What is the application for the op amp?

There are many op amps similar to what you've listed in higher end audio applications. The only place absolute flat linearity with no phase shift would be needed is typically an instrumentation device, and even then, the op amps listed are flat response when used in properly designed circuit.

If the purpose is for a "perfectly flat" audio amplifier, there will be far greater problems finding a BJT or MOSFET that comes close to the range and response of many op amps.

 

Ron H, bertus and papabravo: I will look into it. You see, I am a EE undergrad student so while I can take people's word for the functioning, I also need to know of the whys and the hows.

By the way, will the gain-bandwidth product's knowledge also tell me about the phase shift? Because along with a flat gain, I also need a linear phase shift with frequency.

marshallf3: I'm a little confused. You're saying that expensive op amps like the 2134 are unnecessary and that the tl082 would suffice? The TL082 is listed as a "general purpose" amplifier, and I'm pretty sure the 741 is in the same category. So why the TL082 and not 741? Why the TL082 and not the OPA2134?

Also, while we're at it, could somebody explain to me a little of the op amp input varieties? As in, several of the op amps I have come across mention their input as JFET. Several, I think, mention it as simply FET. Other don't mention the input type at all. What is the difference, what is the impact of these different types? Aren't opamps made out of transistors to begin with? So what new info do we get by mentioning the input as JFET or FET etc.?

 

thatoneguy: Well, basically I'm hoping to make a buffered pre-amp for the guitar and the bass to feed into the line in of the computer. I was hoping to do it with a couple of (distinct) MOSFETs or BJTs, since I recently completed a course on these, but it seems that I've sidetracked into opamps for the same purpose. I think I sidetracked because I've no idea if FETs and BJTs are sold separately and what are their varieties. Whereas the opamps I'm coming across are specially geared for a particular purpose.

 

Do you really think an audio amp should have such a high gain? A 0.1 volt input would result in a 400 volt output or some other equally outrageous number.?

Open loop gain is just for comparison BETWEEN different op amps.

In circuit they are set to practical amounts of gain. Like 2, or 10, in which range they will have a flat freq response way past the human hearing range.

No.
A lousy old 741 opamp has slew-rate limiting that cuts its max output above only 9kHz.
An old LM358 or LM324 opamp has slew-rate limiting cutting its max output above only 1kHz or 2kHz.

 

thatoneguy: Well, basically I'm hoping to make a buffered pre-amp for the guitar and the bass to feed into the line in of the computer.

I think you'd freak out if you saw what was used in the construction of Effects Pedals. Or are these acoustic guitars and you found a microphone that has a flat bandwidth and no phase shift from under 20Hz to 20kHz?

 

thatoneguy: Well, basically I'm hoping to make a buffered pre-amp for the guitar and the bass to feed into the line in of the computer. I was hoping to do it with a couple of (distinct) MOSFETs or BJTs, since I recently completed a course on these, but it seems that I've sidetracked into opamps for the same purpose. I think I sidetracked because I've no idea if FETs and BJTs are sold separately and what are their varieties. Whereas the opamps I'm coming across are specially geared for a particular purpose.

http://www.montagar.com/~patj/smixer.gif

 

The reason you are getting sidetracked into op-amps is that they are ridiculously easy to design excellent audio amps with as demonstrated in post#26.

and yes, fets and bjt's are sold seperately, but you have to put tons more work into them to get something that compares to the quality of an op-amp audio amplifier.

 

and yes, fets and bjt's are sold seperately, but you have to put tons more work into them to get something that compares to the quality of an op-amp audio amplifier.

Just as an example of this look at the spec sheet for an OPA1641 - http://focus.ti.com/docs/prod/folders/print/opa1641.html

 

A FET (or an opamp with a FET input) is recommended as a buffer (or preamp) for an electric guitar pickup. But 50 years ago FET-input opamps were not available so they used discrete FETs.

 

I think you'd freak out if you saw what was used in the construction of Effects Pedals. Or are these acoustic guitars and you found a microphone that has a flat bandwidth and no phase shift from under 20Hz to 20kHz?

Are you saying that they have lots of imperfections in their frequency response? I can understand that they'd have imperfections, but I just want to construct the amplifier as much neutral as I can make it, and without any phase distortion. I can then apply EQ to it myself.

 

A FET (or an opamp with a FET input) is recommended as a buffer (or preamp) for an electric guitar pickup. But 50 years ago FET-input opamps were not available so they used discrete FETs.

May I know why, though? Normal Op amps themselves can be used as buffers too, apparently. http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html#c1

 

A buffer amp is usually used to help match impedances, in your case a pickup may be high and the input to the amp a bit lower.
The better you can match impedances the less distortion you'll end up receiving.

