# amplifiying voltage using op-ampTL0072

Discussion in 'The Projects Forum' started by faris, Nov 19, 2011.

## how to amplify the voltage

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1. ### faris Thread Starter New Member

Nov 19, 2011
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hey guys for my project i want to amplify the voltage in the attached circuit in the circuit the output voltage is 1volt and current 10 mA . but i want to get the output voltage 5volt and more current doesn't matter so what which resistor should i change to get 5 volt and which value should replace .
any help or explanation will be appreciated

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2. ### wayneh Expert

Sep 9, 2010
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You didn't mention your supply voltage. It obviously needs to be greater than 5v if you want an output of 5v.

Without understanding or modifying that circuit at all, you could just add another stage of amplification - a non-inverting op-amp circuit with a gain of 5.

I'll speculate that adding the right resistance in series with those diodes might increase the input voltages seen at that op-amp, and thus the output. The diodes currently cap the voltage there at ~0.7v

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3. ### SgtWookie Expert

Jul 17, 2007
22,183
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A TL072 needs to have a positive supply that is at the very minimum 1.5v higher than the input range or desired output voltage; and that is at low current.

It also needs a negative supply that is 3v lower than the lowest expected input or output.

4. ### faris Thread Starter New Member

Nov 19, 2011
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ya thanx alot dear wayneh ,i ll supply 12 V voltage but u mean right resistance in the series with Rf ?
and what should be the value for that and i have one thing to ask if i inreased the value of Rf like to 500kohm i ll get more voltage ryte ?

5. ### SgtWookie Expert

Jul 17, 2007
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The circuit is more or less a low-pass filter.

If you simply want to amplify the output with a gain of 5, then add another opamp stage; you won't disturb the design of the filter that way.

See the attached.

• ###### Faris opamp.png
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6. ### faris Thread Starter New Member

Nov 19, 2011
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thanks alot SgtWookie i ll try this circuit can u give me the name of simulator u r using to simulate the circuits ?

7. ### SgtWookie Expert

Jul 17, 2007
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I am using LTSpice. It is a free simulator that Linear Technology has available for download from their website.

[eta]
I have modified the schematic a little bit by replacing the TL07x opamps with LT1013 opamps (because they come with LTSpice and TL07x does not) and I've included the model for the 1N4148 diode in the schematic; I don't know for certain if the library comes with that or not anymore.

The schematic is attached. Once you install LTSpice, you should be able to download this .asc file and run the simulation. Click on the wires next to out1 and out2 to see the plots.

• ###### Faris opamp 2.asc
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Last edited: Nov 20, 2011
8. ### faris Thread Starter New Member

Nov 19, 2011
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hmm thanks for ur help SgtWookie

9. ### DickCappels Moderator

Aug 21, 2008
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You can increase the gain by increasing R5.

10. ### Yako New Member

Nov 24, 2011
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Total MådnE§§. You like?

11. ### SgtWookie Expert

Jul 17, 2007
22,183
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Gee, I think this is the first mistake you've made on here, Dick. (if only I had such an enviable record. )

Increasing R5 will decrease the gain.

R5/R6/U3 is the amplifier I added for Av=5.

Faris,
I neglected to mention a couple of things:
1) Use all 20k metal film resistors for R5 and R6; two 20k resistors in parallel for R5, and two 20k resistors in series for R6, preferably using resistors that are all from the same batch. This will help to ensure that your gain is reasonably accurate, and you only need to buy one size of resistor. 40k is not a standard value of resistance, but 20k is. Metal film resistors are less noisy than any type of carbon resistor, and are also more stable.

2) If the circuit is already constructed using a TL072, TL082 or LF353, you could use a TL071, TL081 or LF351 for U3. If it is not built, you could use a TL074 or TL084, both of which have 4 opamp channels. In this case, you will need to connect the unused channel as a voltage follower; output connected to the inverting (-) input, and noninverting (+) input connected to ground. Otherwise, the unused channel may oscillate unpredictably, and cause problems with the other channels.

3) 0.1uF/100nF bypass capacitors across the supply pins of the opamps were not shown. Each opamp needs a minimum of one 0.1uF cap from +V to ground, and one 0.1uF cap from -V to ground. The caps can be ceramic or poly metal film.