operational-amplifier
I saw a post a few days ago.
Power Dissipation of Op-Amp
where I was actually curious about the circuit where they use BJTs all over the circuit but use 2 MOSFETS for the output stage.(also why are they using JFETs?)
In most designs that I have seen online, only BJTs are used.
Is there any advantage of using MOSFETs instead of BJTs for the output?
Are MOSFETs not less linear than BJTs?
If so what is their advantage?
It would be great if someone can shed some light on it.
If you mean something like below, then the currents add:
As Alfred mentions, this is not known as a "two stage CE amplifier" though.
With BJTs, the emitter resistors are needed to prevent thermal runaway. This is caused by the fact that as a BJT gets hotter it passes more current. The emitter resistor provides some negative feedback (higher current = more voltage across resistor = lower Vbe to stop runaway) but wastes power.
FETs don't have this problem and can be connected directly, so are better for parallel operation.
I found the culprit. Actually it is due to ADC's switching when it takes Sample. I disconnected OPamp's Output from ADC by removing the R from RC and the ringing was gone.
Best Answer
The reason could be partly due to the absence of second breakdown. In BJTs the power dissipation capability is often reduced at high voltage as the current tends to be concentrated in localised hot spots. Some MOSFETs do not suffer from this. With this device the safe operating area graph shows a straight line up to 100 V when the device is dissipation limited.