Operational amplifier MOSFET vs BJT

Thread Starter

jsnx

Joined Mar 17, 2022
21
Hello, I have a question regarding operational amplifiers. Whenever I search for some datasheets of high gain and high-speed Op Amps ( BW higher than 50MHz), I mostly find that they are made of bipolar transistors rather than mosfet. Why is it so?

I've done some research and I've found that MOSFET is better than BJT when it comes to input impedance, noise immunity, and offset level, three characteristics that are very important to have an ideal opamp. Also, MOSFET gives more freedom in design since the transconductance and so the gain can be tuned by re-sizing the MOSFETs. If so, why are bipolar transistors widely used for OpAmp?

Thank you in advance for enlightening me on this subject
 

DickCappels

Joined Aug 21, 2008
8,710
It is probably the large capacitances associated with the gate coiupled with the fact that the gate needs to be driven by significant changes in voltage (while bipolars on the other had have relativelysmall swings on the base-emitter junctions that:
A) Slows things down, and
B) Increases power dissipation

I don't design IC's this is just my take on the situation.
 

Audioguru again

Joined Oct 21, 2019
4,688
50MHz is a medium radio frequency. Opamps are usually used for DC and audio frequencies.

Modern rail-to-rail opamps are made with Mosfets and have a max frequency of >50MHz. Look at rail-to-rail opamps in Google and at www.analog.com.
 
Top