05-11-2014, 03:25 PM
(This post was last modified: 05-11-2014, 03:34 PM by HomeMadeAudioProject.)
BJT transistor matching
What to measure / match: hFE
IB = base current
IC = collector current
hFE = DC current gain = IC / IB
The specs of a transistor (including hFE) varies with current draw, temperature, applied voltage, ... so if you want to be precise you should measure under the circumstances the transistor will see in his working environment.
complementary pair
a pair of a coresponding NPN and PNP transistor, e.g. BC550/BC560 or 2n5088/2n5087
how close to match the hFE values
5 is almost perfect. 10 to 20 is just fine. Probably you'll have to accept larger deviations (about 30), even with large quantities from one shop, from one manufacturer and one manufacturing batch matches are not guaranteed
time to settle
When testing there will be some initial figure on the display in the first moment, soon after that the figure changes going up (or down). hFE is affected by temperature, while holding the transistor between your finger tips and applying voltage the transistor heats up and thus it's hFE changes (particulary with small TO-92 parts). Just make sure all transistors have the same time to settle, a few seconds are enough.
Don't become obsessed about matching, I've build my PPAs without having any transistor matched and they are working fine. Probably you can measure the difference but it is unlikely that you can hear the difference.
Furthermore you probably won't find matching pairs among the output transistors anyway. Just pick all transistors from a single order, they should come from a single manufacturing batch. Only when you are going to pick transistors from several orders (and several shops) matching should make a significant difference
When you have complementary pairs of different value, choose the high hFE pair over the middle and low pairs.
the easy way: using a DMM
This is the easiest, fastest and most simple way: use a digital multi meter (DMM) that sports a hFE mode. Just put the transistor to the supplied socket, select hFE mode and read the value on the display. The DMM calculates the hFE for IB being a fraction of a mA but that's precise enough for most applications.
deviations between DMMs
most probable reasons:
different loaded batteries
different current measure points
simple grouping by collector voltage
just measure the collector voltage and group the transistors by their voltage without calculating the hfe (easy + fast)
calculating hFE
extensive and time-consuming, you need to measure two voltages and calculate the hFE values via a spreadsheet
VRB = voltage across base resistor
VRC = voltage across collector resistor
base current IB = VRB / RB
collector current IC = VRC / RC
hFE = IC / IB
hFE = ( VRC / RC ) * ( RB / VRB )
please note that you have to reverse the polarity when switching from npn to pnp transistors and that there is the risk of cooking the transistor when you connect it in a wrong way. Double check the pinout of the transistor (refer to datasheet!) and your wiring before applying voltage.
Links
bipolar junction transistor at Wikipeadia.org
transistor matching thread at head-fi.org
datasheet BC550 / BC560
datasheet 2n5088 / 2n5087
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JFET
IDSS
Testing a JFET's IDSS
take a look at the datasheet of the jfet, determine n- or p-channel and the pinout
set up a solderless breadboard (a.k.a. plugboard) with a jumper across two rows or build a small circuit with sockets on a perforated board (a.k.a. perfboard)
I plug the JFET into the board so that the jumper connects the gate and source pins
connect the negative side of a ~9V power supply to the jumper
connect the negative lead of a milliammeter to the JFET's drain
connect the positive lead of the milliamperemeter to the positive supply lead
the JFET saturates at its IDSS level in this situation, and that current value shows up on the milliamperemeter
To get usable results from this test, your milliammeter has to have a resolution of at least 1 mA, and 0.1 mA or better is perfect. I recommend that you use a current-limiting power supply for this test because it's very easy to short the milliammeter across the power supply with a simple slip of the probe; if your power supply is capable of putting out more amps than your meter can handle, you'll blow a fuse in the meter. (Or if it isn't fused, you could destroy the meter!) DMM fuses are expensive and hard to find, so you don't want this to happen.
Once you have your breadboard set up for testing transistors, I recommend that you test a whole bag of them at once and keep them sorted somehow. That way you only have to do the test once, and then later you can pick matched Q1/Q2 pairs quickly. One way to keep your transistors sorted is to put them on a strip of tape, folded over the heads of the transistors, similar to the way some resistors and diodes come packaged. Masking tape works well for this. Another way is to get a small fisherman's tackle box, and use each compartment to hold parts within a small IDSS range; the kind with lots of small compartments works best.
