# Operational Amplifier Voltages

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#1
My work is paying for me to do an HNC, and I'm really wanting to give up because it's making me feel completely unintelligent. Now I'm stuck between a rock and a hard place because if I give up my employer will think I'm a moron and I'm starting to fully accept I may well be a moron.

I've come to electronics, and I just don't understand it. I've read over and over about op amps and how to calculate voltages but I just don't understand.

In particular I've been asked to calculate i/p or o/p voltages in a simple op amp circuit with the only clue being a ratio of resistances - no resistance values are shown just a doubling of another unknown resistance.

How the hell can you calculate with just that?

I'm despising this distance learning course.
0
3 years ago
#2
(Original post by wtaf)
My work is paying for me to do an HNC, and I'm really wanting to give up because it's making me feel completely unintelligent. Now I'm stuck between a rock and a hard place because if I give up my employer will think I'm a moron and I'm starting to fully accept I may well be a moron.

I've come to electronics, and I just don't understand it. I've read over and over about op amps and how to calculate voltages but I just don't understand.

In particular I've been asked to calculate i/p or o/p voltages in a simple op amp circuit with the only clue being a ratio of resistances - no resistance values are shown just a doubling of another unknown resistance.

How the hell can you calculate with just that?

I'm despising this distance learning course.
Can I ask what the HNC course title is? i.e. is it an electronics or electrical or mechanical course etc? Stick with it and throw yourself into learning. If you were lucky enough to get the position and the company is paying you to gain valuable work experience plus learning lifelong transferable skills, that is something which is way too valuable to throw away at the first few hurdles. People will cut off their left arm for that kind of opportunity.

Op Amps are useful because they are easy to use without needing to understand the complex circuits inside them.

In the simplest configuration using two resistors, they provide voltage gain where the output is the same polarity (phase for ac signals) as the input (non-inverting), or inverting, where the output is the opposite polarity (180o phase reversal for ac signals) as the input.

The two most basic configurations are:

It's critical to notice the input signs on the device itself:

+ = non-inverting input
- = inverting input

and how the resistors are configured.

Then it's simply a case of applying the formula.

You can see that the gain of the circuit is only dependent on the ratio of resistances.

For instance a 10x gain inverting amplifier can be constructed with any two resistances as long as the have a ratio of 10:1. i.e. 10K Ohms and 1K Ohms or 470 Ohms and 47 Ohms. In both of the two examples, an input of 1V will produce an output of -10V.

In practical use, the actual maximum output is limited by the device itself together with the power supply voltages.

Going further, the choice of input resistor for the inverting circuit, determines the load applied to the voltage source. i.e. choosing a 22K Ohms resistor for Rin will load the source with 22K Ohms.

A third very useful circuit used extensively, is a unity gain buffer.

This boosts the current available to a load connected to the output, whilst not loading the input and provides a gain of x1. The output is a replica of the input.

For instance a -3.2V input signal will produce an output of -3.2V. etc.

Further reading and very useful general electronics sites:

http://www.electronics-tutorials.ws/opamp/opamp_8.html
1
3 years ago
#3
Yeah - by the sound of it you just need to work out whether it's an inverting or non inverting configuration and just apply the gain calculation for that type.

fwiw I like this explanation - if you've got 50 minutes to spare watching a video (guy repeats himself quite a bit to make sure everyone gets it - it's not 50 mins full throttle)

1
#4
Thanks for the support both of you, I think I'm looking too deep into op amps and I'm trying to understand what is happening inside too much instead of just applying formula and understanding the rules.

It's a very abstract subject, and I've always found it easy learning with more tangible subjects.

I was also being a bit stupid I think, the resistance values are R1 = R, R2 = R/3 etc. All that was needed was to make R1 = 1 and R2 = 1/3.

I'm doing an electrical HNC because my work wants to promote me to an engineer, and have been a spark for many years, but I've not studied for 15 years and to be honest this is like torture. I've lost all of my study know how.

Again thanks for the replies, distance learning is far more difficult than what I thought it would be - there's no lecturer to get hold of at short notice.
1
3 years ago
#5
(Original post by wtaf)
Thanks for the support both of you, I think I'm looking too deep into op amps and I'm trying to understand what is happening inside too much instead of just applying formula and understanding the rules.

It's a very abstract subject, and I've always found it easy learning with more tangible subjects.

I was also being a bit stupid I think, the resistance values are R1 = R, R2 = R/3 etc. All that was needed was to make R1 = 1 and R2 = 1/3.

I'm doing an electrical HNC because my work wants to promote me to an engineer, and have been a spark for many years, but I've not studied for 15 years and to be honest this is like torture. I've lost all of my study know how.

Again thanks for the replies, distance learning is far more difficult than what I thought it would be - there's no lecturer to get hold of at short notice.
Yeah there are some initially mindbending things about opamps... like the virtual earth... but if you take it slow you'll get it down
0
3 years ago
#6
(Original post by wtaf)
Thanks for the support both of you, I think I'm looking too deep into op amps and I'm trying to understand what is happening inside too much instead of just applying formula and understanding the rules.

It's a very abstract subject, and I've always found it easy learning with more tangible subjects.

I was also being a bit stupid I think, the resistance values are R1 = R, R2 = R/3 etc. All that was needed was to make R1 = 1 and R2 = 1/3.

I'm doing an electrical HNC because my work wants to promote me to an engineer, and have been a spark for many years, but I've not studied for 15 years and to be honest this is like torture. I've lost all of my study know how.

Again thanks for the replies, distance learning is far more difficult than what I thought it would be - there's no lecturer to get hold of at short notice.
No worries.

Like everything else in learning, take it one step at a time and master the basics thoroughly.

If you need help, you only need post here on the Physics forum.

FWIW, I went up the same route as yourself starting out as a spark's apprentice (self employed) many years ago, then landed a job in a drawing office in a medium sized local company (P.C.B. design) and did a day release ONC/HNC in electronics before doing my degree in electronics. Ten years after my degree, I did a masters in Aerospace Systems Engineering. The sky literally is not the limit !

Yup, it's hard to get going at first, but you get used to it and with time, you get to know how to approach a new topic. Every little helps as a certain supermarket likes to advertise.

To be honest, there is now so much information, help and resources out there with great websites like TSR, I somewhat envy your opportunity and journey looking back and knowing the only help we then had (late 70's to early 80's) was the one lecturer for each subject (impossible to get hold of outside lessons) and our one course book. Digital watches and four function calculators were considered neat inventions!

Welcome to TSR !
0
#7

Could someone help me understand the following circuit? I know it's a comparator and I can work out the upper and lower threshold voltages without the lovely little 3V power source they've thrown in there;

(Assume the maximum output voltage swing of the op-amp is ± 3 V for when the supply is ±5 V)

1000/(1000+1000) x 3 = 1.5V Upper

-(1000/(1000+1000) x 3) = -1.5V Lower
0
2 years ago
#8
(Original post by wtaf)

Could someone help me understand the following circuit? I know it's a comparator and I can work out the upper and lower threshold voltages without the lovely little 3V power source they've thrown in there;

(Assume the maximum output voltage swing of the op-amp is ± 3 V for when the supply is ±5 V)

1000/(1000+1000) x 3 = 1.5V Upper

-(1000/(1000+1000) x 3) = -1.5V Lower
Right...
Since it's operating with no -ve feedback the output will be getting driven to either of the supply rail levels (or as close as the op amp is capable - which is +3V or -3V)

Due to the potential divider R1&R2 the voltage on the non inverting input is halfway between the output and 3V...

so when the output is at +3V the non inverting input is at 3V (halfway between 3V and 3V) (CASE 1)

when the output is at -3V the non inverting input is at 0V (halfway between -3V and +3V) (CASE 2)

---
if CASE 1 (output = +3V, non inv IP = 3V)
if the inverting input is >3V... the output is driven to -3V
if the inverting input is <3V... the output is driven to +3V

if CASE 2 (output = -3V, non inv IP = 0V)
if the inverting input is >0V... the output is driven to -3V
if the inverting input is <0V... the output is driven to +3V
--
obviously when the inverting i/p is >3V it is also >0V
and likewise when the inverting i/p is <0V it is also <3V

so inv i/p >3V causes the output to saturate to -3V
and inv i/p <0 causes the output to saturate to +3V

hope that helped...
1
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