Induction Balance Stuff - Single/Multi Frequency Response, GB, Disc, Measurements, Ideas, Fun, etc.

[Aziz]

Missing the LTspice zip file...

PS: Why it is missing? I have uploaded the file.
Or is it owed to the fact of enjoying a Johnnie Walker (20+ years old)?

[Carl-NC]

Here is a direct comparison between a Lee-type CB preamp and a traditional opamp preamp:


The two methods are adjusted to have the same overall output swing, indicating an overall gain equality:

The blue is the std preamp and the green is the Lee/CB preamp. The frequency is 10kHz. The Lee has an offset because N4 hasn't fully settled, that takes ~3-4s and I'm not gonna wait that long on the sim. No big deal. Notice the 90° phase difference; that's because the std method (voltage mode) has a -90° shift due to the induction derivative and the Lee method (current mode) does not. If you have ITMD3, it's all explained in Appendix A.

Here is the output noise for each:

At 10kHz the output-referred noise is about the same. Std = 28nV/rtHz and Lee = 23.2nV/rtHz. Equivalent input-referred noise is 1/20th of these numbers, a little over 1nV/rtHz.

The reason the Lee method isn't much better is because the low input impedance forces the coil into current mode, and the function of the CB amp is to first convert the current mode back to voltage mode. This is something you get for free with the voltage mode connection. If you were to strictly consider the voltage gains of the two methods, the Lee amp has ~30X more voltage gain and would seem to be the hands-down winner, but here it's being used as a transimpedance amp, not a voltage amp.

So, yes, it works, and with some tinkering might do a little better. But I agree with you Aziz, it ain't the way to go.

[Aziz]

Hi all,

well ok, I will increase the dynamic Rin of the amplifier a bit (10 kOhm min). The frequency compensation feedback capacitor Cf must be reduced or totally eliminated in this case.
The bootstrapping method works then too. But this isn't required I think.

PS: Thanks for clarification Carl.
Aziz

[Aziz]

Hi all,

I have seen, that the op-amp OPA1612 can be operated down to 4.5 V supply voltage (single). I am going to use it as it gives very very good performance.
So the amplifier will be optimized for 4.5 V - 6 V supply voltage. It will draw less power too.

Unfortunately, op-amp has 2 V common mode input voltage limit to the power rails (like the NE5532) but it has a rail-to-rail output. The whole transistor stage operating point will be so set, that the common mode input voltage is around mid-level of the supply voltage.

I will add a 9 V supply voltage with NE5532 to the simulation files too.
I am not finished yet.

PS: Increasing the input impedance won't work well. We need the frequency compensation feature for stable operation. It will be around 7 kOhm however.

[Aziz]

Hi all,

the promissed amplifier schematics:
- 9 V Version with NE5532
- 4.5 V - 6 V Version with OPA1612

See the LTspice simulation files please.
Cheers,
Aziz