Links
junction gate field-effect transistor and field-effect transistor at Wikipeadia.org
datasheet 2n5486
datasheet BF245
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MOSFET complementary pair matching
VGS
I = (V - 4) / R1
V = 15
adjusting for about a 4V VGS ⇒ 11V across the resistor R1
input MOSFETs:
current of 5mA ⇒ R1 = 2.2kΩ
Measure the voltage between the gate and the source.
Write it down on a piece of masking tape or a sticky label and place it on the part.
Keep in mind the caveats about electrostatic discharge: touch ground before you touch the parts.
Matching input MOSFETs is critical, because they must share equally the 10mA of bias current from the current source, and they will not do that unless their VGS is matched. At 5mA current, they have an equivalent source resistance of about 15Ω. Assuming we want them to share the current to within 2mA, we calculate the required VGS match as follows. Using the formula V = IR, we see V = 0.002 x 15, which gives us 30mV. The VGS of the input devices should be matched to within 30mV at 5mA current. The matching is only essential within a given pair; you do not have to match the Ps to the Ns, or match to devices in another channel.
If you are unable to find input devices matched to within 30mV, you must insert resistance in the source to make up the difference. The resistance is calculated by the difference of the two values of VGS divided by 5mA. For example, if the difference in VP1GS is 100mV, then 0.1/0.005 = 20Ω. You would then place 20Ω in series with the MOSFET source having the lower VGS.
output MOSFETs
I=20mA ⇒ R1=560Ω
No matching is required for these devices; we are just checking to see that the VGS is between 4-4.6V and that they work.
We will measure the output device VGS at about 170mA. You can achieve this with either a 56Ω at 2W resistor, or two 100Ω at 1W resistors in parallel. We are looking to obtain a reasonable match within a parallel output bank of each polarity of each channel, so we want two groups of 12 with matched N- channel devices, and two groups of matched P-channel devices.
match VGS to within 0.3V If you are unable to find input devices matched to within 30mV, you must insert resistance in the source to make up the difference. The resistance is calculated by the difference of the two values of VGS divided by 5mA. For example, if the difference in VP1GS is 100mV, then 0.1/0.005 = 20Ω. You would then place 20Ω in series with the MOSFET source having the lower VGS.
We also measured the transconductance by taking another reading for each device at a higher current (0.5A), just to see what kind of variation we got. The transconductances measured from a low of 1.19 to a high of 1.56, with the average at about 1.35. Within this amplifier's general operating curve, each output will vary its current by about 1.3A for every volt of its VGS change. For 12 devices in parallel, we expect about 15A for each such volt.
By placing 1Ω source resistors on each transistor, we can assure adequate current sharing for a fairly wide range of VGS. In Class A bias, we will be operating at about 200mA/device, which will place 0.2V across each source resistor. A variation in VGS will cause the bias to be unequally distributed between the devices. For example, for a 4.6V device in parallel with a 4.5V device, the first will run at about 160mA at 6W and the second at about 240mA at 9W.
Links
MOSFET at Wikipeadia.org
MOSFET matching HowTo at PASS labs
practical MOSFET testing for audio [pdf] at PASS labs
datasheet IRFZ24N
datasheet IRF9Z34N
datasheet IRF610
datasheet IRFP240
datasheet IRFP9240
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preuzeto sa: http://www.diamondstar.de/transistor_matching_bjt.html
------------------------------------------------------------
http://www.klaus-boening.de/html/lab_page.html
------------------------------------------------------------
http://www.geofex.com/
kaže ovo:
iliti prevedeno "po naški":
(geo-Guitar Effects Oriented)
Problem koji se pojavljuje svaki put je uparivanje JFET-ova. Ljudi pokušavaju da izrade efekte koristeći JFET-ove kao linearne pojačivače ili promenjive otpornike (u svim tim fazerima/in all that phasers) i na kraju im efekti ne rade kako je planirano. Uzrok su najčešće razlike u karakteristikama JFET-ova. Poredeći ih sa bipolarnim BJT (PNP ili NPN) tranzistorima,JFET-ovi mnogo više variraju. Evo nekoliko najčešće korištenih JFET-ova i njihovih gate-source "cutoff" napona(naponi pri kojima potpuno prestaje protok struje kroz njih).
