Help in understanding Op amp technology

[brainsalad]

Some of the OP amps I am seeing being used/recommended for op amp rolling (ie OPA627 & LT1364 (stock iBasso D1 L/R op amp)) are unity-gain. From what I understand, these provide constant voltage and have no gain.

Others are recommending the AD797 which is a gain device.

How can a unity gain op amp serve as an amplifier for headphones? I can maybe see how the AD797 can since it has gain.

Or am I completely not understanding?

 

[ruZZ.il]

opamp@unity gain = current buffer. If there's a voltage gain op-amp before it, it'll see a much higher impedance which is better for the voltage-gain op-amp. I'm not sure that thats what you're talking about though...

 

[Jambo]

I haven't looked, but I'd be surprised if any opamps were unity gain - if you do some reading you'll find that one of the basic (false) assumptions that you're told to make about opamps is that they have infinite open loop (ie without feedback) gain. They don't, but it's high enough that it usually seems this way and this assumption allows lots of other calculations to work.

However.. you can *configure* any opamp to be unity gain when it will behave as you have described, providing no voltage gain but acting as a current buffer - which is you state is pretty much for impedance matching though for whatever reason I personally don't like to think like that.

Also, all opamps can be configured to give voltage gain (amplification) as you describe.

Does that make anything any clearer?

If you provide a link to wherever you have read that maybe someone can explain it better.

 

[00940]

Quote:

Originally Posted by brainsalad /img/forum/go_quote.gif Some of the OP amps I am seeing being used/recommended (ie OPA627 & LT1364 (stock iBasso D1 L/R op amp)) are unity-gain. From what I understand, these provide constant voltage and have no gain but serve as more of an impedance matching device? But the OPA637 & AD797 are gain device therefore providing amplification? Or am I completely not understanding?

The opa627 is not an "unity gain" device. It is "unity gain stable". Which means that you can use it without gain. It doesn't mean that you can't use it with gain. You can very well use an opa627 with a gain of 10 if you want to.

 

[brainsalad]

I am not sure either. I re-worded my question.

What I am trying to understand:

I have an iBasso D1 and it has quite a tweaker following with people that like op amp rolling. In the D1 it has a L/R buffer section followed by the L/R amps. In the stock D1, it uses lmh6643 as the buffers and lt1364 (unity gain op amp) as the L/R amp.

Is the LT1364 taking the hi impedance of the LMH6643 and matching to the lower impedance for headphone use and since it has an impedance drop it is creating current amplification?


On the other hand there are people recommending AD797's for the L/R Op Amp but the buffer stage will be replaced with bypass wires for the buffer. Is that because the AD797 has voltage gain and not necessarily current gain?

 

[Pars]

Quote:

Originally Posted by 00940 /img/forum/go_quote.gif The opa627 is not an "unity gain" device. It is "unity gain stable". Which means that you can use it without gain. It doesn't mean that you can't use it with gain. You can very well use an opa627 with a gain of 10 if you want to.

As 00940 says, the corollary to this is the OPA637, which is NOT unity gain stable (i.e., must have a gain of 5 or more IIRC). An opamp by definition is capable of gain. Buffers (BUF634, etc.) are designed expressly for use at unity gain (gain = 1).

 

[brainsalad]

Quote:

Originally Posted by 00940 /img/forum/go_quote.gif The opa627 is not an "unity gain" device. It is "unity gain stable". Which means that you can use it without gain. It doesn't mean that you can't use it with gain. You can very well use an opa627 with a gain of 10 if you want to.

As you can tell, I'm a newbie without an EE degree trying to make sense of all this. This definitely helps. I have been trying to read spec sheets and trying to figure out what in the world people are talking about.

It's kind-a-like I can see people swimming and doing things in the water and I can tell they are having fun and I want to have fun. I know I use water, I know water is good, I know I have water all around me, and I know people are changing the water, but I don't know what water is.

 

[brainsalad]

I heard a quote once.

No such thing as a stupid question but there is such a thing as stupid people asking questions.

 

[Jambo]

Apologies, I shouldn't have said that all opamps can be configured for unity gain

Quote:

Originally Posted by brainsalad /img/forum/go_quote.gif On the other hand there are people recommending AD797's for the L/R Op Amp but the buffer stage will be replaced with bypass wires for the buffer. Is that because the AD797 has voltage gain and not necessarily current gain?

I'm not familiar with the circuit, but if two opamps are being replaced by one then the AD797 must be being used to provide both the voltage gain and current capability, which is more than possible.

 

[00940]

Introduction to opamps :

Audio Designs With Opamps

Operational amplifier - Wikipedia, the free encyclopedia

741 Op-Amp Tutorial, op-amps, Operational Amplifier


To know everything about them:

ADI - Analog Dialogue | Op Amp Applications Handbook

 

[DaKi][er]

In the basic opamp circuits we use for audio, the gain of the opamp is determined by the resistors we use around it, and if you take one of the most basic amps, the cmoy to learn from, the gain of this circuit is determined by 1+R4/R3

Unity gain is as you know, when you have a gain of 1 but not all opamps are designed to be stable when run with a gain of 1 and the datasheets will tell you what the minimum gain you must have to ensure stability.

All opamps have gain, just limits of what that gain can be

 

[brainsalad]

As I have been reading the posts and reading some of the recommended links, I am starting to understand. Now my original question(s) look quite odd but I was trying to make sense out of some very technical jargon that I knew nothing about. The thing that was throwing me off were some of the spec sheets were showing the device as "unity gain stable" and I was interpreting that as it was a unity gain device vs a gain device. I then thought that there must be "unity gain" op amps and also "gain" op amps. Now I think that I understand that unity gain is a function of the circuit design not the device. I also understand that all op amps by definition have gain but not all op amps are stable in a unity gain circuit.

Now I almost have enough knowledge to be of danger and I certainly have more than enough knowledge to be either an audio consultant/expert or an audio reviewer. Maybe I could go to work for Stereophile now.

 

[ruZZ.il]

"All op-amps"..
Well, we usually only deal with a single type of op-amp for audio amplification, a non-inverting one.
Take a look here at the non-inverting, and many other types!
Not trying to confuse you or anything, but they're handy little buggers in many different flavors.