24-02-2018, 01:59 PM
MOS-FETs don't draw gate current*, so the hFE is arguably infinite 
Yes, they have transconductance, but much less than a typical bipolar transistor. If you can live with the base current - and that's the way to regard base current (an inconvenience, rather than something that can be depended on) - then bipolar transistors will often do a better job. However, FETs are more marketable in some circles for whatever reason... And easier to understand than valves IMHO.
*Of course, that's DC current. At AC the gate will draw current because it looks like a capacitor.
Any PSU can be negative or positive, assuming one end hasn't been connected to earth. That's certainly the case with bench power supplies, where the outputs deliberately float. However, in lieu of that, then use a variable resistance in series with a milliampmeter. As you vary the resistance, the voltage across it varies in sympathy. Note the points where the milliamp reading changes, and measure the voltage across the current source... Watch for power dissipation in the resistance, of course. A decade box is nice to use if you have one, but a pot will do.
Again, Zout is almost certainly irrelevant for your application, but to measure it, you apply an AC voltage to the current source (technically a "sink" here) via a series resistor, and sweep the frequency. You can use a DMM to measure that voltage if you know that it accurate at the frequencies of interest; otherwise use a pair of 'scope probes to perform a differential measurement. I'll leave it as an exercise to work out how to crunch the numbers.
Ripple rejection is easy. Let the current source/sink establish a flow of current in something like a 1k resistor. Monitor the AC voltage across the resistor (hopefully it'll stay at 0V). Then, look to see how much ripple you have on the negative rail feeding your current sink. From there, you can do 20*log(Vr/Vs) (where Vr is the AC voltage across the resistor, and Vs is the AC voltage at the supply rail). I'll leave it to you to decide if you need to consider the value of the load resistor in those calculations (hint: you might want to turn that AC voltage into an AC current). If you find that the AC voltage across the 1k resistor is hard to measure on a 'scope (and it ought to be), then try increasing the resistor... You can also increase the ripple on the supply rail - either by reducing the value of the smoothing capacitor(s), or by deliberately drawing a large current from the rail with a power resistor.
Yes, they have transconductance, but much less than a typical bipolar transistor. If you can live with the base current - and that's the way to regard base current (an inconvenience, rather than something that can be depended on) - then bipolar transistors will often do a better job. However, FETs are more marketable in some circles for whatever reason... And easier to understand than valves IMHO.*Of course, that's DC current. At AC the gate will draw current because it looks like a capacitor.
Any PSU can be negative or positive, assuming one end hasn't been connected to earth. That's certainly the case with bench power supplies, where the outputs deliberately float. However, in lieu of that, then use a variable resistance in series with a milliampmeter. As you vary the resistance, the voltage across it varies in sympathy. Note the points where the milliamp reading changes, and measure the voltage across the current source... Watch for power dissipation in the resistance, of course. A decade box is nice to use if you have one, but a pot will do.
Again, Zout is almost certainly irrelevant for your application, but to measure it, you apply an AC voltage to the current source (technically a "sink" here) via a series resistor, and sweep the frequency. You can use a DMM to measure that voltage if you know that it accurate at the frequencies of interest; otherwise use a pair of 'scope probes to perform a differential measurement. I'll leave it as an exercise to work out how to crunch the numbers.
Ripple rejection is easy. Let the current source/sink establish a flow of current in something like a 1k resistor. Monitor the AC voltage across the resistor (hopefully it'll stay at 0V). Then, look to see how much ripple you have on the negative rail feeding your current sink. From there, you can do 20*log(Vr/Vs) (where Vr is the AC voltage across the resistor, and Vs is the AC voltage at the supply rail). I'll leave it to you to decide if you need to consider the value of the load resistor in those calculations (hint: you might want to turn that AC voltage into an AC current). If you find that the AC voltage across the 1k resistor is hard to measure on a 'scope (and it ought to be), then try increasing the resistor... You can also increase the ripple on the supply rail - either by reducing the value of the smoothing capacitor(s), or by deliberately drawing a large current from the rail with a power resistor.