uređaj minimalni/ uobičajeni/ maksimalni
MPF102 not specified not specified -8.0V
2N4416 not specified not specified -6.0V
2N4416A -2.5V not specified -6.0V
2N3819 not specified -3.0V -8.0V
2N5484 -0.3V not specified -3.0V
2N5485 -0.5V not specified -4.0V
J201 -0.3V not specified -1.5V
BF244A -0.4 not specified -2.2V
BF244B -1.6 not specified -3.8V
BF244C -3.2 not specified -7.5V
Da vidimo sad... Ovi naponi su ti kod kojih potpuno prestaje protok struje kroz JFET-ove. Specifikacije o pojedinom JFET-u su vam garantovane od strane proizvođača. Oni npr, garantuju da će svi J201 JFET-ovi potpuno prekinuti protok struje pri
-1,5Vgs (tj. -1,5V između geta i sorsa,Ugs) . Takođe garantuju da NI JEDAN J201 NEĆE prekinuti protok struje ako je Ugs manji od -0,3V. Sve dok JFET-ovi rade u granicama garantovanim od strane proizvođača-to su dobri i ispravni JFET-ovi.
Sad da vidimo ovako...hm...1,5 podeljeno sa 0,3 jednako je...pa to je odnos razlika od 5 naprema 1! Strašno!!! Ako pogledate gornju tabelu,situacija postaje još lošija! 2N5484 varira između 0,3 i 3(apsolutne vrednosti uzete u obzir) ,to je 10 naprema 1! Ako pogledamo 2N4416A videćemo da je razlika "tek" nešto više od 2...uh,napokon jedan deo sa "tesnom" specifikacijom. Specijalan slučaj je 2N4416 koji čak i nema minimalne (pa ni srednje) specifikacije. Usput rečeno,tamo gde se navodi "nije specifikovano" (određeno) to znači da je proizvođač nevoljan da kaže bilo šta o tom parametru. Minimum nije određen,i to je to. Minimalan cutof napon može biti i 0,jednostavno neće vam to reći. A šta to "uobičajen"(typical) znači za 2N3819?
A šta je sa tim 2N3819-icama i MPF102-kama? Ne može se garantovati da će prekinuti protok sa manje od 8V,pa ni slabija baterija od 9V ne može da to uradi! A i BF244A, B, i C,to su tri JFET-a kojima se preklapaju minimumi i maksimumi. O čemu se zapravo radi?
Radi se o tome da proizvođači proizvode sa inherentnim(pretpostavljam da to znači neizbežnim) razlikama. Naime,mnogo mnogo teže je proizvesti identične/što sličnije JFET-ove nego bipolarne tranzistore. Podrazumeva se,pošto su toliko različiti,da bi bilo preskupo da se uparuju u uskim granicama/rasponima. Tako nešto se dešava i sa BF244,proizvođač ih pravi u velikim količinama,odredi granice za A B C -tipove,testira proizvedeno i onda obeležava po grupama u čije kriterijume se uklapaju. Kandidati za BF244B koji nisu uspeli sa -3,8V stavljeni su u grupu BF244C.
To nama koji pravimo gitarske efekte, čini život zanimljivijim. Često želimo da koristimo JFET-ove kao promenjive otpornike,naročito u "fazerima" kao što je PHASE 90. Pošto znamo da JFET ima minimalan otpor kad je Ugs=0, i da otpor ide skoro do u beskonačno pri Ugs=U"cutoff" ,gore prikazani rasponi napona Ugs odgovaraju kontrolnim naponima koje ćemo morati dovesti na gate JFET-ova,da bi od njih dobili promenjive otpornike negde između Rdson(obično oko par stotina Ohm) i otvoreno kolo. Obično želimo da postavimo neke kontrolne napone na njih i da dobijemo određene razlike otpora. Opseg vrednosti napona Ugs je prevelik da bi prosto samo zamenili određeni JFET bez nekih prethodnih merenja.
NAPOKON
To je ono što ovaj strujni krug čini. Upoređivač podešava napon jednak polovini napona napajanja uz pomoć dva 10kOhm otpornika sa leve strane,a zatim JFET i drugi 10kOhm otpornik da uporedi sa referentnim naponom. Posao op-ampa ovde je da uporedi dva napona i napravi napon na gejtu JFET-a upravo onolikim da izgleda kao da je JFET zapravo 10kOhm otpornik(isti napon i struja) u ovom sistemu. Nema ničeg magičnog oko vrednosti od 10kOhm za Rset,to je samo pogodna vrednost za većinu strujnih krugova koje ćemo koristiti.
