Frequency compensation

With frequency compensation is defined as the internal or external circuit of an electrical amplifier, such as an operational amplifier, on the one hand to ensure a stable operation without oscillations and on the other hand, to reduce overshoot, when it is operated with negative feedback.

Motivation

Amplifiers such as operational amplifiers are comprised of several internal amplifier stages, which in turn are composed of transistors and resistors. Signals pass through transistors but not as rapidly as desired, but the (enhanced ) output voltage with a short delay of a few nanoseconds follows the input voltage. OPs also have internal capacity, which only need to be charged and discharged and can delay the phase by up to 90 °. Both effects together cause a phase shift that is becoming more serious with increasing signal frequency and number of stages and at high frequencies ( in the picture at 7 MHz ) and many amplifier stages which can exceed 180 ° brand significantly.

In practical use, an amplifier is made of the use of negative feedback, also referred to as negative feedback. For example, a voltage divider of resistors from the output back to the inverting input.

At high frequencies, but the intended effect is reversed: the recycled output voltage occurs due to the internal phase difference of the operating too late, from the negative feedback, a feed forward (positive feedback ), and the circuit is now an important characteristic of an oscillator.

Oscillation condition

An amplifier can be changed by feedback in an oscillator. Harry Nyquist was around 1930, the conditions under which a permanent oscillation occurs in the output signal. These are known as Nyquist criterion:

Can not change the increasing phase shift with increasing signal frequency, unless you reduce the number of amplifier stages internal to one. So you can build but no amplifier. Thus, one must reduce the loop gain to less than 1 before the phase-shift reaches the critical value.

Compensation

The usual measure is a small capacitor of a few picofarads between the collector and base of a transistor in a common emitter circuit, which reduces the Miller effect because of the gain with increasing frequency.

• This capacitor can be either built into the operating room, then he is like the OP27 fully compensated. That is, it can be used in circuits which have a gain of 1, but has a lower gain-bandwidth product, in this case 8 MHz.
• Or this capacitor must be externally supplemented, then he is like the OP37 partially compensated. It is stable in the circuits having an overall gain of at least 10, but has a higher gain-bandwidth product, in this case 63 MHz.

A ring oscillator is basically a negative feedback 3 - or 5 -stage amplifier without frequency compensation in order to operate as an oscillator. From the generated frequency, one can calculate the cutoff frequency of the individual stages, where the any possible transit times with respect to the phase shifts are not dominant.