# What is the true circuit behind an opamp?

So, first year EE student, and I just learned about op-amps. I understand the ideal model, and know how to analyze them, and understand the idea behind them/the circuit that we were shown that is inside them. Except, that's not the real circuit, it has a dependent source. My question is, what is actually inside an op-amp? If we were to replace the dependent source with real sources, what would we see? (I guess this is also more of a question about 'What are dependent sources, really?'). I have searched everywhere, and I always find the same answer 'Dependent sources are useful tools to model a circuit'. But what are they really?

Here is a \$35 kit you can make, which ends up being the equivalent of a 741 op-amp using discrete 13 2N3904 and 7 2N3906 transistors. It has eight binding posts representing the eight pins of the device.

Here is a link to the datasheet, which includes the schematic for the kit (shown below) and a BOM.

Compare that to a "real" 741 out of the TI datasheet:

They are virtually the same, even down to the resistor values.

There is also an 11 page "Principles of Operation" which goes into quite a bit of detail on how it works. And finally, they have a Wiki.

• Also note that it will not be as good as a 741 in several areas, especially offset voltage and drift. The input transistors Q1 and Q2 are neither matched nor thermally coupled, both of which are important to modern (IC) op amps. Commented Apr 11, 2016 at 3:55
• @WhatRoughBeast That's why I posted a link to the datasheet for the kit, so he can compare the parameters with the TI datasheet. Still I thought it's a pretty cool way to learn about the guts of an op-amp, especially following through the circuit as explained the the Principles of Operation. It handles topics like Offset voltage, including use of the Offset Null pins 1 and 5. Commented Apr 11, 2016 at 3:59
• "We're gonna need a bigger breadboard." Commented Apr 11, 2016 at 18:27
• This is my favorite answer by far of the whole site! Now I have to find a circuit. Commented Apr 11, 2016 at 19:44
• Where can I buy this toy? Commented Apr 12, 2016 at 9:14

"Dependent sources are useful tools to model a circuit'. But what are they really?"

Regarding "dependent sources": We discriminate between four different controllable (dependent) sources:

Voltage-controlled voltage source (VCVS), Current-controlled voltage source (CCVS), Voltage-controlled current source (VCCS) and current-controlled current source (CCCS).

Examples:

• Transistors (bipolar and FET): VCCS

• Operational amplifier: VCVS

• Operational transconductance amplifier (OTA): VCCS

• Current conveyor (second generation, CCII): CCCS.

In reality, all dependent sources are non-ideal (finite input and output impedances, frequency-dependent). That means: Real dependent sources can be modelled using ideal dependent sources in conjunction with "parasitic" elements (resistors, capacitors)

• Thanks! This actually makes sense, finally. I don't understand why we never go over this stuff in my classes... Would definitely make understanding the subject a lot easier Commented Apr 11, 2016 at 11:41
• @MahmudAssamaray You're clearly, like me, a theory-based learner. You have to understand the principles behind something before you can really understand it. Unfortunately, we're in a minority and most teaching is aimed at people who learn the surface detail more easily and find the theory behind that detail too abstract to understand without practical experience first. Try looking through a few textbooks other than the ones you're using, you might find one that has a teaching style more appropriate to your learning style. Commented Apr 11, 2016 at 18:41
• Aren't bipolar transistors also CCCSes? Commented Apr 11, 2016 at 23:41
• No - that is a common misconception (unfortunately, to be found even in some textbooks). Physically spoken - according to Shockleys famous equation, the current Ic is controlled by the base-emitter voltage Vbe only. How could - for example - three positively charged carriers from the base release 333 charged carriers (negative) from the emitter (assuming a beta value of 300) ?
– LvW
Commented Apr 12, 2016 at 7:17
• @Jules obligatory 'there are literally dozens of us'... It's annoying being the only one in class with my learning style, especially when I ask a question, my professors usually wave it off and say 'thats too deep of a question for the scope of this class'. Self learning/the internet/stackoverflow are a blessing. I've never actually thought at looking for other textbooks, thanks for the tip! Commented Apr 12, 2016 at 22:29

Other answers have suggested looking at the implementation of real op-amps like a 741, but from a perspective of learning how they work, the best way to start is with a simplified system. The core of an op-amp is long-tailed pair. This can be built and operated or analysed in isolation to the rest of an op-amp, and provides the basic fundamentals of what an op-amp is. Looking at the 741 schematic, note that the transistor pairs (Q1,Q3) and (Q2,Q4) are substituting for the single transistors Q1 and Q2 in the diagram on wikipedia. The resistors in the wikipedia circuit are subsituted with transistors to allow the behaviour of that core amplifier to be optimized. The rest of the 741 circuitry is basically designed to improve on the response of this amplifier (removing offsets, increasing gain, improving frequency response, etc), and isn't strictly necessary to do the basic job.

Search for 'LM709 schematic', or 'LM 741 schematic'. Those were some of the first opamps available and have reasonably simple schematics. Modern opamps are based on similar principles, but generally have more complex circuits (because transistors are much cheaper now, and performance requirements keep increasing).

On the web sites of chip manufacturers you may find extensive documentation on most IC's, including the opamps that you actually use in your electronics lab. Such documentation often include the schematic diagram. Especially for opamp chips.