# Single Supply inverting Op Amp ?Problem?

Discussion in 'General Electronics Chat' started by kaning, Aug 25, 2014.

1. ### kaning Thread Starter Member

Jun 2, 2014
43
0
Hello guys, I've been reading about op amps. To start i want to invert a dc signal from 5v to -5v or close to that with single supply. (i want to drive P channel mosfet).

Im using TL071 op amp, i saw some videos and tl071 datasheet and i designed this circuit for my purpose.

From everything i've read, this is a single supply inverting circuit with a 2,5v Vref on + input and a gain of 2, right?. So, i should be getting - signal on the Vout with this, but im not! no matter what Vin i use, i keep getting 2,49v wich is understandable if i use a Vin of 1,25v or greater but even with less voltage values this is happening. why?

with this circuit i know i cant get -5v, anyway my main goal here is why im not getting -Vout..

what am i missing?? how i can get -Vout ? many thanks in advance

2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
To get Pin6 to go below 0V (gnd), you would have to power pin 4 with a negative supply voltage, like -10V.

Note that unless an opamp is advertised by the manufacturer as being rail-to-rail output, the output pin (6, in this case) cannot pull higher than (+supply-~1.5V) or 3.5V if +supply is +5V, and pull lower than (-supply+~1V), or 1V if your -supply pin is grounded.

Best advice: Get and read the datasheet for the specific opamp you are thinking about using for a project. Read all of parameters on the data sheet, and get to point where you understand what they mean...

Last edited: Aug 25, 2014
3. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,327
593
hi kanning,
An inverting OPA means it inverts the 'sense' of the input signal, not the polarity.

eg:
Hi in , Low out
Lo in Hi out
E

4. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,907
1,789
An op amp does not make up voltages, in a real sense it redirected the voltage from the power pins to the output.

So to get -5V out you need to put -5V in. Actually a little bit more.

What you really want to use here is a "Switched-Capacitor Voltage Inverter" such as the LT1054

Last edited: Aug 25, 2014
5. ### #12 Expert

Nov 30, 2010
17,899
9,318
The TL-071 is not built for 5 volt supplies. It's built for audio frequencies at +/- 12 to 15 volts.
Wrong chip, or wrong power supply.

6. ### kaning Thread Starter Member

Jun 2, 2014
43
0
all the purpose of this is to drive P channel mosfet on my portable project powered by a lipo battery... i will search about that "Switched-Capacitor Voltage Inverter".

All i want is a at -4v signal to drivethe mosfet. many thanks for the replies

7. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
What does the control signal come from?
Do you have a u-controller?
What supply voltages do you have?
Tell us more about what the PFET is supposed to switch?

8. ### kaning Thread Starter Member

Jun 2, 2014
43
0
Since my goal is a portable device, i dont have negative supply voltage. thats exactly my goal, if i would have negative voltage supply i wouldnt need this circuit :b So, doesnt virtual ground solve this negative supply issue?

I tried this time with 12v Vcc and on +Input 6v. So its the same as Vcc+=6v and Vcc-=-6v and + Input=0v, right?

As i said i tried now with 12Vcc and 6v +Input but anyway no matter what Vin (i tried up to 1,6v) and i keep getting always 6v output :/

9. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
Forget the opamp approach. Answer my questions independent of your proposed solution...

10. ### kaning Thread Starter Member

Jun 2, 2014
43
0
sorry i said signal and i shouldnt, i want to use the voltage from the portable device power supply (lipo battery) to drive the P Mosfet.

on this project only one (lipo battery)

PFET will act as a normal switch. probably i will not use as this neither its important anymore. by now i only want to get over this problem xb and learn with it.

edited: solar panel will directly charge phone; battery will directly charge phone; solar panel/USB will charge battery. So this PFET is a security precaution for when battery is charging phone, the battery charger IC will not get current. maybe a schematic would be better?

Last edited: Aug 25, 2014
11. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
Ok, I give up.

Jun 2, 2014
43
0

13. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
One schematic is worth 1000s of words...

14. ### kaning Thread Starter Member

Jun 2, 2014
43
0
http://imgur.com/S8KGSIr

this is a diagram i made while i was thinking about it. so the main goal is when a positive voltage is detected on PFET gate, it means phone is charging so no voltage will be going to MCP73831 (battery charger IC)

15. ### wmodavis Well-Known Member

Oct 23, 2010
739
150
Now if you'd have started with that last post and explained what you were trying to do the attempts to help might have progressed much better.

So much less confusion and guessing.

16. ### kaning Thread Starter Member

Jun 2, 2014
43
0
what im trying to do doesnt matter anyway because the point of this thread was only how to make the op amp circuit work, whatever the purpose is. On the first post i just mentioned the PFET and i didnt mention it as a pirority of this thread. i see no confusion on what my goal is on this thread

17. ### wmodavis Well-Known Member

Oct 23, 2010
739
150
I understand your position but how many responses did it take before you got a useful answer?
Clear and complete communication is a key factor especially in a technical field.

If people have to guess what you really want or ask many questions for clarification before understanding, what does that suggest about the communication process?

18. ### BobTPH Active Member

Jun 5, 2013
906
161
Do you think a negative voltage is needed on the gate of a P-channel MOSFET?

It is not.

The voltage must only be more negative than the voltage on the source.

So if your P-channel MOSET has the source connected to, say 5V, and you put 0V on the gate you can turn the MOSFET on. However, you will need a "logic-level" MOSFET that turns on with whatever battery voltage you are using.

Bob

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19. ### kaning Thread Starter Member

Jun 2, 2014
43
0
i know that, but isnt the very first post clear enough? :/ nevermind. anyway i understand your point

20. ### #12 Expert

Nov 30, 2010
17,899
9,318
Maybe you broke the chip? Still, you need a different chip.
Here's a nice one. The important words being, "R-R I-O" Rail to Rail input and output.
No promise that it's the best chip. It's just an example of one that will work.

• ###### sbos703a-346551.pdf
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2.1 MB
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Last edited: Aug 25, 2014
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