If you have been used to designing with digital electronics, you would have been used to the almost block diagram “plug ‘n play” way that digital IC’s connect the pin to pin with very few extra components needed. A few chips and wires, and you might have a working digital clock. But in looking at analog circuits and, in particular, op-amps, this is not so.
If you ignore the inner workings of op-amps, you can still treat them as three-terminal block diagrams with some success. With a few resistors and capacitors, op-amps can be tamed into performing many useful and functional tasks.
The name op-amp, or operational amplifier, was coined in early uses to do analog mathematics such as integration and differentiation. First, op-amps were vacuum tube types. Early IC op-amps were the µA709 and the LM101, but there was a big evolution due to their enormous usefulness and popularity.
One of the most famous of these was the 741. You will still see many circuits and applications for this. Fortunately, manufacturers have kept the pins the same for the ever-increasing and improving range of op-amps. Because the external feedback components largely determine the behavior of op-amp circuits, you can generally substitute unless it is a particular and critical application such as ultra-high-frequency response or very low noise or offset.
We will look at op-amps here and in the next two articles: Part 1 – a general overview of how they work and how they get connected; part 2 – some useful linear applications; and part 3 – some non-linear applications.
Shown above are some of the forms op amps come in, including single, dual and quad versions. We will not get bogged down in mathematical derivations of gain equations or worry about microscopic offset voltages and currents at this early point.
How Op Amps Work and What’s Under the Hood
Shown above is a simple three-terminal device (ignoring, for now, the power supply).
Below are two input pins, and on the right is an output pin. The two inputs are called the –ve or inverting input and the +ve or non-inverting input. These two inputs are connected to a differential amplifier which may be bipolar transistors or FETs (see below for a differential amplifier), followed by more differential amplification stages.
A level shifting stage enables the output to be set to zero as all the stages are DC coupled, and finally, a low output impedance amp to drive the load and prevent any changes on the output from affecting the inputs. There are, of course, + and – power pins and often, but not always, pins to connect a trimpot to do the zero output shift and two pins for a phase compensation capacitor.
In the differential amplifier, which makes up the input stage (shown above), we find what is also called a long-tailed pair. Q1 and Q2 are a pair of very closely matched FETs or bipolar transistors. They are connected to –ve via Q3 which is configured as a constant current source.
The constant current source helps to improve the CMRR or common-mode rejection ratio. Keeping this high means any signal applied to both inputs simultaneously will result in zero output. This is a high-gain stage (gain is the ratio between an output signal to the input signal, often expressed in dB). Any difference between the + and – inputs will result in a proportional swing in the collectors or drains of Q1 and Q2. This gain is huge and not all that useful as it depends on the raw properties of the transistors and frequency.
As we have not tried to tame or control it yet, it is called the open-loop gain of the op-amp and is an important parameter to know. Also, any difference in the raw gains of Q1 and Q2 will result in a large unwanted output swing. This all sounds like bad news, but it can all be brought under our control and made predictable by adding a few components to provide negative feedback between the input and output.
The power supply is generally shown as + and – 15V, but the op-amp will work down to quite small supplies, e.g., +/- 9V, limiting the output swing.
Showing How it Works
Let us wire up our 741 and see what it can do.
Shown below is the circuit for an inverting amplifier with a gain of 100. Inverting means the signal at the output is 180deg out of phase with the input. We will look at a non-inverting one just now. If we were to feed the input with a small signal of 0.05V AC at 100Hz, we would see a signal at the output of 100X0.05, or 5V.
We could lower the frequency down to 1Hz or even DC with no problem and a constant output of 5V. However, if we increase the frequency, we would find that it starts to fall off in level at a certain point. Let’s see why.
If we were to remove R2 in the circuit above, we would have no feedback. This is called an open loop.
The gain is now a function of the op-amps top gain, and in the figure on the left, you can see it starts at a huge 100dB. But at only 10Hz, it starts to fall off rapidly at 20dB/decade, and hence, the voltage at the output is 1/100 of what it was every time the frequency is increased 10 times.
If this were an audio amplifier, this would be disastrous. On the straight part of the slope, the gain multiplied by the frequency at any point is a constant known as the gain bandwidth product. Gain is inversely proportional to bandwidth.
Replacing R2, we now have negative feedback. Unfortunately, this reduces the gain and allows us to actually control the level to 100 times or 40dB.
Note where it starts to roll off now—about 37kHz, which would now be okay for audio purposes. If we wanted to push it a bit further, we would have to increase the feedback and reduce the gain. If the gain gets too low, we might have to add a second stage of amplification.
Other Important Specified Parameters
THD + Noise: The total of the unwanted products at the output. As a ratio, it is normally expressed as a figure in dB. THD is total harmonic distortion, i.e., the 3rd, 5th, etc. harmonics at the output that was not present at the input, noise generated by the op-amp itself. An ideal voltage source voltage is in series, with the input pins representing the internally generated noise.
Offset voltage: The DC voltage that, when applied between the input pins, will cause a DC output voltage of zero. If both inputs were grounded, the output voltage of the op-amp would not be zero.
Slew rate: The time taken for the output to change for a given input. Specified as V/mS.
Equiv input noise voltage: The noise performance of the op-amp. An ideal voltage source voltage is placed in series with the input pins that represent the internally generated noise.
Common mode rejection ratio: The ability of the op-amp to reject signals appearing on both inputs at the same time. Particularly important for differential amplifier applications.
By now, you should have a better idea of how op amps work and how to choose and wire up one. In the next article, we look at practical examples of some linear circuit applications.