Question

What is the purpose of a bootstrapped input stage in an op-amp?

Answer 1

A bootstrapped input stage in an operational amplifier (op-amp) is used to improve the input impedance and reduce the loading effect on the previous stage.

Here’s how it works and why it’s useful:

Purpose of Bootstrapping:

1. Increase Input Impedance:
   

   Bootstrapping helps to maintain a high input impedance, which means the op-amp draws very little current from the input signal. This is important in preventing the op-amp from loading down the signal source, ensuring that the signal isn't significantly altered by the op-amp’s input stage.

1. Minimize Signal Distortion:
   

   By preventing excessive current draw from the input, bootstrapping reduces the likelihood of signal distortion. This ensures that the op-amp faithfully amplifies the input signal without introducing unwanted changes.

1. Improve Common-Mode Rejection Ratio (CMRR):
   

   Bootstrapping can also enhance the common-mode rejection ratio, which means the op-amp is better at rejecting unwanted signals that are common to both input terminals (like noise or interference) and focusing on the desired differential signal.

How It Works:

In a bootstrapped input stage, a capacitor or a combination of components is used to “boost” or stabilize the voltage at the input terminal of the op-amp. This makes the effective impedance looking into the input much higher than it would be in a non-bootstrapped configuration.

In simpler terms, bootstrapping makes the op-amp behave like it has a very high resistance at its input, meaning it won’t draw much current from the signal source and will not affect the signal as much.

This technique is commonly used in high-precision applications where it’s important to minimize interference and signal loss.