Evo šta sam napravio za 76 komada 2N5485 kad sam ih kupio. Isekao sam traku za obeležavanje na 76 malih pravougaonika,nalepio ih na JFET-ove koje ću testirati i krenuo sa merenjem sve do jednog. Kad sam to uradio,nacrtao sam tabelu sa 76 polja i dve kolone,u jednu kolonu ispisao redom brojeve od 1 - 76 ,a u drugu izmerene vrednosti napona Ugs. Evo šta se desilo:
fabričke specifikacije za napon Ugsoff = -0,5V/-4V
izerene vrednosti su bile sledeće:
najniža -1,342V najviša -2,72V Izdvojile su se dve grupe,jedna od oko -2,5V i jedna od -1,6V. Evo slike rasporeda.
http://www.geofex.com/vgsdistr.gif
Sortirane su po naponu Ugs,i to tako da se najniže vrednosti Ugs nalaze u donjem desnom uglu. Mesta gde su dva JFET-a imali isti Ugs ostavljena su prazna. Mesta gde mnogo variraju su strma,tako da dobre parove možemo lako uočiti po praznim mestima. Pošto većini ljudi trebaju četiri JFET-a za Phaze90,tražio sam komlet od četiri komada. Našao sam četiri-pet komada dobo uparenih i to sa naponom Ugs od -2,5V do -2,6V. Bilo je još nekoliko koliko-toliko dobro uparenih setova koji bi se najverovatnije dobro pokazali pri uprebi,i nekoliko setova koje bi upotrebljavao u "pinch-u"(ne znam tačan prevod ovoga,pretpostavljam da se radi o nekom aparatu-uređaju). Bilo kako bilo,ako ih ne kupite gomilu i uparite,nećete dobiti phaser koji će raditi kako treba. Ne oslanjajte se na sreću i na JFET-ove "iz iste serije".
Phaser-ima su potrebni JFET-ovi koji su uvek u otpornom režimu,istovremeno ni potpuno "on" (uključeni) ni potpuno "off"(isključeni) . Uz malo sirove(loše,rekao bi) sreće,dok je vaš LFO(ne znam konkretno o čemu se radi-samo sam doslovno prepisao) u upotrebi,phaser bi potpuno uključivao i isključivao JFET u različito vreme,pa baš i ne biste dobili mnogo "phase-iranja"(oprostite na eventualno lošem prevodu pojedinih izraza i pojmova).
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Nema potrebe za daljnjim prevođenjem,jer se dalje tekst(kao i deo već prevedenog) ne odnosi na uparivanje JFET-ova.
What to measure / match: hFE
IB = base current
IC = collector current
hFE = DC current gain = IC / IB
The specs of a transistor (including hFE) varies with current draw, temperature, applied voltage, ... so if you want to be precise you should measure under the circumstances the transistor will see in his working environment.
complementary pair
a pair of a coresponding NPN and PNP transistor, e.g. BC550/BC560 or 2n5088/2n5087
how close to match the hFE values
5 is almost perfect. 10 to 20 is just fine. Probably you'll have to accept larger deviations (about 30), even with large quantities from one shop, from one manufacturer and one manufacturing batch matches are not guaranteed
time to settle
When testing there will be some initial figure on the display in the first moment, soon after that the figure changes going up (or down). hFE is affected by temperature, while holding the transistor between your finger tips and applying voltage the transistor heats up and thus it's hFE changes (particulary with small TO-92 parts). Just make sure all transistors have the same time to settle, a few seconds are enough.
Don't become obsessed about matching, I've build my PPAs without having any transistor matched and they are working fine. Probably you can measure the difference but it is unlikely that you can hear the difference.
Furthermore you probably won't find matching pairs among the output transistors anyway. Just pick all transistors from a single order, they should come from a single manufacturing batch. Only when you are going to pick transistors from several orders (and several shops) matching should make a significant difference
When you have complementary pairs of different value, choose the high hFE pair over the middle and low pairs.
the easy way: using a DMM
This is the easiest, fastest and most simple way: use a digital multi meter (DMM) that sports a hFE mode. Just put the transistor to the supplied socket, select hFE mode and read the value on the display. The DMM calculates the hFE for IB being a fraction of a mA but that's precise enough for most applications.
