# How to amplify from a positive ground?

Discussion in 'General Electronics Chat' started by brucester, Jan 20, 2013.

1. ### brucester Thread Starter New Member

Dec 24, 2012
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0
Hi,

I have a simple circuit set up that generates a clippped voltages +/- 0.8 volts using 2 diodes on a positive 9 volt signal.

For example, 9 volts is ground, signal varies between 9.8 and 8.2 volts.

I want to amplify the millivolt part. If ground was zero i could just use a standard op amp with 100 gain and end up with a signal +/-9 volts.

But if i do it here, it multiplies the original 9 volts, plus the extra. So 9.1 volts become 910 volts.

Is there a way to do this?

Thanks

2. ### bertus Administrator

Apr 5, 2008
18,500
3,567
Hello,

You could use a summing amplifier.
Put the signal to be measured on one input and a fixed reference signal on the other input.
If the measurement signal is + 9 Volts and the reference is - 9 Volts (and the gain of both inputs is the same) the output will be 0 Volts.

Bertus

3. ### MrChips Moderator

Oct 2, 2009
16,887
5,200
Is the millivolt part a changing signal at some reasonable frequency, say greater that 10Hz?
Then you can AC couple the signal to your amplifier.

4. ### brucester Thread Starter New Member

Dec 24, 2012
19
0
Basically, i am trying to achieve the N-Mosfet version of this setup:

Circuit

It works great here when using a P mosfet because the load is under the fet and runs to ground.

But when i do it with an N mosfet, the load is above and ground is positive.

This circuit amplifies the last 800 millivolts of an inductors flyback.

I want to do the exact same thing but with an n mosfet.

5. ### bertus Administrator

Apr 5, 2008
18,500
3,567
Hello,

Can you post the schematic in a graphics format like .png?
The falstad simulator will hang my browser.

Bertus

Dec 24, 2012
19
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7. ### GopherT AAC Fanatic!

Nov 23, 2012
8,025
6,785
What is the goal of your circuit?

What op amp are you using?

I am not sure you (want to / need to) tie your inputs to the + and - rails.

Capacitor coupling your signal (AC) can be used to remove a 9 volt DC offset. Capacitor value will be determined by the AC frequency of your signal (if it is AC).
Or,
Like Bertus said, you can adjust the dc level going into your non-inverting input to null the 9vdc offset.

8. ### brucester Thread Starter New Member

Dec 24, 2012
19
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It pulses a coil and amplifies the final stages of the flyback. Beginning sage of a pulse induction metal detector.

I'll read up on adjusting dc levels.

Dec 24, 2012
19
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10. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,432
1,312
Since this an AC signal you just want to amplify (and not measure) it is standard practice to "capacitor couple" it to the amplifier. ie; all you need is a capactor to run the signal through. That allows the AC voltage signal through but will ignore the DC level.

Dec 24, 2012
19
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12. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
727
Do you have an image of the schematic? It's not loading for me.

Dec 24, 2012
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14. ### brucester Thread Starter New Member

Dec 24, 2012
19
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I get how the cap only lets AC through, but my signal is the voltage, so 9+ volts still happens so i'm not sure if this is what i need.