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university:courses:electronics:text:choosing-transistors [14 May 2014 15:36]
_deletedUser0_ [CHOOSING A TRANSISTOR]
university:courses:electronics:text:choosing-transistors [19 May 2014 17:13] (current)
JamesBryant [TRANSISTORS]
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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<​sub>​fe</​sub>​ is the //current gain// of the transistor.) The input impedance is neither low nor linear so we can also view a BJT as an I<​sub>​out</​sub>/​V<​sub>​in</​sub>​ (transconductance) amplifier with a silicon diode as its input device. It is clear that the greater the value of ß the better the current amplifier. For most applications a minimum value of 80-100 is adequate but higher values to a few hundred are not uncommon. ("​Super-beta"​ transistors with ß up to several thousand are possible, but they have a very narrow base region and low breakdown voltages and are so fragile that they are rarely used except within analog integrated circuits.) 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<​sub>​fe</​sub>​ is the //current gain// of the transistor.) The input impedance is neither low nor linear so we can also view a BJT as an I<​sub>​out</​sub>/​V<​sub>​in</​sub>​ (transconductance) amplifier with a silicon diode as its input device. It is clear that the greater the value of ß the better the current amplifier. For most applications a minimum value of 80-100 is adequate but higher values to a few hundred are not uncommon. ("​Super-beta"​ transistors with ß up to several thousand are possible, but they have a very narrow base region and low breakdown voltages and are so fragile that they are rarely used except within analog integrated circuits.)
  
-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<​sub>​out</​sub>/​V<​sub>​in</​sub>​) devices.+There are two types of FET, junction FETs (JFETs) and  ​Insulated Gate FETs (IGFETs), more commonly, but less accurately, called ​Metal Oxide Silicon FETs (MOSFETs) ​which is the name I shall use here, 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<​sub>​out</​sub>/​V<​sub>​in</​sub>​) devices.
  
 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<​sub>​ds</​sub>/​ΔV<​sub>​gs</​sub>​ when the device is turned on and the drain circuit is not current-limited. It is measured in siemens (S) (or, for traditionalists amongst us, in mhos or reciprocal ohms, which are the obsolete name and symbol for exactly the same thing). Small-signal FETs and MOSFETs may have g<​sub>​fs</​sub>​ as low as a few mS but larger ones can have gains of large fractions of a siemens to several siemens or more.+**Forward transconductance. gfs **The forward transconductance of an FET is the ratio of ΔI<​sub>​ds</​sub>/​ΔV<​sub>​gs</​sub>​ when the device is turned on and the drain circuit is not current-limited. It is measured in siemens (S) (or, for traditionalists amongst us, in mhos or reciprocal ohms [Ʊ], which are the obsolete name and symbol for exactly the same thing). Small-signal FETs and MOSFETs may have g<​sub>​fs</​sub>​ as low as a few mS but larger ones can have gains of large fractions of a siemens to several siemens or more.
  
 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//​ is specified on their data sheet. Ohm's law applies - the voltage drop is proportional to the current and the on resistance, and their dissipation is I<​sup>​2</​sup>​R. **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//​ is specified on their data sheet. Ohm's law applies - the voltage drop is proportional to the current and the on resistance, and their dissipation is I<​sup>​2</​sup>​R.
  
-**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,​ both BJTs and FETs, have their noise figure specified and guaranteed by their manufacturers. When comparing the noise figures of different devices it is very important that the noise figures should have been measured with the same source impedance. If the transistors are intended for use in radio systems it is likely that their NF will have been measured at 50O and so comparison is simple, but it is meaningless to compare the NFs of two devices whose NFs were measured at different impedances. A paper associated with an earlier RAQ<​sup>​7</​sup>​ covers this and other noise issues in detail and should be consulted if you are interested in the topic.+**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,​ both BJTs and FETs, have their noise figure specified and guaranteed by their manufacturers. When comparing the noise figures of different devices it is very important that the noise figures should have been measured with the same source impedance. If the transistors are intended for use in radio systems it is likely that their NF will have been measured at 50Ω and so comparison is simple, but it is meaningless to compare the NFs of two devices whose NFs were measured at different impedances. A paper associated with an earlier RAQ<​sup>​7</​sup>​ covers this and other noise issues in detail and should be consulted if you are interested in the topic.
  
 **Transition Frequency. ft** The f<​sub>​t</​sub>​ of a BJT is the frequency at which the current gain, with a short circuit (at HF) output, is unity. Again I do not propose to discuss how this may be measured<​sup>​8</​sup>​ but simply to observe that f<​sub>​t</​sub>​ is the most widely used figure of merit for comparing the frequency response of BJTs. Most TUNs and TUPs will have f<​sub>​t</​sub>​ well over the 100 MHz minimum but high power and high voltage transistors will often have rather lower values. **Transition Frequency. ft** The f<​sub>​t</​sub>​ of a BJT is the frequency at which the current gain, with a short circuit (at HF) output, is unity. Again I do not propose to discuss how this may be measured<​sup>​8</​sup>​ but simply to observe that f<​sub>​t</​sub>​ is the most widely used figure of merit for comparing the frequency response of BJTs. Most TUNs and TUPs will have f<​sub>​t</​sub>​ well over the 100 MHz minimum but high power and high voltage transistors will often have rather lower values.
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 gain and higher V<​sub>​ce(sat)</​sub>​ or R<​sub>​on</​sub>​ and are sure to be a bit more expensive.)\\ gain and higher V<​sub>​ce(sat)</​sub>​ or R<​sub>​on</​sub>​ and are sure to be a bit more expensive.)\\
 **Maximum current:​- **Select a value =33% above the maximum expected collector/​drain current. **Maximum current:​- **Select a value =33% above the maximum expected collector/​drain 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:​-** What package, //and pinout//, do you require? **Package:​-** What package, //and pinout//, do you require?
 (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:​-** What is the maximum dissipation?​ **Power:​-** What is the maximum dissipation?​
-(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,​ and select devices which meet your requirements. Consider all the remaining list but only specify the ones you actually care about:- 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,​ and select devices which meet your requirements. Consider all the remaining list but only specify the ones you actually care about:-
university/courses/electronics/text/choosing-transistors.txt · Last modified: 19 May 2014 17:13 by JamesBryant