V}\) with the difference between \(1 \mathrm{~V}\) and \(2 \mathrm{~V}\). In the positive \(V_{G S}\) region, does the drain current increase at a significantly higher rate than for negative values? Does the \(I_{D}\) curve become more and more vertical with increasing positive values of \(V_{G S} ?\) Is there a linear or a nonlinear relationship between \(I_{D}\) and \(V_{G S}\) ? Explain.
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