deviations between DMMs
most probable reasons:
different loaded batteries
different current measure points
simple grouping by collector voltage
just measure the collector voltage and group the transistors by their voltage without calculating the hfe (easy + fast)
calculating hFE
extensive and time-consuming, you need to measure two voltages and calculate the hFE values via a spreadsheet
VRB = voltage across base resistor
VRC = voltage across collector resistor
base current IB = VRB / RB
collector current IC = VRC / RC
hFE = IC / IB
hFE = ( VRC / RC ) * ( RB / VRB )
please note that you have to reverse the polarity when switching from npn to pnp transistors and that there is the risk of cooking the transistor when you connect it in a wrong way. Double check the pinout of the transistor (refer to datasheet!) and your wiring before applying voltage.
Links
bipolar junction transistor at Wikipeadia.org
transistor matching thread at head-fi.org
datasheet BC550 / BC560
datasheet 2n5088 / 2n5087
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JFET
IDSS
Testing a JFET's IDSS
take a look at the datasheet of the jfet, determine n- or p-channel and the pinout
set up a solderless breadboard (a.k.a. plugboard) with a jumper across two rows or build a small circuit with sockets on a perforated board (a.k.a. perfboard)
I plug the JFET into the board so that the jumper connects the gate and source pins
connect the negative side of a ~9V power supply to the jumper
connect the negative lead of a milliammeter to the JFET's drain
connect the positive lead of the milliamperemeter to the positive supply lead
the JFET saturates at its IDSS level in this situation, and that current value shows up on the milliamperemeter
To get usable results from this test, your milliammeter has to have a resolution of at least 1 mA, and 0.1 mA or better is perfect. I recommend that you use a current-limiting power supply for this test because it's very easy to short the milliammeter across the power supply with a simple slip of the probe; if your power supply is capable of putting out more amps than your meter can handle, you'll blow a fuse in the meter. (Or if it isn't fused, you could destroy the meter!) DMM fuses are expensive and hard to find, so you don't want this to happen.
Once you have your breadboard set up for testing transistors, I recommend that you test a whole bag of them at once and keep them sorted somehow. That way you only have to do the test once, and then later you can pick matched Q1/Q2 pairs quickly. One way to keep your transistors sorted is to put them on a strip of tape, folded over the heads of the transistors, similar to the way some resistors and diodes come packaged. Masking tape works well for this. Another way is to get a small fisherman's tackle box, and use each compartment to hold parts within a small IDSS range; the kind with lots of small compartments works best.
Links
junction gate field-effect transistor and field-effect transistor at Wikipeadia.org
datasheet 2n5486
datasheet BF245
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MOSFET complementary pair matching
VGS
I = (V - 4) / R1
V = 15
adjusting for about a 4V VGS ⇒ 11V across the resistor R1
input MOSFETs:
current of 5mA ⇒ R1 = 2.2kΩ
Measure the voltage between the gate and the source.
Write it down on a piece of masking tape or a sticky label and place it on the part.
Keep in mind the caveats about electrostatic discharge: touch ground before you touch the parts.
Matching input MOSFETs is critical, because they must share equally the 10mA of bias current from the current source, and they will not do that unless their VGS is matched. At 5mA current, they have an equivalent source resistance of about 15Ω. Assuming we want them to share the current to within 2mA, we calculate the required VGS match as follows. Using the formula V = IR, we see V = 0.002 x 15, which gives us 30mV. The VGS of the input devices should be matched to within 30mV at 5mA current. The matching is only essential within a given pair; you do not have to match the Ps to the Ns, or match to devices in another channel.
If you are unable to find input devices matched to within 30mV, you must insert resistance in the source to make up the difference. The resistance is calculated by the difference of the two values of VGS divided by 5mA. For example, if the difference in VP1GS is 100mV, then 0.1/0.005 = 20Ω. You would then place 20Ω in series with the MOSFET source having the lower VGS.
output MOSFETs
I=20mA ⇒ R1=560Ω
No matching is required for these devices; we are just checking to see that the VGS is between 4-4.6V and that they work.
We will measure the output device VGS at about 170mA. You can achieve this with either a 56Ω at 2W resistor, or two 100Ω at 1W resistors in parallel. We are looking to obtain a reasonable match within a parallel output bank of each polarity of each channel, so we want two groups of 12 with matched N- channel devices, and two groups of matched P-channel devices.
match VGS to within 0.3V If you are unable to find input devices matched to within 30mV, you must insert resistance in the source to make up the difference. The resistance is calculated by the difference of the two values of VGS divided by 5mA. For example, if the difference in VP1GS is 100mV, then 0.1/0.005 = 20Ω. You would then place 20Ω in series with the MOSFET source having the lower VGS.
