Assignment 1: Amplify millivolts x1000

With an sinewave input of 1 mVpp, amplify the signal to 1 Vpp using a generic opamp.

This is the circuit I will be using, a simple inverting amplifier.

The gain for this amplifier is $G = -{R_2 \over R_1}$. For a gain of 1000, I will use an R1 of 100 and R2 100k.

I will be using a TL072 opamp. This IC needs a supply of at least 10V between the VCC+ and VCC- . This is the pinout:

This is my first test, as a start I tested a lower gain of 10 using a 1k and 10k resistor.

| AWG | Siglent SDG1032X | | --------- | ---------------- | | Frequency | 1kHz | | Amplitude | 100 mVpp | | Waveform | Sine |

That went well. Now let's try a x100 amplification. I will use 1k for R1 and 100k for R2.

| AWG | Siglent SDG1032X | | --------- | ---------------- | | Frequency | 1kHz | | Amplitude | 10 mVpp | | Waveform | Sine |

This does not look so good. What might have gone wrong?

• The blue waveform is clipping at around 8 Vpp; evidently the opamp cannot go higher with its current -5/+5 V supply.
• The yellow waveform is jittery. Perhaps the AWG can not produce clean waveforms at such low voltages? Or could this have to do with the cabling?
• The gain is not as expected. With 10 mVpp x100 I expected a blue waveform of 1Vpp.

I try connecting my AWG directly to my scope and this is the result:

So far I have tried a different cable and channel on both the scope and the AWG without any change. Now I will try and see if using a voltage divider to get a lower voltage will improve it.

Let's say I output a 100 mVpp from my AWG and use a voltage divider to split it into 90 mVpp and 10mVpp. If this produces a clearer result then I can probably say the cause of the 'blur' is the AWG and not the scope. Credit to the writer of this post for an easy way to create a 1:10 voltage divider.

It looks...different now. Quite a bit sharper, but now it's a bit 'stuttery'. Oops! my probe wasn't connected properly; it's showing a 50Hz frequency sinewave...which of course is net mains being picked up by the cabling. But when I do connect it properly the waveform is rubbish:

I decide to use my Fluke 87 DMM to check the voltages and am suprised to read 35.7V coming from the AWG (which is still set to 100mVpp). Could this have to do with the 'RMS'? Yes! I don't feel like getting my calculator, but this online tool tells me the Vrms for 100 mVpp is equal to 35.35 mV using this formula: $$ V_{rms} = \frac{1}{2\sqrt{2}}V_{P-P} = 0.353V_{P-P} $$ And indeed when I measure between the 0 V and 0.01 V spot on the breadboard the DMM shows 3.5 mV so the voltage divider works properly.

Okay, back to the 10 mVpp directly from the AWG. When I zoom in I see a distinct waveform on top of the existing wave. What is the frequency of this and what might be the source?

I enabled the cursor and placed the vertical lines so that 10 periods fall between. The duration of 1 period is: $$ duration = {{90.80 + 66.40} \over 10} = 15.72 \micro s $$ Which means the frequency is: $$ frequency = {1 \over 15.72*10^{-6} } \approx 63613 Hz $$ On the internet here I find this might be an AM radio signal. Not sure what I can do about that. Perhaps I can test shielding the cable with aluminium foil?