# inverting amp question

#### smoking 555

Joined Sep 21, 2008
25
hello how can i use a 7.2 battery pack to get a tl071 op amp to invert the voltage? it seems like a stupid question but i dont understand how to get the negative voltage. please help!!!!!

#### beenthere

Joined Apr 20, 2004
15,819
If the op amp has only a single supply, then it can only output positive voltages. To invert a signal, the op amp has to be given an artificial ground that is halfway between Vcc and 0 volts. The signal is only relatively negative, though.

It is common to use a voltage divider to make the offset voltage. It is applied to the non-inverting input of an op amp. That op amp's output sources the reference level for all the other op amps in the circuit. Signals may need to be coupled in and out through capacitors to remove the DC bias.

#### smoking 555

Joined Sep 21, 2008
25
so this could be done with two resistors i think and grounding the positive?

#### scubasteve_911

Joined Dec 27, 2007
1,203
I think that he wants just a negative voltage from the battery pack, rather than trying to make an inverting amplifier with a VCC/2 virtual ground.

To use a battery to make a negative voltage, call the positive terminal ground, then the negative terminal will be -Vbat.

Steve

#### SgtWookie

Joined Jul 17, 2007
22,210
If the current requirements are very low, the OP might use an Intersil ICL7660S or Maxim MAX1044 DC-DC converter IC to generate a negative supply from the positive supply.

The easiest solution would be to use two 7.2v battery packs; one pack's + terminal connected to the other's - terminal to create a virtual ground.

#### Wendy

Joined Mar 24, 2008
22,155
The easiest solution would be to use two 7.2v battery packs; one pack's + terminal connected to the other's - terminal to create a virtual ground.
If you use 2 battery packs it isn't virtual anymore, it is a dual power supply, which is what op amps like best. It's all in what you call ground.

I think we'll need to find out what the original poster (OP) wants.

#### smoking 555

Joined Sep 21, 2008
25
its a tl071 op amp and i need it to send a signal to a 555 for a gas engine sound project i am working on. i need the signal to be negative going into pin 5 of a 555 ic. but i think the split voltage trick with the 2 resistors is going to do the trick.

#### SgtWookie

Joined Jul 17, 2007
22,210
OK.
Keep in mind that the virtual ground using two resistors won't be very stable nor efficient.

Power op amps like Fairchild's L272/L272A are dirt cheap (under \$0.75/ea), and will provide a relatively stable virtual ground. This will help a lot to extend the battery life; they won't be using any more power than they have to in order to keep the virtual ground stable and centered. The KA334 is in "lifetime buy" phase, so I won't recommend it.

#### smoking 555

Joined Sep 21, 2008
25
would i wire the L272 the same way as tl071? meaning that in the L272 i could invert it to a negative signal by doing the resistor voltage split on the negative side? or just hook my inputs to the negative side?? also this is a stupid question but for my input coming in it will be tapped off of a dc motor with + and - wire leads going through a bridge rectifier. the input into the amp would be the positive side of the motor leads and the negative sides to the ground??? i'm really confused with the l272??

#### SgtWookie

Joined Jul 17, 2007
22,210
An L272 is a power operational amplifier. It can output up to 0.7A, which is quite a lot. Most opamps are in the 10mA to 40mA range.

Now I am not sure what you are really trying to do.

You have a battery that's 7.2v, and you wanted to get a positive signal input converted to a negative signal output.

Are you attempting to use a DC motor as a generator?

Have you drawn up a schematic of what you're trying to do?

Have a look at the attached; it's just a simulation of one half of an L272 creating a virtual ground, and the other half inverting a signal. The input signal is a 2v peak to peak signal with a 1.5v offset. You see the output is the same as the input, but it's a mirror image voltage-wise.

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#### Ron H

Joined Apr 14, 2005
7,014
its a tl071 op amp and i need it to send a signal to a 555 for a gas engine sound project i am working on. i need the signal to be negative going into pin 5 of a 555 ic. but i think the split voltage trick with the 2 resistors is going to do the trick.
Pin 5 on a 555 cannot handle a negative voltage.

#### smoking 555

Joined Sep 21, 2008
25
heres a schematic of what i have tried to create 3 times. my friend thinks its got something to do with the tl071 not being properly grounded on pin 4 of the non inverting side. its been gone over with a fine tooth comb with all power hooked up and it just doesnt work. can you spot any errors with it? your right Ron H the 555 cant handle the negative. it says that it creates an offset voltage, "TL071 average offset of 1.15 volts was ideal to maintain an acceptable idling speed at zero volt reference input from the rails ." this is for a model train but i want to use it for a car. I tried using 12 volts regulated from a pc power supply then 12 volts from a train power pack but still no go. if i could get it to pop with those power supplies i was going to play with the resistors to get it to run on 7.2 volts dc

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#### SgtWookie

Joined Jul 17, 2007
22,210
There is a wire missing on the 555CMOS timer between pin 2 and pin 6.

#### smoking 555

Joined Sep 21, 2008
25
sorry bout that. it lists it in the pcb but not in the schematic. included the jumper for 2 and 6 on the 555, still didnt work. do you think its the ground for pin 4 of the tl071?

#### Ron H

Joined Apr 14, 2005
7,014
sorry bout that. it lists it in the pcb but not in the schematic. included the jumper for 2 and 6 on the 555, still didnt work. do you think its the ground for pin 4 of the tl071?
See the red ellipses for the obvious omissions. There may be other errors.

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#### smoking 555

Joined Sep 21, 2008
25
omg that might be it!!!!! so should i make the following changes:

1. make sure that pin 4 on the op amp is grounded?
2. make sure that pin 2, 6, and 5 are all in tied together
3. not to sure about the vcc marking, do you mean just make sure they get power? sorry novice newbie here.

#### SgtWookie

Joined Jul 17, 2007
22,210
No, pin 5 of the 555CMOS timer must not be connected to pins 2 and 6 of the same IC.
Pin 5 of the 555CMOS timer gets connected to TL071 pin 6, and one side of R5 and the + side of C3.

It's not just pin 4 of the TL071 that gets grounded; pin 1 of the 555 and the junction of R2/R4 also gets grounded - and it also serves as a 0v reference for the input of the track voltage - which changes the sound via PWM.

The 4017 pin 16 and the LM386 pin 6 (which isn't circled) should be connected to S1 pin 2; that way the ICs will only receive power when S1 is ON.

#### VenomBallistics

Joined Sep 24, 2008
2
its a game of "connect the dots" if there isnt a dot ... dont connect it, its an overlap.
a circuit may be built in three dimensions, but only drawn in two

#### SgtWookie

Joined Jul 17, 2007
22,210
its a game of "connect the dots" if there isnt a dot ... dont connect it, its an overlap.
Normally, what you say is true. Unfortunately, in this circuit that is not the case. Look at pin 7 on the 555 timer; it's connected to the junction of R6 and R7, but there is no dot; there should be one. The circuit won't function without a connection there.

[eta]
OK, so I guess this was the page where you originally found the circuit:
http://members.shaw.ca/roma/train.html
It would've helped a great deal if you'd included the page reference in the beginning.

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