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Join Date: Apr 2007
Posts: 24

Variable Gain Amplifier

06/26/2007 7:53 PM

Hi
I have passed electronic 1 and know how BJT & JFET transistors work. plus analyzing small signals and amplifiers. Now I need to design a VGA ( variable gain amplifier) as my project.
First I need to know how they work. I have used Google, but has not find a good definition and tutorial yet.
Could you please help me with some ( preferablly, easy to understand) resources?
Thanks

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Power-User
United States - Member - USA Engineering Fields - Control Engineering - Never enough money

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#1

Re: variable gain amplifier

06/26/2007 8:22 PM

Try looking at "Operational Amplifiers" for inspiration also called op-amp.

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Guru
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#2

Re: variable gain amplifier

06/26/2007 11:25 PM

I have used Google, but has not find a good definition and tutorial yet.

Really?! I typed "variable gain amplifier" and got 250,000+ hits. Wikipedia and Answers.com described them well enough.

Typical variable gain amplifiers change their gain by changing the resistance of the feedback resistor. This can be an ordinary potentionmeter or a light-dependent-resistor (LDR) or even parallel connected resistors that are switched in and out using solid-state switches.

If you're looking for a book, try "IC Op-Amp Cookbook" by Walter Jung. This book has been around for a long time and it explains how Op-Amps work and how to use them in easy and simple to understand terms.

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Active Contributor

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#3

Re: variable gain amplifier

06/27/2007 12:23 AM

Thanks. yes there are google results for VGA but I thought they'll just give an overview, not explaining how the circuit works, like what I experienced about TTL and ECL. because I need to design a 4-bit VGA for my project and need to fully understand how it works

Is there any free e-book?

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Guru
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#5
In reply to #3

Re: variable gain amplifier

06/27/2007 2:54 AM

Going through 250,000+ hits will take a couple of days to go through so you'll have to be creative or get pieces of information from different sources to get the bigger picture.

I Googled, "variable gain amplifier" filetype:pdf. That gave me 28,000 pdf files with VGA in them. Next, I typed "variable gain amplifier" +tutorial. This got me 530 hits.

I typed "variable gain amplifier" +ebook. Now I have just 32 hits. Unfortunately, I don't have time to go through all of them for you. You'll have to do it yourself.

Don't' despair if you can't find one. Data-sheets can be very informative. Most of the time they have application notes which can be very helpful in understanding them. Take a look at this:

http://www.analog.com/UploadedFiles/Data_Sheets/AD603.pdf

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Guru
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#6
In reply to #5

Re: variable gain amplifier

06/27/2007 11:44 PM

I agree that AD603 is the easiest thing to use. Other things he can try are DAC with four quadrant multiplier external reference where he can feed-in the signal and can set gain from DAC digital input. FET as variable resistor is also a very simple gain control device. For cheapest experiment one can change the operating point of the class A transistor amplifier to get gain change for AC signal, but not usable for DC signal.

I think tutorials and application notes from manufacturers are best way for the start up to learn and try. These notes also give you sources of likely problem and some graphs to get feeling and some time designers kits or evaluation kits that you can buy and try.

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Anonymous Poster
#4

Re: variable gain amplifier

06/27/2007 1:28 AM

I know three common ways of achieving variable gain:

1)using an amplifier + an electronically controlled attenuator made of a DC-biased ring diode bridge(such as Mini Circuits)

2)ancient video amplifiers by Motorola(MC1445) provides variable gain of -30dB to +10dB with a bandwidth of #100MHz

3)lower bandwidth (a few megahertz)can be achieved with OTA(operationnal transconductance amplifiers) made by the former RCA,in their op amp brochure.
Many references available,and there should be one at National Semiconductors too.

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Anonymous Poster
#7

Re: Variable Gain Amplifier

06/28/2007 2:41 AM

Variable gain amplifier is a really simple concept. Like your stereo is a variable gain amplifier. It has a "volume" (Gain) control. Any amplifier with a variable voltage divider at the input, or variable feedback, or switchable, or variable emitter resistor in a common emitter class a circuit is a variable gain amp. Almost everyone else is talking about non-linear and or compression or expansion schemes, where gain varies as a function of input amplitude or excursion. Try reading a book on electronics, like Joint Army Navy "Basic Electronics" or Electricity.

This is an example In which searching the web leaves you enormously confused. It is just not a subject that people usually think about in the way you expressed it. I think, that by talking about four bits whoever asked you to do this project would like sixteen switchable steps of gain. Any of the four quadrant multiplier schemes will give you non-linear gain, all the time. A photo resistor or photo transistor used as an emitter resistor in a common emitter class a amp with four LEDs spaced successively further from the photo-resistor so that each next one in the sequence results in half the effect of the one infront of it, and some kind of binary counter firing the LEDs at the user's discretion would seem to me to fullfill the requirement. Somehow this all reminds me that ( in my opinion) our culture has turned us all into specialists who in aggregation really have a lot of difficulty tying our shoe laces!!!!

Geoffrey Reed

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