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Author Topic: 741 with 50 ohms input  (Read 1638 times)
9H1FQ
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Posts: 192




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« on: July 11, 2010, 02:38:16 AM »

Hi all,

How can you make a 741 type op amp with a 50 ohms input z ?
Any interesting websites or ideas please
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An unknown gem of the Mediterranean sea is the Island of Malta. blessed with warm sun , all year round.
In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ
K2DC
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« Reply #1 on: July 11, 2010, 03:31:43 AM »

I found a page on op-amp basics that includes a section on input and output impedances.  take a look at http://zebu.uoregon.edu/~rayfrey/431/notes9.pdf

GL & 73,

Don, K2DC

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AA4PB
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« Reply #2 on: July 11, 2010, 05:10:02 AM »

Put a 50 Ohm resistor across the input. The input impedance of the actual 741 is quite high so the resistor will set the circuit input impedance.
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Bob  AA4PB
Garrisonville, VA
G3RZP
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« Reply #3 on: July 11, 2010, 07:31:14 AM »

use a series 50 ohm resistor to the inverting input. Choose a feedback resistor to give the desired gain and ground the non-inverting input.

Suggest you read some basic books on op amps: at one time - about 40 years ago! - the National Semiconductor Applications book had a whole chapter on various circuits for doing things with op amps, including gyrators, log amps and so on.
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9H1FQ
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« Reply #4 on: July 11, 2010, 09:55:25 AM »

Hi Guys

Thank you all for your help. Much appreciated

Paul
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An unknown gem of the Mediterranean sea is the Island of Malta. blessed with warm sun , all year round.
In just one day, you can see places which will make you travel in history from the Prehistoric temples throu the middle ages, the Knights of Malta, all the way to world war two. All the major civ
W3JJH
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« Reply #5 on: July 11, 2010, 03:59:05 PM »

The 741 can only drive loads of about 2 k or greater, and the feedback resistor takes part of the output current so it's part of the load.  If you use a 50 ohm resistor at the non inverting input and the gain is too low, you will see clipping in the output signal.
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TANAKASAN
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« Reply #6 on: July 13, 2010, 05:11:33 AM »

You really should use a better opamp than the 741, this is a design way past its end of life. If you don't mind surface mount try using a SN10501 device which can work well past 50 MHz.

Tanakasan
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