|
The Symmetrical
and the Non-Symmetrical
Note that in the previous circuit both MOSFETs are of N-channel flavor. This will upset many, as they have been raised on symmetrical solid-state output stages. Yet, there are many advantages to not going symmetrical. While a few N-channel MOSFETs have P-channel equivalents, the match is seldom that good. The transconductances differ as do the input capacitances. I understand that when Hitachi made their famous complementary MOSFETs, they started by making the best Plate-channel MOSFET they could and then they crippled a N-channel MOSFET until it matched it. On the other hand, inexpensive N-channel MOSFETs from the same run will match each other at least in transconductance and capacitance. And matching is not that difficult, requiring only a low voltage source, a voltage meter, a few resistors and a potentiometer. The disadvantage to using only one type of MOSFET is that we must ensure that both are treated equally, i.e. that both see an identical gate-to-source voltage and that power supply noise relations are fully understood.
In this amplifier, the top and bottom MOSFET are being used in exactly the same way: as grounded source amplifiers. The top MOSFET may look as if it is used in a source follower configuration, but it is not, as the 100 ohm resistor is attached to the output. In other words, the top MOSFET offers no output resistance, as the gate will follow the source's movements. For example, +1 volt pulse applied to the output will force the 100 ohm resistor up 1 volt, which the source will dutifully follow. On the other hand, if the 100 ohm terminated into ground and not the output, then that same pulse would force the top MOSFET to stop conducting altogether, as its gate would move effectively -1 volts relative to its source. The output impedance in this case will equal the inverse of the transconductance of the top MOSFET. And of course, the bottom MOSFET offers no resistance to the output's voltage movement, as its gate is entirely immune to any voltage movements at the output.
Adding a feedback loop is what will lower the output impedance and gain of the amplifier. With the feedback loop in place, the same pulse will force the input tube to conduct more current, which will impose a negative pulse at the top MOSFET's gate and a positive pulse at the bottom MOSFET's gate.
|