We also measured the transconductance by taking another reading for each device at a higher current (0.5A), just to see what kind of variation we got. The transconductances measured from a low of 1.19 to a high of 1.56, with the average at about 1.35. Within this amplifier's general operating curve, each output will vary its current by about 1.3A for every volt of its VGS change. For 12 devices in parallel, we expect about 15A for each such volt.
By placing 1Ω source resistors on each transistor, we can assure adequate current sharing for a fairly wide range of VGS. In Class A bias, we will be operating at about 200mA/device, which will place 0.2V across each source resistor. A variation in VGS will cause the bias to be unequally distributed between the devices. For example, for a 4.6V device in parallel with a 4.5V device, the first will run at about 160mA at 6W and the second at about 240mA at 9W.
Links
MOSFET at Wikipeadia.org
MOSFET matching HowTo at PASS labs
practical MOSFET testing for audio [pdf] at PASS labs
datasheet IRFZ24N
datasheet IRF9Z34N
datasheet IRF610
datasheet IRFP240
datasheet IRFP9240
------------------------------------------------------------
preuzeto sa: http://www.diamondstar.de/transistor_matching_bjt.html
------------------------------------------------------------
http://www.klaus-boening.de/html/lab_page.html
------------------------------------------------------------
http://www.geofex.com/
kaže ovo:
Quote:An issue that comes up all the time is matching JFETs. People try to build effects using JFETs as a linear amplifier or as a variable resistor (in all those phasers) and find that the effect doesn't work as planned.
What's wrong is usually the variability of JFET characteristics. Compared to bipolar (NPN or PNP) transistors, JFETs vary all over the map. Here are some commonly used JFETs and their gate-source cutoff voltages:
Device Minimum Typical Maximum
MPF102 not specified not specified -8.0V
2N4416 not specified not specified -6.0V
2N4416A -2.5V not specified -6.0V
2N3819 not specified -3.0V -8.0V
2N5484 -0.3V not specified -3.0V
2N5485 -0.5V not specified -4.0V
J201 -0.3V not specified -1.5V
BF244A -0.4 not specified -2.2V
BF244B -1.6 not specified -3.8V
BF244C -3.2 not specified -7.5V
Let's do some interpretation. This voltage is that voltage that causes the JFET to cut off completely - no current flows. The spec sheet is a guarantee that the manufacturer of the transistor makes to you. They will guarantee, for instance, that all J201's will cut off completely when you put -1.5V from the gate to the source of the device. They also guarantee that no J201's will cut off completely with less than -0.3Vgs. As long as the devices you get fall in that range, they are good parts (to this particular spec, anyway).
So let's see... um... 1.5V divided by 0.3V... that's a five to one variation! That's terrible! Looking up the chart, things get worse. A 2N5484 varies from 0.3V to 3.0V, ten to one. If we look at the 2N4416A, we see that the variation is only a little over two to one... phew, finally a tightly specified part. Especially compared to the plain vanilla 2N4416, which specifies no minimum at all. By the way, where it says "not specified", the manufacturer is unwilling to say what the parameter is. If no minimum is specified, it's just that. The minimum turn off voltage might be zero - they won't say. And what does that 2N3819 "typical" mean?
And how about those 2N3819's and MPF102's? They can't be guaranteed to turn off with less than 8V - a weak 9V battery can't turn them off at all. And the BF244 A, B, and C - three parts with overlapping minimums and maximums. What's happening there?
What's going on is that the makers are coping with the inherent variation in the JFET process. It is much, much harder to make consistent JFETs than to make consistent bipolars devices. And unstated, but implied is that if they vary this much, then the testing to sort them into nice, tightly defined bins of parts would cost too much. That's what's going on with the BF244's - the maker has made batches of JFETs, shot at the middle of the distribution for the A, B, and C parts, and then tested them. BF244B candidates that didn't make the max -3.8V spec could at least be dropped in the BF244C bin.