 

A buffer amp is usually used to help match impedances, in your case a pickup may be high and the input to the amp a bit lower.
The better you can match impedances the less distortion you'll end up receiving.

Yes, I get that. So is your point that FET-input opamps have higher input impedances than regular ones to better serve my purpose? And what exactly does an FET-input opamp even mean? I thought all op amps were constructed out of FETs or BJTs, along with resistors and capacitors, so obviously they would all have FET/BJT inputs and outputs and everything else inside.

 

Fez,
Please answer the very important question: Are you using an acoustic guitar with a microphone or are you using an electric guitar with a magnetic pickup?

An audio opamp like a TL071 (which is a TL081 selected for low noise) has an absolutely flat frequency response and no phase shift from DC to 100kHz when its voltage gain is only 10. Its distortion is 0.003%. An OPA134 costs much more and has the same spec's except its nose is less and its distortion is only 0.00008%.

Both of these opamps have Jfet inputs so have the high input impedance needed for a guitar-pickup preamp but they can also be used for a microphone preamp.

 

Audioguru: Really? Now I'm confused again. I thought all these people were telling me that I could get the job done with almost any op amp with closed loop gain? So why the TL071 especially?
I am using an electric guitar. Ideally I'd want to expand it for electric bass and mics too. I just need to capture the sound for now.
I googled the TL071 and TL081. They are under the "standard linear amplifer" category on ti.com, not under the low noise or high precision categories. Could you offer some explanation please? And once again I ask, are you suggesting the JFET input op amps because they have higher input impedances as compared to regular-input op amps?


And please, I ask again, could someone tell me the difference between regular op amps and JFET input op amps???! I thought they are constructed entirely out of transistors, so why specifically mention an op amp to have "JFET inputs"?

 

And please, I ask again, could someone tell me the difference between regular op amps and JFET input op amps???! I thought they are constructed entirely out of transistors, so why specifically mention an op amp to have "JFET inputs"?

You could look into the datasheet, where sometimes is a simplified schematic of the opamp. FET opamps like TL072 have the input pair of transistors made from JFETs, so their input impedance is very high. Regular or bipolar opamps have the input pair made of standard PNP or NPN bipolar juctnion transistor, so they have lower input impedance.

 

Ah, so what you're saying is, regular op amps are BJT-only? And FETs only when mentioned. I see.

 

Using Bipolar Junction Transistor (BJT) input stages on an op amp make the "equivalent circuit" a current controlled voltage source. Since current is controlling it, it has low input impedance, it acts like a load to the signal source, in your case, the guitar.

Using Field Effect Transistor (JFET, FET) input stages inside the op amp make the "equivalent circuit" a voltage controlled voltage source. Since very, very little current is used at the input, the voltage level is unchanged from the source, and is a very high impedance "load" to the guitar.

There are "high output" op amps that can act somewhat as current sources instead of voltage sources, the above is a simplification to help you understand the concept.

An Ideal Operational Amplifier has infinite input impedance (shows no load to the signal source), and zero output impedance (can source/sink infinite current). The Ideal Op Amp exists only for theory, but JFET input op amps are the closest affordable approximation.

The slope you are referring to as "not flat" isn't exactly a graph of frequency response, it is a graph of the maximum gain for a given frequency. If the gain of an op amp is set at 10, the bandwidth is quite wide and flat. If the gain is set at 1000, there will be lower amplification of higher frequencies. For a usable design, a JFET input op amp with a gain of 10 would feed another op amp with the required gain for line level output. You may need more than one op amp after the input stage, but as long as they aren't the LM741, you should be fine.

The LM741 is dis-recommended for audio, it was designed in the late 1960's and still has the limits of silicon ability of that time. There are far far better options for new designs, though not all older designs are "bad". The LM741 is better used in an analog computer rather than the input of an audio amplifier. The "improved" 741's have different part numbers. The reason the 741 "sucks" but is still around is A) It is very popular in schematics from people who didn't design with a bettter op amp, or B) Older circuits may rely on some quirks of suckiness provided by the 741, such as limiting high frequencies, preventing oscillations.

 

Do you want the guitar to sound like a guitar or to sound like a telephone (no high audio frequencies)?

For the fourth or fifth time:
Are you using an acoustic guitar with a microphone or an electric guitar with a magnetic pickup?

 

I am using an electric guitar with a magnetic pickup.


Are there any alternative opamps to the TL071? I will be heading to the IC shop soon, and should have some alternate opamps to ask for if they don't have the TL071.