What's All This Matching Stuff About? - part 1

People talk about matching transistors, but what do they mean?

There are a few instances in pedal circuits where matching transistor parameters to each other or to circuit requirements is essential for proper performance.

1. FET-based Phase-Shifters
Many Phase Shifters used JFETs as voltage-controlled resistors. The Phase 90 is a prime example. This schematic is from electrosmash.com. They have a very good explanation of how Phase-Shifters work, so I won't repeat all that here. They link to an article by R. G. Keen on JFET matching that is well worth reading.


The transistor parameter that requires matching is the JFETs' Vp, the pinch-off voltage. Vp varies widely for the same part number. The trimmer resistor is there to adjust for the JFET's Vp, but there are four JFETs and only one trimmer, so matching Vp between the JFETs is essential for the circuit to function. How closely should they be matched? The closer the better; if you get all four withing 0.1V you're going to have a great Phase-Shifter. Get beyond 0.5V or so and it will be crap. If there is one take-away here it is that unless you buy a matched set, you will need to buy at least 5x as many as you need and test them yourself to end up with a good matched set.

2. Differential-transistor octave circuits
There are five types of octave-up circuits on PedalPCB. One of those circuits uses a pair of transistors to perform frequency doubling. An example is the Propolis.


Q3 and Q4 act as a full-wave rectifier, switching between two copies of the signal that are 180° out of phase. Without getting into the finer details of how or why this works, suffice it to say the two copies need to be identical for the strongest octave-up content. for that to happen, Q3 & Q4 need to be matched for Vbe so that they turn on and off symmetrically. In addition, some other parts need to be matched, specifically R7 & R8, R9 & R14, R10 & R13. If you use 1% resistors, you don't need to match them by hand. If you use 5% or 10% carbon comp resistors, get out your DMM. Also, Q2 needs to have hFE >100 so that the signals at the emitter and collector are the same amplitude.

 

[Chuck D. Bones]

I was saving that for Part 2. I'm sure there will be plenty of discussion on the topic of biasing a Fuzz Face because it is very subjective. The trivial answer is you build a Fuzz Factory and turn knobs until you get the sound you like.

 

[Chas Grant]

I was saving that for Part 2. I'm sure there will be plenty of discussion on the topic of biasing a Fuzz Face because it is very subjective. The trivial answer is you build a Fuzz Factory and turn knobs until you get the sound you like.

That would be the easy way, I will let that be part 2 then! So I guess @cooder will have to wait?

 

[cooder]

Oh yeah more suspense... and I can wait...
.... kind of... twiddling thumbs....
Many thanks guys!
Cool place to hang out, that Boneyard.

 

[Jbanks]

People talk about matching transistors, but what do they mean?

There are a few instances in pedal circuits where matching transistor parameters to each other or to circuit requirements is essential for proper performance.

1. FET-based Phase-Shifters
Many Phase Shifters used JFETs as voltage-controlled resistors. The Phase 90 is a prime example. This schematic is from electrosmash.com. They have a very good explanation of how Phase-Shifters work, so I won't repeat all that here. They link to an article by R. G. Keen on JFET matching that is well worth reading.

View attachment 7968
The transistor parameter that requires matching is the JFETs' Vp, the pinch-off voltage. Vp varies widely for the same part number. The trimmer resistor is there to adjust for the JFET's Vp, but there are four JFETs and only one trimmer, so matching Vp between the JFETs is essential for the circuit to function. How closely should they be matched? The closer the better; if you get all four withing 0.1V you're going to have a great Phase-Shifter. Get beyond 0.5V or so and it will be crap. If there is one take-away here it is that unless you buy a matched set, you will need to buy at least 5x as many as you need and test them yourself to end up with a good matched set.

2. Differential-transistor octave circuits
There are five types of octave-up circuits on PedalPCB. One of those circuits uses a pair of transistors to perform frequency doubling. An example is the Propolis.

View attachment 7969
Q3 and Q4 act as a full-wave rectifier, switching between two copies of the signal that are 180° out of phase. Without getting into the finer details of how or why this works, suffice it to say the two copies need to be identical for the strongest octave-up content. for that to happen, Q3 & Q4 need to be matched for Vbe so that they turn on and off symmetrically. In addition, some other parts need to be matched, specifically R7 & R8, R9 & R14, R10 & R13. If you use 1% resistors, you don't need to match them by hand. If you use 5% or 10% carbon comp resistors, get out your DMM. Also, Q2 needs to have hFE >100 so that the signals at the emitter and collector are the same amplitude.