This makes our lives more interesting as effects makers. We often want to use a JFET as a variable resistor, particularly in phasers like the Phase 90. Since we know that a JFET has it minimum resistance when Vgs is zero and that goes to a maximum of almost infinite when the device is cut off, the ranges of Vgs off shown above correspond to the ranges of control voltages we have to put on the JFET gates to get them to be a variable resistor somewhere between Rdson (usually a few hundred ohms) and an open circuit. We usually want to put some control voltage on them and get some resistance variation. The range of the Vgs values is too large to cope with in most simple circuits without some further selection.
That's what this little circuit does. The matcher sets up a middle-of-the-power-supply voltage on the left side with two 10K resistors, and then a JFET and another 10K resistor to compare to the reference voltage. The opamp's job is to compare the two voltages and make the voltage on the JFET gate just right so it looks like a 10K resistor (same voltage and current) in this setup. There's nothing magic about the 10K value for Rset, it's just a convenient value that's going to be about where a lot of our circuits will wind up.
iliti prevedeno "po naški":
(geo-Guitar Effects Oriented)
Problem koji se pojavljuje svaki put je uparivanje JFET-ova. Ljudi pokušavaju da izrade efekte koristeći JFET-ove kao linearne pojačivače ili promenjive otpornike (u svim tim fazerima/in all that phasers) i na kraju im efekti ne rade kako je planirano. Uzrok su najčešće razlike u karakteristikama JFET-ova. Poredeći ih sa bipolarnim BJT (PNP ili NPN) tranzistorima,JFET-ovi mnogo više variraju. Evo nekoliko najčešće korištenih JFET-ova i njihovih gate-source "cutoff" napona(naponi pri kojima potpuno prestaje protok struje kroz njih).
uređaj minimalni/ uobičajeni/ maksimalni
MPF102 not specified not specified -8.0V
2N4416 not specified not specified -6.0V
2N4416A -2.5V not specified -6.0V
2N3819 not specified -3.0V -8.0V
2N5484 -0.3V not specified -3.0V
2N5485 -0.5V not specified -4.0V
J201 -0.3V not specified -1.5V
BF244A -0.4 not specified -2.2V
BF244B -1.6 not specified -3.8V
BF244C -3.2 not specified -7.5V
Da vidimo sad... Ovi naponi su ti kod kojih potpuno prestaje protok struje kroz JFET-ove. Specifikacije o pojedinom JFET-u su vam garantovane od strane proizvođača. Oni npr, garantuju da će svi J201 JFET-ovi potpuno prekinuti protok struje pri
-1,5Vgs (tj. -1,5V između geta i sorsa,Ugs) . Takođe garantuju da NI JEDAN J201 NEĆE prekinuti protok struje ako je Ugs manji od -0,3V. Sve dok JFET-ovi rade u granicama garantovanim od strane proizvođača-to su dobri i ispravni JFET-ovi.
Sad da vidimo ovako...hm...1,5 podeljeno sa 0,3 jednako je...pa to je odnos razlika od 5 naprema 1! Strašno!!! Ako pogledate gornju tabelu,situacija postaje još lošija! 2N5484 varira između 0,3 i 3(apsolutne vrednosti uzete u obzir) ,to je 10 naprema 1! Ako pogledamo 2N4416A videćemo da je razlika "tek" nešto više od 2...uh,napokon jedan deo sa "tesnom" specifikacijom. Specijalan slučaj je 2N4416 koji čak i nema minimalne (pa ni srednje) specifikacije. Usput rečeno,tamo gde se navodi "nije specifikovano" (određeno) to znači da je proizvođač nevoljan da kaže bilo šta o tom parametru. Minimum nije određen,i to je to. Minimalan cutof napon može biti i 0,jednostavno neće vam to reći. A šta to "uobičajen"(typical) znači za 2N3819?
A šta je sa tim 2N3819-icama i MPF102-kama? Ne može se garantovati da će prekinuti protok sa manje od 8V,pa ni slabija baterija od 9V ne može da to uradi! A i BF244A, B, i C,to su tri JFET-a kojima se preklapaju minimumi i maksimumi. O čemu se zapravo radi?
Radi se o tome da proizvođači proizvode sa inherentnim(pretpostavljam da to znači neizbežnim) razlikama. Naime,mnogo mnogo teže je proizvesti identične/što sličnije JFET-ove nego bipolarne tranzistore. Podrazumeva se,pošto su toliko različiti,da bi bilo preskupo da se uparuju u uskim granicama/rasponima. Tako nešto se dešava i sa BF244,proizvođač ih pravi u velikim količinama,odredi granice za A B C -tipove,testira proizvedeno i onda obeležava po grupama u čije kriterijume se uklapaju. Kandidati za BF244B koji nisu uspeli sa -3,8V stavljeni su u grupu BF244C.
