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university:courses:electronics:text:choosing-transistors [14 May 2014 15:32] – created Doug Mercer | university:courses:electronics:text:choosing-transistors [19 May 2014 17:13] (current) – [TRANSISTORS] James Bryant | ||
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From figure 2 we see that a BJT is a current amplifier - the output current is ß times the input current, and ß may vary slightly with the base current so that the amplifier is not quite linear. (The ß or h< | From figure 2 we see that a BJT is a current amplifier - the output current is ß times the input current, and ß may vary slightly with the base current so that the amplifier is not quite linear. (The ß or h< | ||
- | There are two types of FET, junction FETs (JFETs) and Metal Oxide Silicon FETs (MOSFETs), and both come in either polarity (N-channel for positive supply, P-channel for negative). FETs have very high input resistance (but their input capacitance may be quite large - tens or even hundreds of pF) and are therefore transconductance (I< | + | There are two types of FET, junction FETs (JFETs) and |
Today the MOSFET is the commoner device. The N-channel version consists of a strip of P-type silicon with two N-type diffusions. Over the strip between the diffusions is a very thin layer of silicon dioxide (or some other insulator) covered with a conducting film (usually aluminum or polycrystalline silicon). A positive potential on this conducting gate causes the P-type material just under the insulator to become N-type, joining the drain and source diffusions and allowing a current to flow. The amount of current varies with the applied voltage so the device works as an amplifier as well as a switch. | Today the MOSFET is the commoner device. The N-channel version consists of a strip of P-type silicon with two N-type diffusions. Over the strip between the diffusions is a very thin layer of silicon dioxide (or some other insulator) covered with a conducting film (usually aluminum or polycrystalline silicon). A positive potential on this conducting gate causes the P-type material just under the insulator to become N-type, joining the drain and source diffusions and allowing a current to flow. The amount of current varies with the applied voltage so the device works as an amplifier as well as a switch. | ||
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Since some of the emitter current must flow in the base the collector and emitter currents of a BJT are not identical, which means that the current output stage in figure 10 should be made with a MOSFET rather than a BJT since MOSFETs have virtually zero gate current. | Since some of the emitter current must flow in the base the collector and emitter currents of a BJT are not identical, which means that the current output stage in figure 10 should be made with a MOSFET rather than a BJT since MOSFETs have virtually zero gate current. | ||
- | **Forward transconductance. gfs **The forward transconductance of an FET is the ratio of ΔI< | + | **Forward transconductance. gfs **The forward transconductance of an FET is the ratio of ΔI< |
In general a few volts change of gate voltage is sufficient to change the drain current from minimum (off) to its absolute maximum value. It is also important to know at what gate voltage conduction starts - see:- | In general a few volts change of gate voltage is sufficient to change the drain current from minimum (off) to its absolute maximum value. It is also important to know at what gate voltage conduction starts - see:- | ||
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**On Resistance. Ron** MOSFETs do not saturate because they are majority carrier devices. When they are turned hard on with a gate voltage well above their gate threshold voltage they behave as low value resistors and their //on resistance// | **On Resistance. Ron** MOSFETs do not saturate because they are majority carrier devices. When they are turned hard on with a gate voltage well above their gate threshold voltage they behave as low value resistors and their //on resistance// | ||
- | **Noise Figure. NF** The majority of transistor applications are relatively high-level and noise is not an issue. Where it is an issue, though, it is critically important. Many transistors, | + | **Noise Figure. NF** The majority of transistor applications are relatively high-level and noise is not an issue. Where it is an issue, though, it is critically important. Many transistors, |
**Transition Frequency. ft** The f< | **Transition Frequency. ft** The f< | ||
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gain and higher V< | gain and higher V< | ||
**Maximum current: | **Maximum current: | ||
- | (You may need to consider peak transient currents as well as maximum steady state currents.) | + | (You may need to consider peak transient currents as well as maximum steady state currents.)\\ |
**Package: | **Package: | ||
(If a device comes in several packages the absolute maximum current and power ratings may vary with the | (If a device comes in several packages the absolute maximum current and power ratings may vary with the | ||
package chosen - check this. Also the parametric selection guide may not provide pinout details.)\\ | package chosen - check this. Also the parametric selection guide may not provide pinout details.)\\ | ||
**Power: | **Power: | ||
- | (Remember that a switch dissipates very little power when off, and when it is on most of the power is in the load, not the switch itself. During switching dissipation is higher but this is only important if the device is continually switching at a high rate.) | + | (Remember that a switch dissipates very little power when off, and when it is on most of the power is in the load, not the switch itself. During switching dissipation is higher but this is only important if the device is continually switching at a high rate.)\\ |
It is necessary to decide the above parameters whenever we choose a transistor. The remaining ones may be critical in some applications and unimportant in others, so you must decide for yourself which ones matter in your application, | It is necessary to decide the above parameters whenever we choose a transistor. The remaining ones may be critical in some applications and unimportant in others, so you must decide for yourself which ones matter in your application, | ||
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Calshot - England | Calshot - England | ||
April 2014 | April 2014 | ||
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+ | **Return to [[university: | ||
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+ | **Go to [[university: | ||
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+ | **Return to [[university: | ||
====References==== | ====References==== |