TL072 as a 1MHz amplifier

[JohnSym]

Hi

I am trying to use a TL072 as an amplifier with a gain of 10, and i cannot get it to produce a sine wave output.

I have chosen the TL072 because it worked well for me as a sine wave oscillator, so it can handle 1MHz. 

Ultimately, i wish to put a sine wave of 1MHz 2.1uV (pk) into the amplifier, and have it output a sine wave of 1MHz 21uV (pk). Can anyone offer a suggestion as to why i am getting this output? Is the TL072 the correct chip for this? Have i connected it correctly?

Any assistance would be greatly appreciated.

[JohnSym]

Here is the multisim file

[johnsold · Accepted solution]

JohnSym,

I do not have Multisim so I cannot look at your circuit but I have designed quite a few circuits with the TL07x amplifiers.

1. The typical Gain-Bandwidth product is specified at 3 MHz. That means that the open loop gain is only 3 at 1 MHz.

2. The Input noise specification is 18 nV/root(Hz) above 1 kHz or 4 uV over a 10 kHz bandwidth.  For a 1 MHz bandwidth the noise density translates to 18 uV of input noise.

So you would need at more than three amplifier stages to get a gain of ten at 1 MHz. Consider four stages with gain of 2.5 each. Input noise at first stage is 18 uV. At output noise is 45 uV. Do an rms sum of the noise at the next stage input.  Its input noise is ~48.5 uV. Output noise is 121 uV. After the fourth stage the noise is > 750 uV.

This is not an appropriate device for uV signals at 1 Mhz.

Lynn

[JohnSym]

Wow Lynn thats amazing, thanks.

You've given me more of an idea of what to look for; I'll try out some other op-amps now.

Thanks again

[johnsold]

The information is all in the datasheets. Sometimes you need to dig a little or do some calculations to interpret the data sheet values.

The other thing to remember is that simulation models are often not set up to accurately represent the behavior of the device when you get close to the limits. So a model for a TL072 which accurately simulates the behavior at 1 kHz with a 1 V signal might be completely useless at 1 MHz and a few uV.

Lynn

[JohnSym]

Yes it's just interpreting the datasheets that can sometimes be an issue for beginners. I found that exact behaviour you mentioned - i could get it to work at a certain frequency and voltage, but not at others. Ultimately i was unsure whether this was becuase i was asking it to do something it couldnt do, or whether i had made an error with the circuit somewhere.

Thanks again for clarifying.