amplifiercircuit analysismosfettransistors
I was wondering why would anyone use a MOSFET amplifier?
Since a MOSFET amplifier is nonlinear, it produces significant amount of distortion.
To minimize distortion, textbooks suggest using small voltage variation around the Q-point, which makes amplifier very inefficient.
Why not then use a standard BJT amplifier instead?
Your first two transistors look incorrectly biassed so that Ve is about (0.5*Vcc - 0.7V) in Q0, Q1 maybe slightly better.
This means (thanks to symmetry between Re and Rc (e.g.R0 and R3) that Vc is about Vcc/2+0.7V, in other words Vce=1.4V approx, and the transistor is close to saturation.
First step : run a DC analysis to confirm this for both common-emitter stages (the emitter follower is fine).
To maximise the linear dynamic range you want to maximise the possible collector swing, so as a first pass, re-bias for Vc=9V, Ve=3V, i.e. Vb = 3.7V, calculate the base resistors for that - and see if it's improved.
This gives you up to 6V p-p swing at the collectors. It's not the best you can do as you have less AC swing at the emitter therefore you can increase headroom further, but I'll stop here.
The anode-cathode current for the GU-81M is modulated by the FET. All the current from the valve flows through the FET and it's DC configuration (using the potentiometer) will set the operating point cathode voltage. The center tapped heater has no influence and the heater/cathode is not at zero volts.
Yes, this would be considered cascode operation as in this audio amplifier with two triodes:
In your case since the FET modulates the operating current (and therefore the C-G) of the pentode it changes the amplitude of the RF output.
If you look at the datasheet for your FET:
Excuse my mouse scribbling on top of the graph, but here you can see as the Vgs varies, the current through the valve will vary (and the cathode voltage).
I've no idea where you operating point for the GU-81m is so cannot work out the valve current, but you should be able to.
Note that your grids are at very low voltages, so as the cathode rises they are effectively negative biased. It's doubtful you'd see more than 120-150 V on the cathode so well with the ratings of the FET.
Best Answer
Well, BJT are not linear too, that's why we usually add a negative feedback path.
MOSFETs have a huge gate impedance which is useful in many occasions. Also their distortion behavious is… well different. MOSFET also need a lot less bias current so they are useful in battery applications.
As discrete component you almost never use a mosfet amplifier at low power (except in special cases, like some RF applications). However in power circuits it's a different story, efficiency it's usually better.
The real importance of mosfet in linear mode is in IC design where the process simplifies mixed signal design (logic and analog on the same chip). When you use opamps these days you are either using a mosfet amplifier or some hybrid (like BiCMOS processes).
Both MOSFETs and BJTs (JFETs, too!) have their use in both low power and high power applications. It's just that discrete designs are not prevalent these days.