EveryCircuit - Transformer Coupled Class A Amp

8 years ago

With that high supply voltage and the fact that it still works if you replace the NPN transistors with N-channel MOSFETs, remove the PNP transistor, and short the 2.2kOhm resistor at the power MOSFETs source (is that it's version of the BJT collector?), it seems to be a vacuum tube amplifier.

jason9

8 years ago

I found a slight issue. Run the circuit as it is like normal, but set the simulation speed to 100uS/S.

thebugger

8 years ago

Yeah in reality this wouldn't happen. A real transformer causes a phase shift at the upper frequency range and the 150pF/10k RC chain causes a phase advance at these frequencies to negate the phase shift of the transformer. Usually the value of the RC chain is properly calculated by first examining the frequency-phase characteristics of the audio transformer. I just placed it as an example. In reality instead of causing instabilities, it should cure some at high frequencies.

jason9

8 years ago

Is this actually a vacuum tube amp? Also, is the source a MOSFET the equivalent to the collector of a BJT? And the drain the equivalent to the emitter? And the gate the equivalent to the base?

thebugger

8 years ago

No, they're not equivalent. MOSFETS are driven by voltage and BJT's are driven by current. There's also the V-I characteristics that differ. A MOSFET resembles a triode, and a BJT resembles a pentode, although all tubes are voltage driven (like a MOSFET). There really isn't a direct equivalent for vacuum tubes. The closest should be a JFET in depletion mode

WTFCircuit

8 years ago

What are deplation mode mosfet and jfet used for? I have never see any

thebugger

8 years ago

https://en.m.wikipedia.org/wiki/Depletion_and_enhancement_modes

jason9

8 years ago

I don't mean equivalent as same, but rather like base-emitter current controls a BJT and gate-drain voltage controls a MOSFET, and the majority of the current flows into the collector, and all the current flows into the source, and all the current flows out of the emitter, and all of the current flows out of the drain. I guess by "equivalent", I mean "most similar to". With this meaning of "equivalent", what are the answers to the three questions in my other comment? Is it yes, yes, and yes? Or am I getting some of the MOSFET terminals mixed up?

thebugger

8 years ago

Emitter would be source, collector - drain and base - gate. N channel would be NPN and P channel - PNP

jason9

8 years ago

I see, I seem to have mixed up source and drain. Why does all the current go into the drain and out of the source? It would seem to make more sense if it went into the source and came out of the drain. Or is it that the drain drains down electricity creating a lower voltage as in a class-A preamp, and the source acts as a source of electricity as in a linear regulator. What's your opinion on why it's various terminals are named the way they are? The BJT seems to have much clearer terminal names.

thebugger

8 years ago

http://everycircuit.com/circuit/4790061663256576