4.4 Summary
DE- and E-MOSFET devices may be used to create both common source voltage amplifiers and common drain voltage followers. The common source amplifiers may be swamped or non-swamped, depending on the bias form used. If the bias type does not utilize a source resistor, swamping is not available. This includes zero bias for the DE-MOSFET and voltage divider bias for both the DE- and E-MOSFET.
Like their JFET counterparts, MOSFET common source amplifiers exhibit moderate inverting voltage gain, very high input impedance and moderate output impedance. The input impedance is a function of the biasing resistor configuration situated in front of the gate as the impedance looking into the gate itself is very, very high at low frequencies.
The MOSFET common drain followers also behave similarly to the JFET version. Again we see a non-inverting voltage gain approaching unity, a very high input impedance and a low output impedance. The higher the transconductance is, the closer the gain will be to unity and the lower the output impedance will be.
Review Questions
- How well does the MOSFET voltage amplifier compare to its JFET counterpart?
- How well does the MOSFET source follower compare to its JFET counterpart?
- What are the practical differences between a voltage amplifier using a DEMOSFET versus using an E-MOSFET ?
- It has been stated that a source follower cannot be made using a standard zero biased DE-MOSFET. Why is this?
- It has been stated that a swamped voltage amplifier cannot be made using a standard voltage divider biased E-MOSFET. Why is this?