Chuck,

amazing job explaining all this to all of us less literate builders out there. ?

I know you were mostly discussing matching phasor for transistors but for SMD components like a JFET201, will I still need to test them or are they robotically built to enough spec to be matched because it’s SMD?

thanks in advance!

 

[Chuck D. Bones]

Thanks, Man.

J201s, whether they are SMD or TO-92, are the same die, tested to the same specs at the factory. Like all JFETs, the Vp and Idss limits are pretty loose. What happens to them after they leave the factory depends on the vendor. Even an in-spec J201 will not work in every pedal. Whether they need to be selected or matched depends on the specific circuit. The Calamity is very picky about J201s. Most of the ones you buy will probably NOT work in that pedal. I have not seen any phase-shifters with J201s in them, I suspect it's because their Vp is so small. I doubt that PedalPCB screens or sorts their J201s. They may spot check a few to make sure they're getting a good batch. If you're building pedals with JFETs in them, your best bet is to buy a lot more than you need, test them all and sort them by which pedal needs which specific Vp range. Cross you fingers and hope you get a decent yield. That's what the professional builders do, the smart ones anyway. You way wonder, what do the pros do with the JFETs that they can't use? They might sell them to a dealer at a discount who then tries to unload them on eBay.

I test all of my JFETs before installing them because it's easy, I know what I want for each circuit and it avoids unpleasant surprises.

One more thing... it's really not necessary to quote one of my articles in its entirety (or at all) in your response. My shit is pretty long-winded. I'm not bitching or criticizing, just offering some friendly advice.

 

[cooder]

More great advice there, cheers!
How would we know which pedal likes or needs a specific Vp range?
Do you test smd jfets with the chinese component tester and how many have you dropped being lost forever in the carpet?

 

[Chuck D. Bones]

How would we know which pedal likes or needs a specific Vp range?

You have to analyze the circuit.

Do you test smd jfets with the chinese component tester and how many have you dropped being lost forever in the carpet?

I have tested a few, mounted on the little adapter boards. So far, none have been eaten by the carpet. I feed it a steady diet of trimmed leads and bits of wire insulation.

 

[Kroars]

People talk about matching transistors, but what do they mean?

There are a few instances in pedal circuits where matching transistor parameters to each other or to circuit requirements is essential for proper performance.

1. FET-based Phase-Shifters
Many Phase Shifters used JFETs as voltage-controlled resistors. The Phase 90 is a prime example. This schematic is from electrosmash.com. They have a very good explanation of how Phase-Shifters work, so I won't repeat all that here. They link to an article by R. G. Keen on JFET matching that is well worth reading.

View attachment 7968
The transistor parameter that requires matching is the JFETs' Vp, the pinch-off voltage. Vp varies widely for the same part number. The trimmer resistor is there to adjust for the JFET's Vp, but there are four JFETs and only one trimmer, so matching Vp between the JFETs is essential for the circuit to function. How closely should they be matched? The closer the better; if you get all four withing 0.1V you're going to have a great Phase-Shifter. Get beyond 0.5V or so and it will be crap. If there is one take-away here it is that unless you buy a matched set, you will need to buy at least 5x as many as you need and test them yourself to end up with a good matched set.

2. Differential-transistor octave circuits
There are five types of octave-up circuits on PedalPCB. One of those circuits uses a pair of transistors to perform frequency doubling. An example is the Propolis.

View attachment 7969
Q3 and Q4 act as a full-wave rectifier, switching between two copies of the signal that are 180° out of phase. Without getting into the finer details of how or why this works, suffice it to say the two copies need to be identical for the strongest octave-up content. for that to happen, Q3 & Q4 need to be matched for Vbe so that they turn on and off symmetrically. In addition, some other parts need to be matched, specifically R7 & R8, R9 & R14, R10 & R13. If you use 1% resistors, you don't need to match them by hand. If you use 5% or 10% carbon comp resistors, get out your DMM. Also, Q2 needs to have hFE >100 so that the signals at the emitter and collector are the same amplitude.

People talk about matching transistors, but what do they mean?

There are a few instances in pedal circuits where matching transistor parameters to each other or to circuit requirements is essential for proper performance.

1. FET-based Phase-Shifters
Many Phase Shifters used JFETs as voltage-controlled resistors. The Phase 90 is a prime example. This schematic is from electrosmash.com. They have a very good explanation of how Phase-Shifters work, so I won't repeat all that here. They link to an article by R. G. Keen on JFET matching that is well worth reading.

View attachment 7968
The transistor parameter that requires matching is the JFETs' Vp, the pinch-off voltage. Vp varies widely for the same part number. The trimmer resistor is there to adjust for the JFET's Vp, but there are four JFETs and only one trimmer, so matching Vp between the JFETs is essential for the circuit to function. How closely should they be matched? The closer the better; if you get all four withing 0.1V you're going to have a great Phase-Shifter. Get beyond 0.5V or so and it will be crap. If there is one take-away here it is that unless you buy a matched set, you will need to buy at least 5x as many as you need and test them yourself to end up with a good matched set.

2. Differential-transistor octave circuits
There are five types of octave-up circuits on PedalPCB. One of those circuits uses a pair of transistors to perform frequency doubling. An example is the Propolis.

View attachment 7969
Q3 and Q4 act as a full-wave rectifier, switching between two copies of the signal that are 180° out of phase. Without getting into the finer details of how or why this works, suffice it to say the two copies need to be identical for the strongest octave-up content. for that to happen, Q3 & Q4 need to be matched for Vbe so that they turn on and off symmetrically. In addition, some other parts need to be matched, specifically R7 & R8, R9 & R14, R10 & R13. If you use 1% resistors, you don't need to match them by hand. If you use 5% or 10% carbon comp resistors, get out your DMM. Also, Q2 needs to have hFE >100 so that the signals at the emitter and collector are the same amplitude.

Love reading your articles man. Please keep em coming!

 

[Chuck D. Bones]

Thanks!

 

[peccary]

Hello Chuck,

I had a question about the Mofeta: it calls for four 2N5457's - should these be matched in this circuit? I've not seen matching mentioned with it, but I wanted to be sure.

Also, just a point of clarification: Are Vgs(off) and Vp the same thing? I have a lot of sorting to do and want to make sure that I don't have to do it twice, thanks!

 

[Chuck D. Bones]

Matching the JFETs is not necessary. Just make sure that Q4's Idss is at least 5.5mA. Alternatively, you could bump R10 up to 10K, then Idss can be as low as 1.8mA.

Vp and Vgs,off are the same thing. Us lazy people would rather type two characters instead of seven.

It might make a difference if Q5 and Q6 are matched for Vbe and HFE, but I would not obsess over it.

[CORRECTION]
I had previously stated that 2N5246 is rated for 15V max. That is incorrect, it is rated for 30V max.
If you use 2N5246s, be aware that they are rated for 15V max. It's a conservative rating and they should be good for at least 20V at room temp. The safe thing to do is keep the drain voltages below 15V by starting with the BIAS trimmers at maximum resistance and adjusting from there. Not trying to scare you, just doing my due diligence.

 

[fig]

Well, he should have plenty

 

[peccary]

Matching the JFETs is not necessary. Just make sure that Q4's Idss is at least 5.5mA. Alternatively, you could bump R10 up to 10K, then Idss can be as low as 1.8mA.

Vp and Vgs,off are the same thing. Us lazy people would rather type two characters instead of seven.

It might make a difference if Q5 and Q6 are matched for Vbe and HFE, but I would not obsess over it.

If you use 2N5246s, be aware that they are rated for 15V max. It's a conservative rating and they should be good for at least 20V at room temp. The safe thing to do is keep the drain voltages below 15V by starting with the BIAS trimmers at maximum resistance and adjusting from there. Not trying to scare you, just doing my due diligence.

Thanks for the reply, Dave.

I have a good number of 5457's from the previous group order that I am planning on using for the Mofeta, but I had considered trying the 5246's to see what the difference might be, so I appreciate the heads-up!

 

[owlexifry]

Just make sure that Q4's Idss is at least 5.5mA.

if the datasheet indicates a max of 5ma, how likely is it that i’m actually going to find a 5457 with idss > 5.5ma?

just got hold of a peak23 sot23 socket thing to test some jfets.
so far everything is around 2.5-3 ma (mmbf5457)

 

[Chuck D. Bones]

I'd say not very likely. I should have done a bit more research and said: "I don't know why EAE would design a circuit that way." Maybe they want Q4 to clip asymmetrically. I have not breadboarded the MOFETA. If and when I do, I'll increase R10 and make sure Q4 can operate without hitting the Idss limit.