Amplifier Basics - How Amps Work (Part 6)

Main Index Articles Index

• Introduction
• Part 1 - Valves (Vacuum Tubes)
• Part 2 - Bipolar Transistors
• Part 3 - Field Effect Transistors and MOSFETs
• Part 4 - Operational Amplifiers (Opamps)
• Part 5 - Some Basic Linear Circuit Building Blocks
• Conclusions
• References
• Copyright Info
• Update Info

Section 5 is the last of the technical pages in this series, and this page finalises the topic at this level - at least until such time as I find (or someone points out) a mistake or major omission that I will then have to fix, otherwise there will be no further updates.

The articles in this series describe the essential building blocks of nearly all circuits in common use today.  There are others (of course) but they are most often combinations of the above - for example, a LTP (long-tailed pair) stage can be built using two cascode circuits, a current source and a current mirror.  The resulting circuit looks complex, but is simply a combination of common circuits such as those shown.

Other circuits are modification of the basic stages to exploit what might otherwise be seen as a deficiency - for example circuits that deliberately exploit the temperature dependency of a BJT can be used as high gain thermal sensors, or to stabilise the quiescent current in a power amplifier.

There are also some bizarre combinations possible.  A valve and BJT operating in cascode would be interesting, and would no doubt have some desirable characteristics (and I have seen this particular combination used in a power amplifier).  Likewise, a valve with a transistor current source instead of the load resistor has far better linearity and more gain than a simple resistor loaded version.

In many cases, ICs are available to accomplish the functions described.  Opamps are an obvious one, but there are also IC current sources, transistor arrays (ideal for current mirror applications because of the excellent thermal tracking), plus quite a few others.

There are countless different IC power amplifiers, many of which have very high performance.  There are several ESP projects that use 'power opamps' ... my terminology, because most are used just like any other opamp, but with higher voltages and the ability to drive loudspeaker loads.  Complete ICs are even available for Class-D amplifiers, which combine just about every technique described in this series, but with even more circuit concepts.  As you'd expect, these are also covered in separate articles.

None of the techniques described here is just for audio.  The same (or very similar) circuitry is used in industrial control systems, radio frequency amplifiers and any number of diverse fields.  While you could be forgiven for thinking that everything is now 'digital', that's not the case.  Analogue circuitry will be around for a very long time yet, and will probably never go away.  Even the most sophisticated digital process controller still has to interface with the 'real world', which is 100% analogue!

I hope that I have shed some light on the subject, and that you get some benefit from the information presented.  Please be aware that this series is intended as a very basic introduction only, and (almost) every configuration discussed here is fully explained elsewhere on the ESP site.  There are whole articles on designing with opamps, current sources, sinks and mirrors, and there's even a section dedicated to valves (vacuum tubes).

1. Philips 'Miniwatt' Technical Data, 7th Edition, 1972
2. RCA Receiving Tube Manual, 1968
3. Basic Electronics - Grob, McGraw Hill, 1971
4. Radiotron Designer's Handbook - Langford-Smith, AWV Pty. Ltd, 1957
5. Analysis and Design of Electronic Circuits - P.M. Chirlean, McGraw Hill, 1965
6. Data Sheets, various

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