Slightly off topic but I was just looking at how you used a MOSFET similar to how I have used a bipolar transistor in the input circuit of a MOSFET. Shown is a page from my notebook as I was learning/testing a way to use a non logic level MOSFET in the output of a MC when no logic level device was on hand. As I proved to myself, a non logic level device wont work with a low signal input but can be made to work with a few additional components to boost the input signal. I'll have to play w/ MOSFETs more!
Hi AGCB,
You are obviously are an engineering type.
The IRF9520 is a nice PMOSFET, but it has a relatively high gate threshold voltage of 4V worst case, and a high drain/source on resistance of 0.6 Ohms worst case, compared to the latest PMOSFETS. But the IRF9520 is still a very useful device: 100VDS, 6.8A ID, 0.4 Degrees C Watt thermal resistance (extraordinarily low for a medium power MOSFET), and available in an easy-to-use TO220 case.
A comparable later PMOSFET, would typically have a worst case gate threshold of 2.5V and a much lower on resistance of around 0.05 Ohms. But the maximum drain source voltage would be less at 20 to 50V, with 20V being very common. The new devices are only available in surface mount cases though.
Yes, as you show with your notes, you can make a voltage amplifier/inverter/translator with a bipolar junction transistor (BJT). In fact, Q2 in the circuit of post #13 could be replaced by an NBJT and two resistors.
Your NBJT translator has an input voltage threshold of around 400mV (base/emitter turn on for silicon transistor) and a fully conducting input voltage of 600mV (ignoring base current). You can change the input voltage threshold of a BJT translator by using two resistors on the input rather than one. In your translator, if you added a 10K resistor from the base of the transistor to 0V, the input threshold would be multiplied by two, so would become 0.8V. Of course, you have to ensure that there is sufficient base current drive to generate the required collector current.
With a MOSFET there is no gate current (at DC) to worry about, but the gate threshold voltage is not as well defined as for the base threshold voltage of a BJT. Typically a MOSFET gate threshold has a two to one variation, from device to device. For example, the gate threshold voltage spread for an IRF9540 is 2V to 4V. You can also multiply the threshold voltage of a MOSFET translator with two resistors.
MOSFETS are dead easy, but the fun starts at high frequencies, when you have to cater for the massive parasitic capacitances, and at high currents where you have to watch out for heating and secondary break-down. It is ironic that, although a MOSFET gate takes no current at DC, at high frequencies a MOSFET may take more gate current than an equivalent BJT would require base current.
Special gate driver chips are produced to cater for the effective high input capacitance of MOSFETs. There driver chips have an extraordinarily high current drive capability of 1A to 7A (see LM5110 data sheet below).
Incidentally, the two PMOSFETS in the circuit of post #13, could be replaced by BJTs, but not without some other circuit modifications.
End of lecture.
spec
DATASHEETS
https://www.vishay.com/docs/91074/91074.pdf
https://www.ti.com/lit/ds/symlink/lm5110.pdf