Author Topic: Op amp Boost. Help needed  (Read 1458 times)

ElectricDruid

Re: Op amp Boost. Help needed
« Reply #40 on: February 11, 2019, 06:03:52 PM »
It's easier to think about if you look at the "split" version as making a cheap +/-4.5V supply. That midpoint voltage isn't really 4.5V! No, actually it's "virtual ground". Since voltage is all relative, that means the black wire on the battery is -4.5V, and the red wire is +4.5V. Bingo! It's a bipolar supply! Well, it's mostly close enough, anyway.

So now convert that +/-4.5 to +/-15V. Anything that used to go to battery+ now goes to +15V. Anything that used to go to battery- now goes to -15V. Anything that used to go to Vref now goes to Gnd.
The only thing that really changes is that our input and output will be referenced to ground rather than the battery-. That means you can leave out R1 and R5. And since you don't need the split supply now you have a real bipolar supply, R3, R4 and C2, C3 can all go too. R7 now goes to ground, according to the rules above.

Converting between single-supply and bipolar supply and vice-versa can be a confusing pain in the neck, but it gets easier the more you do of it! Having said that now, someone will come along and point out the glaringly stupid mistake I've invariably made in what I just said...which is fine.

HTH,
Tom

PRR

Re: Op amp Boost. Help needed
« Reply #41 on: February 11, 2019, 09:44:30 PM »
> Whats the formula for calculating the gain in this circuit?

It's a standard opamp inverter. Hasn't that formula been published somewhere?

Buffalo Tom

Re: Op amp Boost. Help needed
« Reply #42 on: February 12, 2019, 02:51:04 AM »
It's easier to think about if you look at the "split" version as making a cheap +/-4.5V supply. That midpoint voltage isn't really 4.5V! No, actually it's "virtual ground". Since voltage is all relative, that means the black wire on the battery is -4.5V, and the red wire is +4.5V. Bingo! It's a bipolar supply! Well, it's mostly close enough, anyway.

So now convert that +/-4.5 to +/-15V. Anything that used to go to battery+ now goes to +15V. Anything that used to go to battery- now goes to -15V. Anything that used to go to Vref now goes to Gnd.
The only thing that really changes is that our input and output will be referenced to ground rather than the battery-. That means you can leave out R1 and R5. And since you don't need the split supply now you have a real bipolar supply, R3, R4 and C2, C3 can all go too. R7 now goes to ground, according to the rules above.

Converting between single-supply and bipolar supply and vice-versa can be a confusing pain in the neck, but it gets easier the more you do of it! Having said that now, someone will come along and point out the glaringly stupid mistake I've invariably made in what I just said...which is fine.

HTH,
Tom

Thanks a lot. Will try this

Buffalo Tom

Re: Op amp Boost. Help needed
« Reply #43 on: February 12, 2019, 06:36:19 AM »
> Whats the formula for calculating the gain in this circuit?
It's a standard opamp inverter. Hasn't that formula been published somewhere?

After two hours on youtube+google I think I found what I'm looking for. LOG 20 formula. So If I got this right it should look like this:

Current version with 10K pot and 1K resistors.
10000/1000 = 10
20 LOG 10 = 20dB

So for 25 dB boost/attenuation I change to 560 ohm resistors.

10000/560 = 17.86
20 LOG 17.86 = 25dB

Am I on the right track?
Thanks

antonis

Re: Op amp Boost. Help needed
« Reply #44 on: February 12, 2019, 07:25:09 AM »
> Whats the formula for calculating the gain in this circuit?
It's a standard opamp inverter. Hasn't that formula been published somewhere?
I found what I'm looking for. LOG 20 formula.
......................................
Am I on the right track?
Thanks
Of course you are but only in EE manner..
IMHO, it should be more convenient to think of Gain as Vout/Vin ratio (e.g. Output voltage 100 times greater than Input voltage) instead of +40 db, as long as you aren't familiar with semi/full log coordinates..

P.S.
For "normal" signal level margins, the above should result in distortion/muting without special precautions taken care of..