Hi,

I found this document describing the comparison between bipolar, JFET and CMOS input Operational Amplifiers.

The example devices given in the document are OPA2210 (bipolar), OPA828 (JFET), and OPA2156 (CMOS).

https://www.ti.com/lit/ab/sboa355/s...63212&ref_url=https%3A%2F%2Fwww.google.com%2F

I am still not getting what are their differences ? In which application which of the three type of Operational Amplifier is suitable. Kindly someone please describe for better understanding.

 

The TI application note gives some insight to the differences of the three op amps, bipolar, JFET, and CMOS.
The main differences are in noise, bandwidth, temperature drift, input impedance, and power consumption.

If you want very high input impedance, then you would choose CMOS or JFET,
However, if you want lower noise, go with bipolar.

 

In which application which of the three type of Operational Amplifier is suitable.

There is no clear-cut division as to when you use a particular type, so it mostly depends upon what characteristics are most important in the application.

• Bipolars can be the fastest, and some can operate from high voltages, but not all are. They are rather the generic type device and are likely the cheapest and most commonly used.
• JFETs have low input bias current and some have low noise. They are not that commonly used except perhaps in some audio applications for their low noise.
• CMOS types are relatively new, also have low input currents, and work well to make rail-rail type input and output op amps. One of my favorites for general use is the low-cost rail-rail TLV925x series which has low input offset, relatively low noise, and better frequency & slew-rate response characteristics than the typical general purpose op amp.

 

What I have heard before or I knew something before is that with BJTs one can work at higher voltages compare to CMOS/MOSFET. Is that true ?
Also switching is faster in CMOS/ MOSFET compared to BJTs. Kindly correct me here. Thanks in advance.

 

The TI application note gives some insight to the differences of the three op amps, bipolar, JFET, and CMOS.
The main differences are in noise, bandwidth, temperature drift, input impedance, and power consumption.

If you want very high input impedance, then you would choose CMOS or JFET,
However, if you want lower noise, go with bipolar.

You need to qualify that:
if you want lower noise for a low impedance source use bipolar
if you want lower noise for a high impedance source use JFET
MOSFETs are noisier than either.

If you want low offset voltage, bipolar are usually the best, but may have high bias currents.
JFET and MOSFET have low bias currents.

 

What I have heard before or I knew something before is that with BJTs one can work at higher voltages compare to CMOS/MOSFET. Is that true ?
Also switching is faster in CMOS/ MOSFET compared to BJTs. Kindly correct me here. Thanks in advance.

op-amps don't switch. You can get high speed in all three types. High speed usually means a larger quiescent current.
MOSFET are generally low supply voltage. Higher supply voltages (up to 40V) can be both JFET and Bipolar.
Then there's the AD275 which has a Butler input stage, which is a combination of JFET and bipolar.

 

What I have heard before or I knew something before is that with BJTs one can work at higher voltages compare to CMOS/MOSFET. Is that true ?

Not necessarily. The maximum source/drain and gate voltage of a MOSFET can be varied. As can the threshold voltage.

Also switching is faster in CMOS/ MOSFET compared to BJTs.

If by switching you mean slew rate, that's a function of any internal compensation and the current available to charge it.

 

This I have found "Switching speed: BJTs are slower whereas MOSFETs are faster. Power consumption: BJTs consume more power and MOSFETs consume less power. Temperature sensitivity: BJTs are more temperature-sensitive than MOSFETs". And I understand this well.

The main question was how about amps, bipolar, JFET, and CMOS.

So JFET and CMOS input OpAmp are slower then BJT input OpAmp even though BJT is slower in switching then MOSFET.

 

This I have found "Switching speed: BJTs are slower whereas MOSFETs are faster. Power consumption: BJTs consume more power and MOSFETs consume less power.

Yes, because bipolar transistors have a delay when coming out of saturation

Temperature sensitivity: BJTs are more temperature-sensitive than MOSFETs". And I understand this well.

No - you are wrong. Both are temperature sensitive.

So JFET and CMOS input OpAmp are slower then BJT input OpAmp even though BJT is slower in switching then MOSFET.

Again. Op-amp speed is determined by the compensation components and the power consumption. There are high speed and low speed op-amps in bipolar , JFET and CMOS

 

Kindly let me know if that is true. The BJT input OpAmp has higher quiescent current comapred to JFET and CMOS input OpAmp.

 

Kindly let me know if that is true. The BJT input OpAmp has higher quiescent current comapred to JFET and CMOS input OpAmp.

False. There are low quiescent current op-amps in bipolar, JFET and CMOS technology.

 

"Switching speed: BJTs are slower whereas MOSFETs are faster.

You are comparing apples and oranges.
Switching speed for digital circuits has little to do with the op amp speed, which is more determined by its internal design.

You are trying to put the op amps in neat categories as determined by the type of their internal transistors, but there are too many op amp variables, and overlap between the performance of different op amps to do that.
You just need to look at the op amp characteristics you need for the application, and then pick the op amp that best meets those requirements, independent of its transistor type.

 

Study the data sheets for the opamps you are considering and choose the one next that best matches the needs of your circuit.

 

Can we summarize like this:

If we have higher source impedance sensor, then we should use CMOS OpAmp / JFET OpAmp because they have higher input impedance and very low input bias currents IB.

If we lower source impedance sensor, then we should use BJT OpAmp.

BJT OpAmp: Low Input Impedance, High input bias current IB, Smaller voltage noise, Higher current noise, Can operate from high voltages.

CMOS OpAmp / JFET OpAmp: Very High Input Impedance, Low input bias current IB, Make rail-rail type OpAmp.

Which one are faster ? CMOS OpAmp / JFET OpAmp or BJT OpAmp ?

 

Which one are faster ? CMOS OpAmp / JFET OpAmp or BJT OpAmp ?

See posts 6 and 9 for the answer.

 

Which one are faster ? CMOS OpAmp / JFET OpAmp or BJT OpAmp

Which is faster, a Ford or a Chevy, or a Dodge?

The fastest are BJT, but many are slower than some CMOS or JFET devices.
Speed is obviously only one of many considerations when selecting an op amp.

 

How fast is fast?
Give some numbers to your requirements.

 

How fast is fast?
Give some numbers to your requirements.

Yes, define "fast". If you need a hole digging, a JCB will dig it faster than a Ford, a Chevy or a Dodge.

 

JCB

Had to look that one up! I would have said Caterpillar, but that would likely have been misinterpreted.

 

Had to look that one up! I would have said Caterpillar, but that would likely have been misinterpreted.

In Britain "JCB" is to "backhoe loader" as "Hoover " is to "vacuum cleaner".