Linear Current Amplifiers, hFE, and Parts Substitutions for Vector Monitors
Two of the substitutes worked. (Plus, of course, the original, giving us three choices.) One of them doesn’t, quite. But why not?
The answer lies in a rarely used parameter from the datasheet: current gain, abbreviated as hFE. (It’s usually buried somewhere on the second or third page, amongst other secondary parameters rarely viewed as important. However, several of them turn out to be very important when designing or retrofitting a vector monitor, due to the special demands it places on the output stage!)
Like most specifications, hFE varies a great deal between individual transistors of the same part number. As Agilent states in this application note, “It is not unusual to find a 2 or 3 to 1 ratio in device hFE.“ The specs we’re given are usually in the form of “minimum, typical, maximum” – and often maximum or typical is omitted, leaving only a guaranteed minimum value.
(Since hFE varies so much, it’s bad form to build a linear amplifier that depends on the hFE of a transistor to determine its gain. In practice, gain is always determined by some other property of the circuit – usually feedback between the output of a gain stage and its input. Vector monitors are no exception. They feature very high negative feedback, which helps them remain resistant to power supply ripple and noise, as well as interference from the HV supply and other sources. For more information on hFE, also abbreviated as β, how it varies, and how it’s tested, read this excellent article.)
Another issue is that hFE declines with increasing current. For high-power BJT transistors, such as used in vector monitor power stages, it’s usually flat, or close to it, until current hits 6-10A…at which point it declines steeply.
Why is this important?
- Vector deflection amplifiers are current amplifiers, not voltage amplifiers.
- The deflection is proportional to the current running through the yoke.
- Thus, if current cannot be sufficiently amplified, the beam can’t reach the edge of the screen.
In my next post, I’ll show you exactly what happened, and why.