BJT, Mosfet, JFET, etc - Can Someone Explain the Datasheets for Pedal Building?

[BuddytheReow]

I've got a transistor tester coming in the mail in a few days. Can someone explain to me or at least point me in the right direction what to look for when testing? On top of that can someone give me a breakdown of what I should be looking for in the various datasheets and why those are important for circuit design/building? I thought posting it here in the Test Kitchen would be beneficial for others as well.

Chuck's got a few good articles on some of these specs so it's a start.

What's All This hFE Stuff About?

We see lots of discussion here and on other forums regarding hFE, but I get the feeling that many of us don't really know what it means or why it's important. First, a little background. Bipolar transistors are current-mode devices. We have to supply base current to get collector current...

forum.pedalpcb.com

Another Way to Measure Transistor Gain

We've talked about hfe, which represents current gain, but there is another way to measure gain in a transistor. Some devices, like vacuum tubes or FETS, don't draw any input current, so the concept of current gain makes no sense. For those devices, engineers use the term "tranconductance,"...

forum.pedalpcb.com

JFET Biasing - part 1

I spent a fair amount of time trying to make this as concise and readable as I could. Intro In theory, setting the bias on JFETs should be easy. In practice, not so much. Here are the details on why that is and what we can do about it. Let's review some theory and get the math out of the...

forum.pedalpcb.com

Biasing BJTs - part 1

There's a lot to cover, so I'll break this up into parts. This discussion will focus on the DC operating point of the transistor. First, some basics. Many of you will already know this stuff. Vbe is the base-emitter junction voltage. It's around 0.6V with silicon and around 0.15V with...

forum.pedalpcb.com

Biasing BJTs - part 2

Before I go any further, let's talk about negative feedback. People don't always like negative feedback in their personal lives, but we absolutely depend on it in electronics. Stephen S. Black is credited with discovering the negative feedback amplifier in 1927 while working at Bell Labs. The...

forum.pedalpcb.com

Biasing BJTs - part 3

This time we'll focus on the Fuzz Face. The Fuzz Face has a long and colorful history that is beyond the scope of this discussion. The original FFs were built with PNP Germanium transistors because that's what the pedal builders had to work with. In this discussion, I'll talk about NPN...

forum.pedalpcb.com

 

[Coda]

Which tester?

 

[BuddytheReow]

Just an el cheapo TC1 from Amazon. I know it won’t test everything, but I can use the Geofex method for testing germaniums

 

[mdc]

I believe you'll want to look for the open source firmware upgrade for that to make it give you pedal-useful information, but it's been a year since I looked into it.

 

[Chuck D. Bones]

On top of that can someone give me a breakdown of what I should be looking for in the various datasheets and why those are important for circuit design/building?

Bear with me while I digress..
The circuit design process is supposed to work like this:

1. unit-level requirements. The pedal will do this and this. It will have these controls. I will fit in this sized box.
2. Conceptual design. There will be these functional blocks (booster, clipper, filter, etc.) It will use these technologies (JFETs, opamps, Germanium, etc.)
3. Detailed circuit design. Each functional block it designed down the component level. This is where you decide which components to use. Now we start looking at datasheets.
4. Circuit simulation and breadboarding. Does the design actually do what we want? Are there places where we need to make changes?
5. Iterate steps 2-4 as required to achieve the desired result.

Not too many people apply this level of discipline in their pedal designs and it shows. It may look like the hard way to do things, but just the opposite is true. A methodical approach will get you to the finish line with a good product and the least amount of swearing (from you and your customers). The reward for taking short-cuts is doing it over.

OK, now about datasheets...

The gist of what I just said is if you're designing a circuit, then you will know which datasheet parameters matter based on the circuit requirements. Modding a circuit is the same as designing a circuit because you have to know what you want the modded circuit to do and how it works. Lotta people talk about what HFE you need for this Fuzz Face or which opamp sounds better in a Tube Screamer but in my mind, most of that is more noise than signal.

Then there is the problem of the datasheet not telling you what you really need to know. For example, a BJT datasheet will give a min and max HFE at one or two collector currents, but those collector current might be nowhere near the collector current in the pedal circuit you're building. Good luck even finding datasheets for vintage Ge transistors. Extracting the info you need from a datasheet is hard work even for an experienced designer.

You will get the most benefit from a transistor or diode datasheet when you use it to compare with test data. You want to know if the parts you bought are in-spec or not. For BJTs, look at HFE and Icbo because that's what the tester will tell you. Again, bear in mind that the tester will measure HFE at one collector current. If you're lucky, it will be near the spec'd collector current in the datasheet. For JFETs look at Vgs,off and Idss. For MOSFETs, look at Vt. For diodes, look at Vf and Ir.