Compensation Methods Electronic Engineering

The following presentation is a part of the level 5 module -- Electronic Engineering. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1 st year undergraduate programme.Engineering (foundation degree, BEng and HN) The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments. Contents Instructions Gain Curve Compensation Methods Dominant Pole Compensation Frequency Compensation Lead Lag Compensation. Summary Credits In addition to the resource below, there are supporting documents which should be used in combination with this resource. Please see: Clayton G, 2000, Operational Amplifiers 4th Ed, Newnes James M, 2004, Higher Electronics, Newnes Compensation Methods

For our amplifier we are going to apply negative feedback to reduce the gain to 55dB. Comment on the amplifiers stability. Compensation Methods

Gain Curve f C1 f C2 Phase Curve -40dB/dec

For our amplifier we are going to apply negative feedback to reduce the gain to 55dB. Comment on the amplifiers stability. Unstable as it crosses the Gain Curve on the -40dB/dec line. The amplifier is therefore unsuitable for this application. Can we do anything about it? Compensation Methods

There are three methods which we will examine. 1. Dominant Pole compensation 2. Frequency compensation 3. Lead Lag compensation. Compensation Methods

Dominant Pole Compensation In this method the Manufacturer introduces an artificial break frequency (pole) which causes the gain to drop to 0dB before the first natural one occurs. f C1 f C2 Unstable Stable New f C Original Curve New Curve gain frequency Desired gain

Try this out on our plot and answer the following questions: At what frequency does the dominant pole need to be placed? What is the Bandwidth of the compensated amplifier? Compensation Methods

Gain Curve f C1 f C2 Phase Curve x 9.5dB – 5.1kHz 29.5dB – 510Hz 89.5dB – 0.51Hz 15dB – 2.8kHz 35dB – 280Hz 55dB – 28Hz

Try this out on our plot and answer the following questions: At what frequency does the dominant pole need to be placed? What is the Bandwidth of the compensated amplifier? 0.51 Hz 28 Hz Compensation Methods

Notes This method is stable for all applications. The user does not need to carry out the compensation exercise. Bandwidths are limited in size using this method. Compensation Methods

Frequency Compensation This method is similar to the first in that a dominant pole is introduced. This time its position is selected by the user and is positioned so that the gain drops to the desired gain at the point the first natural break frequency occurs. f C1 f C2 Unstable Stable New f C Original Curve New Curve gain frequency Desired gain Compensation Methods

A table supplied by the amplifier manufacturer allows the user to convert the new pole frequency measured from the plot into a capacitor value which is connected between two pins on the amplifier. Compensation Methods

Try this out on our plot and answer the following questions: At what frequency does the dominant pole need to be placed? What is the Bandwidth of the compensated amplifier? Compensation Methods

Gain Curve f C1 f C2 Phase Curve x 69.5dB – 3kHz 89.5dB – 300Hz f C1 – 15kHz

Try this out on our plot and answer the following questions: At what frequency does the dominant pole need to be placed? What is the Bandwidth of the compensated amplifier? 300 Hz 15 kHz (f C1 ) Compensation Methods

Notes The user must compensate each amplifier according to its use. The Bandwidth, using this method is always at the first break frequency. Once compensated if the gain is reduced the amplifier will become unstable. Compensation Methods

Lead Lag Compensation. Let us examine the problem we have with our amplifier. f C1 f C2 Unstable Original Curve gain frequency Desired gain The amplifier would be stable if either f C1 occurred at a lower frequency or f C2 occurred at a higher frequency Compensation Methods

We cannot move the two break frequencies as they are inherent parameters of the amplifier. BUT We can make f C1 “look” as though it started at a lower frequency using the circuit below. R1 R2 C V IN V OUT Compensation Methods

Series combination of R2 and C Now we can generate an equation for V OUT in terms of V IN Compensation Methods

Quantative Analysis At low frequencies ω parts << 1 so the gain = 1 phase will be 0° At high frequencies ω parts >> 1 so the gain = R2/(R1 + R2) phase will be 0° In between gain must roll off in some way. e.g. R1 = 10k , R2 = 1k , C = 10nF Plot over the range 100 Hz to 100 kHz

FrequencyGainGain dBPhase

Compensation Methods

Referring to our equation for the network, we will have two break frequencies, one for the top line and one for the bottom. Break frequency where roll off begins f 1 is given by: Break frequency where roll off stops f 2 is given by: Gain after roll-off is given by Compensation Methods

x x f1f1 f2f2 -20dB/dec Compensation Methods

Note The phase is not a concern as it returns to a low value by the time the gain curve stops reducing. Let us return to our amplifier. Compensation Methods

f C1 f C2 Unstable Original Curve gain frequency Desired gain Introduce a lead lag network which starts before f C1 and stops at f C1 f1f1 f2f2 Stable f C1 now looks as though it starts at f 1 Compensation Methods

Try this out on our plot and answer the following questions: What component values do we need – assume C has a value of 100pF What is the Bandwidth of the compensated amplifier? Compensation Methods

Firstly f 2 must equal f C1 Secondly what drop in gain is required Compensation Methods

Gain Curve f C1 f C2 Phase Curve Required drop in gain = -10dB

Firstly f 2 must equal f C1 Secondly what drop in gain is required Finally, what is f 1 Compensation Methods

Gain Curve f C1 f C2 Phase Curve x x f1f1 f2f2 Bandwidth f C2

Try this out on our plot and answer the following questions: What component values do we need – assume C has a value of 100pF What is the Bandwidth of the compensated amplifier? 229k  106k  100pF V IN V OUT f C2 = 300kHz Compensation Methods

Notes The user must compensate each amplifier according to its use. The Bandwidth, using this method is always at the second break frequency. Once compensated if the gain is reduced the amplifier will become unstable. Compensation Methods

Summary No compensation – Unstable Dominant Pole Compensation – B.W. = 28Hz Frequency Compensation – B.W. = 15kHz Lead Lag Compensation - B.W. = 300kHz Compensation Methods