Non-inverting amplifier

Discussion in 'General Electronics Discussion' started by OddOne, Mar 22, 2013.

1. OddOne

42
0
Feb 26, 2013
Hello everyone,

I used the LM741 to make a non-inverting amplifier with a gain of two. The amplification for a square wave works, but it gets a offset and the "down" flanks aren't at 90 degrees.

How can I fix this?

Last edited: Mar 22, 2013
2. Harald KappModeratorModerator

12,330
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Nov 17, 2011
Use a "better" opamp. The offset is an oldie.

You can do with the 741, however, if you know it's limits.
The offset can be nulled using a trimpot, see the datasheet.
There's nothing you can do about the rise and fall time (edges). The 741 has an output slew rate of 0.5V/µs. This means it takes 2µs to generate a voltage difference of 1V. If your signal is e.g. +-10V, then the OpAmp will take 20V/(0.5V/µs)=40µs toi slew from +10V tp -10V or vice versa.
You will need an OpAmp with a much higher slew rate to overcome this.

Apart from that: using an OpAmp to amplify a square wave by 2 in my opinion is overkill. A simple transistor amplifier can do the same - and much faster.

3. OddOne

42
0
Feb 26, 2013
I've tried it, but it didn't worked. I'll try again later.

For some reason there are no problems with the rise time, but only with the fall time.

Are there any websites you can recommend to calculate the values of the components?

Thanks for the help!

12,330
2,930
Nov 17, 2011

5. OddOne

42
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Feb 26, 2013
I've found this transistor amplifier:

Could this work?

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6. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
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Jan 21, 2010
The gain of this stage is approximately the hfe of the transistor.

7. Harald KappModeratorModerator

12,330
2,930
Nov 17, 2011
This circuit is inverting. That may be no problem at all for your application. This type of amplifier can sink considerable current when T1 is on. In the off state of T1, the sourceable current is limited by the resistance of R2.
If you need to sink and source current, use a complementary output stage as for example shown here.

8. OddOne

42
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Feb 26, 2013
Is that the circuit that you mentioned before as a simple transistor amplifier or do you know another transistor amplifier that could work better?

9. Harald KappModeratorModerator

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Nov 17, 2011
This is just one circuit. There are soooooo many.
What does "work better" mean?
More power?
Less distortion?
Higher bandwidth?
...
Have you checked the performance of the circuit I linked? Does it fulfill your requirements? If so, use it. If not, where are the deficits?

10. OddOne

42
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Feb 26, 2013
I was just curious if this was the circuit you had in mind.

11. BobK

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Jan 5, 2010
You haven't given us any details of the range of signals you need to handle as input. This would be all important in designing an amplifier.

Bob

12. OddOne

42
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Feb 26, 2013
I want to amplify a square wave to 12V. The peak voltage is now 10V, but I want to amplify it to 12V. So I was thinking about reducing the peak voltage to 6V and amplify it with a gain of 2.

13. Harald KappModeratorModerator

12,330
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Nov 17, 2011
???

If your current gain is 2, increase the gain to 2*12/10=2.4
If your current gain is anything else , say x, set the new gain to x*12/10=x*1.2

14. OddOne

42
0
Feb 26, 2013
So if I understand it correctly. I need to find a way to get hfe of the transistor to be 10.

15. Harald KappModeratorModerator

12,330
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Nov 17, 2011
The hfe of a typical transistor is much greater than 10.
You need to get yourself acquainted with transistor amplifier basics. You set up the gain of an amplifier by using the correct set of components.

With the simple circuit of yesterday's post of yours, Vout will vary between 0V and Vcc. It will be less than VCC if you apply a load because the load and the collector resistor will form a voltage divider.

If you use a complementary stage, you will have to design that stage to the desired gain. Or use it "as is" and control the input voltage (e.g. using a potentiometer) such that the output has the desired amplitude.

16. BobK

7,682
1,689
Jan 5, 2010
If the input is always a square wave of 10V you could put it through a CMOS inverter powered by 12V to get a 12V square wave. This, however, implies that you will not be drawing much current from it. What is it driving? How much current is required. If more current is required, two discrete MOSFETs would work.

Bob