How to design an op amp.

Let’s start today’s post by showing you how easy it is to design an amplifier circuit with an op amp.

When an op amp is used in a circuit as an amplifier, all you need is two resistors, the op amp and a couple of batteries. You can have the op amp amplify as either an inverting or non-inverting amplifier – meaning that whatever signal you put in can have its polarity or phase intact or flipped over – the same way you can reverse the +/- of your speaker terminals.

So let’s roll our sleeves up and design a preamplifier together with an op amp. We will want to have 20dB of gain in our op amp, meaning whatever we put into it will come out ten times louder.

So here is what an op amp looks like in a schematic: it’s a simple triangle with two audio inputs, one output and the two power supply inputs.

op amp symbol How to design an op amp

The two audio inputs are marked + (non-inverting if we want to preserve the phase) and – (inverting – if we want to flip the phase over) and the output is, well, the output. Simple eh? You have no idea just how simple this thing is. Let’s keep moving.

Here is a drawing of an IC op amp package – a diagram of where each of these terminals are located on the actual device itself. You’ll see some other stuff marked on this one – like offset null – don’t worry, we won’t need to use that or have anything to do with it.

Op Amp 741 300×168 How to design an op amp

The -V and the +V are the battery inputs and between the two batteries we might use (like a couple of 9 volts) we have what’s known as ground – or the center between the two power supplies. Here’s what the two batteries tied together might look like and ground is in the middle between the two where it is written “0V”.

batteries How to design an op amp

The picture I grabbed shows 6 volts – but imagine 9 instead – which BTW was exactly the circuit I first used to power the prototype of the PS Audio phono preamplifier – yep, two 9 volt batteries and a couple of op amps in a Roi Tan cigar box for a chassis.

OK, now let’s make our preamplifier. I think let’s start out making an inverting version, then we’ll go tackle a non-inverting one. As I mentioned all we need is two resistors and because the gain of an op amp is set by the ratio of the two resistors, then all I need to do is have one resistor 10 times bigger than the other. So I could use a 1K (1000 Ohms) and a 10K (10,000 Ohms). These are just little bits I could buy at Radio Shack if I wanted – maybe a whole \$0.10 each. Here’s what our circuit would look like.

inv opamp 300×161 How to design an op amp

We would put the 1K on the in and the 10K between the out and the in. The little triangle symbol you see on the + input is ground (the meeting point between the two batteries). That’s it. There’s not much more. Just connect an RCA input to the input side and another RCA connector to the output and you’re done. Whatever you plug into the input will come out 10 times bigger on the output RCA without any distortion or problem.

If I used quality parts, a PC board to hold everything, a power supply instead of a battery and a fancy chassis I have a commercial preamp. Want a volume control? Just stick a potentiometer on the input and you have it. Want it “high-end”? Use a good op amp and a better power supply and fancy connectors.

But maybe we want to have a non inverting amplifier instead. OK, all we have to do is change where we connect ground up and where the input is. Here, take a look.

non inverting 300×234 How to design an op amp

See? R1 is our 1K resistor and R2 is our 10K. Simple, no?

But this is a single ended preamplifier – where we can only put in the output of an RCA connector. Let’s get super fancy and make it a balanced input amplifier – where you would put in the output of a balanced cable with an XLR on its output – balanced out. We’re all familiar with that.

Here’s what that would look like – please ignore the values because I just grabbed this off of the internet.

balanced 300×242 How to design an op amp

The little symbol that looks like a rake is ground. Cold is the one side of the balanced signal and hot the other.

OK, this is getting lengthy and the purpose isn’t to teach you how to design an op amp but just how danged simple it is – and I am not simplifying anything here.

Tomorrow let’s see what’s inside of these little buggers that makes everything so easy and so widely used.

Paul McGowan – PS Audio, Intl.

Looking deeper

Yesterday I explained why use of the (then) new category of IC op amp was so darned attractive back in the 1970′s and still is today: easy to design with, reliable consistent performance and low cost. Good combinations!

Something else was also attractive to high-end audio designers like Stan and I – they were more like tubes in that they used a standard pinout for their enclosure. If you’ve ever played with tubes you know that not every tube sounds the same even though they are the same style. A 12AX7 tube is perhaps the most popular audio tube ever made. It fits nicely into a socket and it’s easy enough for designers and consumers alike to pop in different brands of 12AX7 tubes and listen to the differences – that’s because the entire amplifying device is contained in one enclosure, just like an op amp.

Now, for the first time in solid state history, audio designers could (if they had a mind to – and most didn’t) pop in any number of op amps into an IC socket and listen to the differences. This took the tube swapping idea to all new heights. With tubes you’re pretty much stuck with swapping out like tubes – 12AX7 was pretty much all you could throw in – but with these new op amps, the sky was the limit. The convention for packages was standardized: 8 pins DIPS (Dual Inline Package meaning two parallel rows of 4 pins) were used for single op amps and 14 pin DIPS were used for dual op amps and all adhered to a standard pin configuration. Wow.

But that’s when the trouble started for those of us who actually listened to our designs in addition to watching our meters. The audible difference in a simple preamplifier or phono stage between a 709, a 301 or a 741 were jaw dropping. Holy baloney! And it got worse: Motorola 709′s sounded decided different than National 709′s and they advertised the same specs! What was happening?

Let’s take a look at a schematic of the internal construction of the famous Widlar designed 709, the first truly great op amp.

709 op amp 300×224 Looking deeper

Now, let’s compare that schematic to the ubiquitous 741 (not designed by Widlar)

741 a 300×184 Looking deeper

Notice any difference? Of course you do they are very different designs – and yes they had very different specs as well – but the point is these seemingly simple building blocks were internally very, very different and they sounded different.

Here is a schematic of a modern day favorite for audio designers and found in many designs, the NE5532. Notice also that as time goes on in the progression of designs the number of components increases? Just look at the amount of parts in the 709 compared to the 5532.

ne5532 300×188 Looking deeper

OK, so tomorrow, let’s show you how all these complicated looking schematics can be easily understood and broken down into simple terms. I think you’ll find tomorrow’s post interesting and easy.

Paul McGowan – PS Audio, Intl.