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 flowing (turn the transistor on). We vary the base current to modulate the collector current and achieve amplification. The definition of hFE (DC current gain) is the DC collector current divided by the DC base current. Notice that the "FE" is upper case, denoting DC conditions. hFE is important for setting the bias on a transistor. The definition of hfe (small-signal AC current gain) is the change in collector current divided by the change in base current. As an example, if a 5uA change in base current causes a 300uA change in collector current, then the hfe is 300/5 = 60. The "fe" is lower case, denoting small-signal AC. If we draw a graph of collector current vs. base current, hfe would be the slope of the curve at any given collector current. hfe is important for determining how much amplification we can get from a transistor, but it's not the only factor. hFE (and hfe) vary considerable from one part number to another and from one manufacturing run to another. The datasheet for 2N3904 says the hfe at 1mA collector current will be anywhere between 100 and 400. This kind of tolerance is typical.
Even for the same transistor, hFE and hfe are not constant, they vary with collector current and how they vary depends on how the transistor was designed and manufactured. When someone says a transistor has an hFE of 100, that means at some collector current the base current is 1/100th of the collector current. With the types of transistors we use and the current levels we have in pedals, when collector current goes up, hFE goes up, but it's not proportional. All this means you can have a transistor that has an hFE of 100 when the collector current is 1mA, but when you run it at 200uA, the hFE will be lower. It's difficult to predict the hFE at various collector currents, so designers depend on testing. Depending on the circuit, cherry-picking transistors may be necessary.
When someone recommends a certain hFE or hfe, it's really just a guideline to get you in the ballpark. Some circuits require hFE to be within certain limits to bias correctly. Tone Benders and Fuzz Faces for example. Other circuits are deliberately designed to not be sensitive to hFE or hfe. The Big Muff Pi is an example of this. Local feedback around each transistor in the BMP stabilizes the bias point and sets the AC gain.
At the beginning of this article, I said we have to supply base current to get collector current flowing. That's only partially true because many germanium transistors, especially vintage ones, are leaky enough to supply their own base current. There's still base current, but it's coming from inside the transistor instead from the circuit outside the transistor. All three stages of the Maestro FZ-1 (and its variants) are examples of leakage biasing. This leakage current makes measuring hFE more difficult because we have to account for the effects of leakage current. DMMs that measure hFE usually do not account for leakage current and as a result, grossly over-estimate hFE. Real transistor testers, like the DCA55, make the measurement correctly.
Variability in leakage and hFE are why some modern germanium fuzz circuits have trimpots in them. When Steve Daniels at Small Bear sells a matched set of Tonebender or Fuzz Face transistors, he also includes the biasing resistors for that set. Those resistors are part of the matching and accommodate the part-to-part variability in the transistors.
So how do you know you have the right hFE (or hfe)? Follow the BOM in the Build Docs, buy quality transistors and cross your fingers. If you're substituting transistors or building with germanium, use sockets and plan on trying a few different ones.
Next time: The Other Way to Describe Transistor Gain.
First, a little background. Bipolar transistors are current-mode devices. We have to supply base current to get collector current flowing (turn the transistor on). We vary the base current to modulate the collector current and achieve amplification. The definition of hFE (DC current gain) is the DC collector current divided by the DC base current. Notice that the "FE" is upper case, denoting DC conditions. hFE is important for setting the bias on a transistor. The definition of hfe (small-signal AC current gain) is the change in collector current divided by the change in base current. As an example, if a 5uA change in base current causes a 300uA change in collector current, then the hfe is 300/5 = 60. The "fe" is lower case, denoting small-signal AC. If we draw a graph of collector current vs. base current, hfe would be the slope of the curve at any given collector current. hfe is important for determining how much amplification we can get from a transistor, but it's not the only factor. hFE (and hfe) vary considerable from one part number to another and from one manufacturing run to another. The datasheet for 2N3904 says the hfe at 1mA collector current will be anywhere between 100 and 400. This kind of tolerance is typical.
Even for the same transistor, hFE and hfe are not constant, they vary with collector current and how they vary depends on how the transistor was designed and manufactured. When someone says a transistor has an hFE of 100, that means at some collector current the base current is 1/100th of the collector current. With the types of transistors we use and the current levels we have in pedals, when collector current goes up, hFE goes up, but it's not proportional. All this means you can have a transistor that has an hFE of 100 when the collector current is 1mA, but when you run it at 200uA, the hFE will be lower. It's difficult to predict the hFE at various collector currents, so designers depend on testing. Depending on the circuit, cherry-picking transistors may be necessary.
When someone recommends a certain hFE or hfe, it's really just a guideline to get you in the ballpark. Some circuits require hFE to be within certain limits to bias correctly. Tone Benders and Fuzz Faces for example. Other circuits are deliberately designed to not be sensitive to hFE or hfe. The Big Muff Pi is an example of this. Local feedback around each transistor in the BMP stabilizes the bias point and sets the AC gain.
At the beginning of this article, I said we have to supply base current to get collector current flowing. That's only partially true because many germanium transistors, especially vintage ones, are leaky enough to supply their own base current. There's still base current, but it's coming from inside the transistor instead from the circuit outside the transistor. All three stages of the Maestro FZ-1 (and its variants) are examples of leakage biasing. This leakage current makes measuring hFE more difficult because we have to account for the effects of leakage current. DMMs that measure hFE usually do not account for leakage current and as a result, grossly over-estimate hFE. Real transistor testers, like the DCA55, make the measurement correctly.
Variability in leakage and hFE are why some modern germanium fuzz circuits have trimpots in them. When Steve Daniels at Small Bear sells a matched set of Tonebender or Fuzz Face transistors, he also includes the biasing resistors for that set. Those resistors are part of the matching and accommodate the part-to-part variability in the transistors.
So how do you know you have the right hFE (or hfe)? Follow the BOM in the Build Docs, buy quality transistors and cross your fingers. If you're substituting transistors or building with germanium, use sockets and plan on trying a few different ones.
Next time: The Other Way to Describe Transistor Gain.
Last edited: