# The use of amplifier stages

1. Jul 3, 2016

### Harrison G

I don't understand the need of amplifier stages if i want for exmple to amplify a small AC signal. Sure i get that it is used to improve the gain, but isnt an RC transistor amplifier made in such a way, that the collector voltage is equal to half Vcc in order to have maximum swing. If so, why do we need more than one amplifiyng stage? To get bigger current? Here's what i know about that: When im building a light sensor for example, i form a voltage divider with a photoresistor and another resistor to ground. The output is connected to the base. I bias it in such a way that the base current is 1/4th of the whole current through the photoresistor and sometimes this doesn't give large enough collector current. This is when i use something like amplifiyng stage. I either use another transistor to increase the gain by connecting them like darlington transistors, or i use the main weak signal to turn off a transistor, which in turn will allow for another transistor to turn on, giving me high enough output. And this is all i use amplifier stages for-to increase a weak current. Is there anything else i dont get here?

2. Jul 3, 2016

### Svein

If you want the amplified signal to have little distortion, you need to linearize the stages - and that linearizing uses negative feedback, removing most of the gain. If you do not care about the relation between the input and output signal, an operational amplifier will usually give you a gain of 10 000 - which means that the output will spend almost all the time close to one of the supply voltages.

3. Jul 3, 2016

### Averagesupernova

Too much gain in a single stage increases the chances of oscillation also.

4. Jul 4, 2016

### davenn

yes, exactly, one of the major reason amplification is done in stages. Keeps everything stable

D

5. Jul 5, 2016

### Baluncore

Devices that amplify are usually specified by a “Gain Bandwidth Product”, GBP = Gain * BW.
If you want a gain of 10 and a bandwidth of 100kHz you will need one device with a minimum GBP of 1MHz.
If you want a gain of 100 and a bandwidth of 100kHz you will need one device with a minimum GBP of 10MHz.
Alternatively, it may be cheaper or more reliable to use two stages, each with a gain of 10 and each with a GBP of 1MHz.

6. Jul 5, 2016

### LvW

Mmmmh...It is a common misconception to believe that reducing the open-loop gain of an amplifier using negative feedback would stabilize such a stage.
Exactly the opposite is true. The stability margin will be reduced correspondingly.
On the other hand, negative feedback enhances bandwidth and dynamic range and reduces harmonic distortions.
Hence, again we have to find a trade-off between some conflicting requirements.
This is normal for electronic design - each good design is a trade-off between several requirements.
Each advantage/improvement must always be paid for by some degradations of other parameters.

Last edited: Jul 5, 2016
7. Jul 5, 2016

### Averagesupernova

There is certainly more to the story than this. There are certainly cases where a lot of gain in a small physical space can cause oscillation. Isolation of several stages can help prevent this. No one here is saying that the only reason for multiple stages is for reduced chances of oscillation.

Last edited by a moderator: Jul 5, 2016
8. Jul 5, 2016

### LvW

I am sure that you will agree with me that oscillations can occur only in the presence of feedback.
A gain stage - even with very large gain like an opamp - will tend to instability only if there is feedback - and it does not matter if the feedback loop is designed or if it is an unwanted (parasitic) effects.

Let me explain a little the background of my comment in post#6:
I have several years experience with teaching analog electronics to students. And very often I have seen that there is a lot of confusion connected with the terms "stability" and "feedback". Why?
Because negative feedback
* IMPROVES DC stability (of the operating point against temperature effects and other uncertainties), and at the same time
* DEGRADES dynamic stability (against oscillations).

At least in one of the answers given in this thread the therms "opamp" and "gain reduction by feedback" are mentioned - and it was my only intention to make clear (and the OP seems to be a beginner) that negative feedback cannot improve dynamic stability at all.
However, negative feedback has many, many advantages - and therefore, as I have tried to explain, a trade-off is always necessary to find the best solution for a specific task (DC stability, AC stability, dynamic range, bandwidth, THD, cost, number of stages, ...).

9. Jul 5, 2016

### davenn

never mentioned any thing about feedback ... just gain
Its standard practice, particularly in RF gear to do amplification in stages for the reasons mention

I stand by my and ASN's comments

10. Jul 5, 2016

### LvW

davenn - did I argue against several stages? In no way! Please read my explanation in post#8.
Primarily, it was my intention to shown that and why a trade-off between conflicting requirements is always necessary.
Perhaps I was not able to express myself very clearly - due to my limited knowledge of your language.

11. Jul 5, 2016

### milesyoung

Can you show the math you're basing this on along with your definitions of 'DC stability' and 'dynamic stability'?

12. Jul 5, 2016

### LvW

I am not sure what you are asking for.
* Do you ask me to proove why negative DC feedback defines and stabilizes the DC operating point of an amplifier?
* Do you ask me to demonstrate in this thread the relationship between the phase margin and the feedback factor ?

13. Jul 5, 2016

### milesyoung

I can see from this what you'd like to show, but I don't think that'll address what you seem to be stating here in general.

Let me ask you this:
If I can show you a system which, in open loop, has a highly oscillatory response to a step input, do you mean to tell me that it's impossible to improve upon its transient response using negative feedback?

14. Jul 5, 2016

### davenn

so why quote me then ????

this wasn't arguing against me ?

if you are not arguing against what I said ... then you shouldn't have quoted me !

15. Jul 5, 2016

### LvW

In case the system has a "highly oscillatory response" it has internal feeedback loops. Thats all.
More than that, I dont think that this case is relevant for our discussion here.

16. Jul 5, 2016

### LvW

davenn - with all respect, dont overrate my answers.
As I have stated already - perhaps I have chosen not the correct words or terms.
Didn`t I try to explain my motivation?
Perhaps I was wrong to quote you in the context of my answer - sorry for that. OK?

17. Jul 5, 2016

### milesyoung

Then please define 'dynamic stability' and show, mathematically, how negative feedback degrades it.

18. Jul 5, 2016

### Averagesupernova

It seems so often negative feedback gets a bad rap because while the intention may be to provide negative feedback, that feedback unintentionally becomes positive feedback at some frequency or frequencies.
LvW, you say here:
You want to be careful with your words here. It looks like you are saying: If I lay out an op-amp on a breadboard and due to sloppy construction practices there was enough parasitic capacitance and no negative feedback that the thing would oscillate and applying negative feedback would not solve this oscillation. It is always assumed that there is always SOME positive feedback that is unavoidable due to parasitic affects. One of the reasons we apply negative feedback and scatter the gain out in stages is so the overall positive feedback is never large enough to start and sustain an oscillation. Try telling a control systems engineer that negative feedback in a PID loop will improve chances of oscillation.

19. Jul 5, 2016

### Svein

Sort of right: Check out http://cds.linear.com/docs/en/application-note/an148fa.pdf.

20. Jul 6, 2016

### LvW

Perhaps you have overlooked that in post#8 I have mentioned "dynamic stability" together with "against oscillation"; more than that, in post#12 I gave an answer using the term "phase margin". I suppose, this should be sufficient to explain what I mean with "dynamic stability" in contrast to "DC stability".

Here is a book chapter (colorado university), which explains to you the term in detail: