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Semiconductors

7 Posts authored by: jc2048
When we are taught about op amps, the usual starting point is a perfect op amp. That amplifier has infinite gain, infinite input resistance on the two inputs, zero output resistance, and so forth. That's done so that the focus can be on understanding the basic operation and how the feedback operates with the various common circuit configurations, without all the messy real- world details that apply to an actual physical circuit. Although real op amps can be very, very good (exceptionally so ...
Introduction   More experimenting. This time I thought I'd have a look at the output characteristics of a JFET. Sometimes these things go well and sometimes they don't. This one didn't go too well but, for all that, I'm going to blog it as I did it rather than try and tidy it up.   The output characteristic is how the drain current behaves with changing drain voltage and is usually shown for a series of fixed gate voltages. Here is an example from a datasheet. This is for a 2SK355 ...
Introduction   Having used JFETs (type BF256B) in some of my test circuits whilst experimenting with MOSFETs, I thought it might be interesting to look at the transfer function and see if I could recreate the curve you sometimes see on a datasheet which relates the gate voltage to the drain current.   If you're not used to JFETs (Junction Field-Effect Transistors), they're similar to MOSFETs but the insulation between the gate and the channel is provided not by an actual insula ...
Introduction   More experimenting with MOSFETs. This time I'm going to have a look at the output characteristics.   On a datasheet, output characteristics are usually drawn something like the following two graphs. The first is for the IRF730 part that I have been experimenting with extensively in this series of blogs and the second is for a small-signal part, a BS170:   Curves of drain current against drain-source voltage are drawn for a set of fixed Vgs voltages. The Vgs ...
  Previous blogs:   Experimenting with MOSFETS: Total Gate Charge Experimenting with MOSFETs: Transfer Characteristic   Introduction   More experimenting with MOSFETs. In the last blog I looked at the transfer characteristics but it wasn't very easy to see from the graph I plotted where the threshold was. The threshold is measured at a drain current of 250uA and that is down in the noise for the current probe I was using. So this is a different approach to finding the ...
This is a short follow-up to Experimenting with MOSFETS: Total Gate Charge where I looked at 'total gate charge' for an IRF730, an n-channel power MOSFET.   Since I still had the test board wired up, I thought it might be interesting to see if could get a quick plot of the transfer characteristic - the way that the drain current varies with gate voltage.   I've taken the previous circuit, bypassed the constant current generator in the drain and then plotted the drain current against ...
Introduction   More experimenting, this time with MOSFETs.   If you've ever designed with a MOSFET, I'm sure you've seen the kind of curves that are usually present in the datasheet showing the relationship between the total gate charge and the gate voltage. Here's one such graph from a datasheet for a Vishay IRF730 device:   They always have the same sort of shape to the curve, though the position of the plateau, halfway up, varies in both voltage and the amount of charge to ...