Operational Amplifiers

The operational amplifier was originally devised in the 1960’s for use in analogue computers.  These are nowadays almost completely obsolete, although a few have some very special applications.  However the operational amplifier still has many uses in control and instrumentation electronics.  Although the original circuits used discrete components and were very expensive, miniaturisation has enabled op-amps to be made as integrated circuits, available for a few pence.

  Operational amplifiers require a dual power supply, which means having a central 0 volts rail, and a + 15 V rail and a – 15 V rail.  The full circuit diagram is shown below, but generally we will ignore the power supply.

Question 5. What is meant by a dual rail supply?  ANSWER

Notice that the op-amp has two inputs and one output.  It is a difference amplifier and amplifies the difference between the inverting input and non-inverting input.  Be careful not to confuse the symbol with a non-inverting gate.  We need to be aware of some definitions to do with op-amps:

• Open-loop voltage gain – ratio of the input to the output voltage with no feedback applied.  It is the D.C. gain of the amplifier or the gain at a frequency of 1 Hz.

• Closed loop voltage gain – the voltage gain with feedback.

• Bandwidth is the frequency range in which the output does not fall by more than 3 dB from its maximum value.

  The ideal op-amp should have the following characteristics:

• Infinite open loop gain

• Infinite input impedance so that no current is drawn

• Zero output impedance so that maximum current can be transferred to the load.

• Very wide bandwidth.

 If you want to review what was covered in Module 1 about op-amps, click HERE.

In practice the maximum open loop gain is 200 000.  Beyond that limit the amplifier goes into saturation which means that the voltage cannot go any higher.  The voltage is limited, of course, by the supply voltages.  In practice the limits are rather lower than this, about 1.5 to 2 V below the value of the supply.  Suppose the supply voltage was 15 V.  The maximum output voltage would be 15 – 1.5 = 13.5 V

Question 6. Can you compare the real op-amp with the ideal op-amp?  ANSWER

The behaviour of the op-amp is shown in this graph.  Notice that the horizontal axis is marked difference in voltage.  The op-amp is a difference amplifier.

The graph tells us:

• The op-amp amplifies the difference between the two input voltages

• When the difference exceeds the limits X and Y, the output is saturated.

• In between the limits the graph is linear, so there is little distortion in these regions.

The characteristic of real op-amps makes them unsuitable for use as amplifiers in open loop form, as clipping will occur and this will distort the signal.  Therefore some of the output is returned to the amplifier by a feedback loop.  This reduces the gain and makes the amplifier more stable.  The amplifier can be used in open loop form as a voltage comparator (Click if you want to revise.)

Question 7  What would the input voltage be to give an output voltage of  ± 13.5 V?  What is the voltage swing needed to go from negative to positive saturation?  ANSWER

The open loop frequency response of the op-amp is not very good:

We can see that the gain starts to fall away quite dramatically above a frequency of only 5 Hz, which is not very high.  It would be quite useless as an audio amplifier.  However the gain can be improved by reducing the gain with the use of negative feedback.

We can see from this graph that the more negative feedback that we apply, the wider the bandwidth.  There is a useful relationship for op-amps:

  bandwidth x gain = constant

  The constant is often called the unity gain bandwidth.  This is the bandwidth at which the gain of the amplifier is 1.

Now try Question 8.

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