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msulak
Joined: 22 Jul 2004
Posts: 3
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 22 Jul 2004 7:54 Differential apmplifier |
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Hello,
Doesn't anybody know how to build a differential amplifier with high input impedance and only two OZ ?
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 22 Jul 2004 13:20 Differential apmplifier |
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| "only two OZ" what is OZ ?
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mazelk
Joined: 23 Apr 2004
Posts: 107
Helped: 2
Location: COLOMBIA
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| 22 Jul 2004 20:16 Re: Differential apmplifier |
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Hi...
A differential amplifier, basically, consists in two input stage, that amplifies the difference between those inputs. Actually, differential amplifiers are made by a chain of stages that work amplifiying and controlling signal and bias.
Youmay read these documents for get a general view:
RF Microelectronics Book By Razavi
http://www.21rf.com/8290/rf_microelectronics_razavi.pdf
Analysis and Design of Analog Integrated Circuits
http://www.edaboard.com/ftopic73630.html
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Kevin Weddle
Joined: 10 Jul 2004
Posts: 101
Location: San Antonio, TX
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| 23 Jul 2004 2:23 Re: Differential apmplifier |
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| You want to make a pushpull amplifier with a low current transistor on the input. You can make a pushpull that will respond to the signal when properly biased. All you need is the two pushpull transistors and a bias transistor that will phase split the voltage. It's the low current transistor that makes the inpedance high.
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msulak
Joined: 22 Jul 2004
Posts: 3
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| 23 Jul 2004 6:22 Re: Differential apmplifier |
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Sorry for a typing error, I meaned OA - operational amplifier.
Basically one is enough if high input impedance is not required.
My only idea is to use three op.amplifiers, one for differential function and two for making high impedance.
| djalli wrote: |
| "only two OZ" what is OZ ? |
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mazelk
Joined: 23 Apr 2004
Posts: 107
Helped: 2
Location: COLOMBIA
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| 23 Jul 2004 19:07 Re: Differential apmplifier |
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Hi...
If you can usse three operational amplifier in order to get more input impedance, you may built an instrumentation amplifier. Also, you may get high CMRR.
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dz
Joined: 29 Apr 2001
Posts: 33
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| 25 Jul 2004 0:59 Re: Differential apmplifier |
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Hi msulak,
op amps are differential amps in a way, it is there first stage internally. for higher input impedance you could select a fet input op amp.
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msulak
Joined: 22 Jul 2004
Posts: 3
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| 30 Jul 2004 17:38 Re: Differential apmplifier |
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Hi Avatar,
If I understand op, I must build feedback with resistors to get some concrete amplifaction factor. This feedback circuit will decrease input impedance and FET inputs will not help with this. Without feedback it is more comparator than differential amplifier.
I want to build a simple amplifier for Hall senzor, the smallest possible becouse I want to place it into lpg multiventil.
Martin
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Sinisa
Joined: 03 Mar 2003
Posts: 305
Helped: 62
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| 31 Jul 2004 2:29 Re: Differential apmplifier |
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| Is there reason why you don't just use integrated diff. amp? I doubt you could build better or smaller discrete solution... Plenty of options.
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rauol
Joined: 02 Oct 2002
Posts: 364
Helped: 11
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| 31 Jul 2004 9:10 Re: Differential apmplifier |
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 I had seen an application note from "Linear technology" in EDN which used three opamp and a referance for Hall sensor .i think u can serach there web site.
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mazelk
Joined: 23 Apr 2004
Posts: 107
Helped: 2
Location: COLOMBIA
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| 31 Jul 2004 20:01 Re: Differential apmplifier |
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| msulak wrote: |
Hi Avatar,
If I understand op, I must build feedback with resistors to get some concrete amplifaction factor. This feedback circuit will decrease input impedance and FET inputs will not help with this. Without feedback it is more comparator than differential amplifier.
I want to build a simple amplifier for Hall senzor, the smallest possible becouse I want to place it into lpg multiventil.
Martin |
You can find out some basics in this link:
http://www.micronas.com/products/overview/sensors/index.php
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Kevin Weddle
Joined: 10 Jul 2004
Posts: 101
Location: San Antonio, TX
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| 21 Aug 2004 18:01 Re: Differential apmplifier |
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| The feeback resistors are the trick to the opamp. The design is such that because of the inversion within the opamp, it will tend to work output to inverting input. I would stick to a pushpull that is biased with another transistor, called the phase splitter, and limit the gain with a collector and emitter resistor. I can't show you the circuit, but if you want to design the basic opamp then you will have to do a bit of studying. The pushpull are two NPN's arranged like a totem pole. If you look at the schematic for an opamp, the ouput is buffered and you have a NPN with a PNP arranged like a totem pole. This is just a voltage follower.
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yeechyan
Joined: 14 Jun 2004
Posts: 111
Helped: 1
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| 24 Aug 2004 1:54 Differential apmplifier |
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| Instrumentation amplifier is a good choice since they use non-inverting inputs as the input terminals. Generally the feedback resistors are connected to the inverting inputs. IN such a way you don't have to worry about the impedance caused by the feedback resistors since you're using non-inverting inputs in instrumentation amplifier.
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savadi
Joined: 14 Sep 2004
Posts: 27
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| 23 Sep 2004 23:54 Re: Differential apmplifier |
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| You must use MOS amplifiers to get high input resistance. for example you can use telescopy, folded cascode amplifier with the second stage like push pull stage to get high gain. you can study the Analog integration circuits written by Razavi, Ken martin, temes.
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