To nama koji pravimo gitarske efekte, čini život zanimljivijim. Često želimo da koristimo JFET-ove kao promenjive otpornike,naročito u "fazerima" kao što je PHASE 90. Pošto znamo da JFET ima minimalan otpor kad je Ugs=0, i da otpor ide skoro do u beskonačno pri Ugs=U"cutoff" ,gore prikazani rasponi napona Ugs odgovaraju kontrolnim naponima koje ćemo morati dovesti na gate JFET-ova,da bi od njih dobili promenjive otpornike negde između Rdson(obično oko par stotina Ohm) i otvoreno kolo. Obično želimo da postavimo neke kontrolne napone na njih i da dobijemo određene razlike otpora. Opseg vrednosti napona Ugs je prevelik da bi prosto samo zamenili određeni JFET bez nekih prethodnih merenja.
NAPOKON
To je ono što ovaj strujni krug čini. Upoređivač podešava napon jednak polovini napona napajanja uz pomoć dva 10kOhm otpornika sa leve strane,a zatim JFET i drugi 10kOhm otpornik da uporedi sa referentnim naponom. Posao op-ampa ovde je da uporedi dva napona i napravi napon na gejtu JFET-a upravo onolikim da izgleda kao da je JFET zapravo 10kOhm otpornik(isti napon i struja) u ovom sistemu. Nema ničeg magičnog oko vrednosti od 10kOhm za Rset,to je samo pogodna vrednost za većinu strujnih krugova koje ćemo koristiti.
Evo šta sam napravio za 76 komada 2N5485 kad sam ih kupio. Isekao sam traku za obeležavanje na 76 malih pravougaonika,nalepio ih na JFET-ove koje ću testirati i krenuo sa merenjem sve do jednog. Kad sam to uradio,nacrtao sam tabelu sa 76 polja i dve kolone,u jednu kolonu ispisao redom brojeve od 1 - 76 ,a u drugu izmerene vrednosti napona Ugs. Evo šta se desilo:
fabričke specifikacije za napon Ugsoff = -0,5V/-4V
izerene vrednosti su bile sledeće:
najniža -1,342V najviša -2,72V Izdvojile su se dve grupe,jedna od oko -2,5V i jedna od -1,6V. Evo slike rasporeda.
http://www.geofex.com/vgsdistr.gif
Sortirane su po naponu Ugs,i to tako da se najniže vrednosti Ugs nalaze u donjem desnom uglu. Mesta gde su dva JFET-a imali isti Ugs ostavljena su prazna. Mesta gde mnogo variraju su strma,tako da dobre parove možemo lako uočiti po praznim mestima. Pošto većini ljudi trebaju četiri JFET-a za Phaze90,tražio sam komlet od četiri komada. Našao sam četiri-pet komada dobo uparenih i to sa naponom Ugs od -2,5V do -2,6V. Bilo je još nekoliko koliko-toliko dobro uparenih setova koji bi se najverovatnije dobro pokazali pri uprebi,i nekoliko setova koje bi upotrebljavao u "pinch-u"(ne znam tačan prevod ovoga,pretpostavljam da se radi o nekom aparatu-uređaju). Bilo kako bilo,ako ih ne kupite gomilu i uparite,nećete dobiti phaser koji će raditi kako treba. Ne oslanjajte se na sreću i na JFET-ove "iz iste serije".
Phaser-ima su potrebni JFET-ovi koji su uvek u otpornom režimu,istovremeno ni potpuno "on" (uključeni) ni potpuno "off"(isključeni) . Uz malo sirove(loše,rekao bi) sreće,dok je vaš LFO(ne znam konkretno o čemu se radi-samo sam doslovno prepisao) u upotrebi,phaser bi potpuno uključivao i isključivao JFET u različito vreme,pa baš i ne biste dobili mnogo "phase-iranja"(oprostite na eventualno lošem prevodu pojedinih izraza i pojmova).
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Nema potrebe za daljnjim prevođenjem,jer se dalje tekst(kao i deo već prevedenog) ne odnosi na uparivanje JFET-ova.
...reci mi Bože,koje si boje kože...